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Alpha Com eclipse observing campaign

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weo
weo's picture
Alpha Com eclipse observing campaign

A campaign is announced to observe the upcoming eclipse of alpha Com. Please see AAVSO Alert Notice 506 for details.

Many thanks, and good observing,

Elizabeth Waagen, AAVSO HQ

lmk
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Vis?

The 0.8 mag drop is well within range of visual observing. Would the PI's accept visual, or just CCD for this campaign?

Mike LMK

 

JimK
JimK's picture
IR Observations

IR observations may provide additional information, as noted in the announcement.  I encourage all PEP observers with SSP4 to participate if possible.  In general a good target for single channel PEP given its brightness.  If we coordinate well we may be able to get pretty good coverage due to our differences in longitude.

Jim

uis01
Good idea Jim.  This target

Good idea Jim.  This target is ideal for PEP observers.

We should get a common comp and check designated for Alp Com for PEP observers.  There is none in the AAVSO list.

 

HQA
HQA's picture
PEP comp stars

Hi John,

The 60/68 suggested comps work well for PEP observers too.  They have close companions, but those companions will be contained within the PEP aperture.  In fact, that is what the researchers are doing, with their long-term monitoring using PEP techiniques.  They are not the most optimal comp stars, but after including their nearby companions, their summed contribution is stable to within a millimag or so over two decades.  What we need to do is include this target in the PEP program. I'll talk to Matt about this on Monday.

Those comps are probably too faint for the SSP-4 folks.  Perhaps Jim has some suggestions as to bright nearby possible comps?

Arne

JimK
JimK's picture
H band comp star

Far away but possible H and J band comparison stars are Regulas and Spica, but Vendemiatrix (Eps Vir) at about 6 degrees away may be a better choice and is a first magnitude star in these bands. It does have a fairly large J-H color index, about 0.4 compared to 0.13 for the trarget.  I will search a bit more to see if I can find a suitable comparison star with a color index a bit closer to the target star.

The target (alf com) may be a bit dim in my setup (14" SCT) as the H magnitude is around 3.3 but I plan to give it a try.  Still expect to get about 100 counts above sky background with a gain of 10 and integration of 10 seconds in the H band. 

As an aside, the Wikipedia article for Coma Bernices states that this star does not likely eclipse, and references Burnham's Celestial Handbook as a reference.  May be a chance for someone to update the article with the new information from the PIs. 

Jim

Carl Knight
Carl Knight's picture
SSP-4...

Hi Jim,

Keep me in the loop. I should be able to provide some coverage in J&H. I'll follow this thread a bit more closely having just caught up on the goings on. SNR gets a bit ropey on my 12" SCT around Mag 4 in J or H when looking at comps.

Cheers.

- Carl

 

JimK
JimK's picture
comp for ssp4

Carl,

Looks like you will be able to reach the target magnitude of around 3.3 in H, and if you use eps vir for your comp you should be OK.  I am asking for other input on comps for the IR bands as well, so stay tuned as we will want to harmonize the selection of an IR comp and check with the other SSP4 observers.  

What is your longitude?  I am trying to see what kind of time zone/weather coverage we might have.  (Looked at your blog, so 175 degrees east, right?)  I am at 73 degrees west and about 45 degrees north latitude.

Jim

Carl Knight
Carl Knight's picture
Hi Jim,I am at 40°08' S,

Hi Jim,

I am at 40°08' S, 175°22' E. Central North Island New Zealand. Alpha Com reaches the heady heights of 32° on the meridian, but observable from my observatory none-the-less.

Touché Jim. Just saw your update about getting my lat/lon from the blog.

Cheers.

- Carl.

 

tcalderw
tcalderw's picture
Two comp 60 stars

There are actually two magnitude 60 comp stars in the size A photometry table (id=13731CKX).  I believe we want 000-BBT-464.  The 60 and 68 comps have B-V very close to the variable, which is nice.

Sebastian Otero
Sebastian Otero's picture
Chart for visual observers

Since this is a very bright star, the three brightest comparison stars in the visual sequence won't plot on a VSP A-scale chart because they are too far away.
They can be plotted if we make a chart using a different position, since all of them lie to the NW.

Make a chart using coordinates.
Center the chart at 13 00 + 21 00 (easy to remember!) and use the largest possible FOV which is 1200 arc minutes.

13641BU is the chart I made. Check the Binocular chart option. Then identify alpha Comae in the chart (it is the bright star at the SE of the chart's center).

Cheers,
Sebastian

Richard Wagner
Richard Wagner's picture
Alpha Com Paper on arxiv.org

I don't know if everybody is already aware but Muterspaugh and Henry's paper on the up-coming eclipse appeared on arxiv.org on Thursday.  http://arxiv.org/abs/1412.1432

Richard Wagner

WCR

weo
weo's picture
paper by Muterspaugh and Henry

Hi Richard,

Yes, this paper is one of the ones I cited in Alert Notice 506 (and excerpted some text from). Thanks for participating in this campaign!

Good observing,

Elizabeth

tcalderw
tcalderw's picture
gold standard

If the PIs are willing to share their data as observing begins, it will be a great opportunity for AAVSO observers to calibrate against a professional system.

HQA
HQA's picture
alpha Comae on BSM monitoring list

I'm adding alf_Com to the BSM monitoring queue, hopefully to be observed by all 6 BSM systems on a nightly basis.  This will be at BVRI, plus I'll get some monitoring grating images.

Does someone want to raise their hand to extract the photometry from these images?  I can have them sent to your VPHOT account, or to our anonymous ftp site.

Arne

spp
spp's picture
BSM photometry

Arne,

I'm raising my hand somewhat tentatively here since the prospect of potentially reducing 24 images per night (if all the sites should have stretches of good weather) I find somewhat daunting, even using VPhot.  If we can split the work load amongst 3 or 4 "reducers" I'd be happy to be one of them. 

Are all the BSM sites current up-and-running?

Phil Sullivan 

WBY
WBY's picture
Spectrograms

This is a nice bright star. What about recording spectra at various stages in and out of eclipse? This may be too bright for many professional telescopes, but should be well suited for amateurs. 

Does anyone know the position angle of the orbital plane?

Another question: VSX lists both stars as F5V but Simbad lists α Com A as F5V and α Com B as F6V. Is this a typo in VSX or is there a difference in classification between sources?

 

Finally, how does one insert Greek letters in Drupal (this forum)? They aren't included in the special character list that displays and the list doesn't scroll to show additional characters.  Currently I compose in Word, copy and paste if I want to include Greek letters. 

Brad Walter, WBY

SET
SET's picture
Cart

Sebastian,

I put your set up into VSP. I do not get any comp stars brighter than 4.9. Where is the marked star brighter than 4.9? I need 1 or 2 brighter than 4.3. There are 2 brighter stars at the east (left) edge of the chart, right by the East marker. Is 1 of these them? If so, which one and what is its magnitude? Perhaps you can attached a copy of your chart. Thanks.

 

Chris Stephan   SET

Wooster, OH USA

weo
weo's picture
chart for alf Com

Hi Chris,

I sent you this off-line but wanted to answer here for everyone. If you use VSP and put in Sebastian's chart code 13641BU (don't put in the name or coordinates), you get Sebastian's  chart with the 43, 44, etc. and alf Com at the center. (If you specify binocular chart, you will still get Sebastian's chart but with just the brighter comps.)

Good observing,

Elizabeth

Sebastian Otero
Sebastian Otero's picture
Bright comp stars

Hi Chris,

Elizabeth already gave you the answers.
There are 43, 44 and 47 stars in that chart. Maybe you were centering the chart around alpha Comae?

About brighter comparison stars, they won't fit in any VSP chart.
The 43 comp star has V= 4.25 so it is brighter than alpha Comae by 0.07 mag. (alp Com is 4.32) so I don't think that you need more comparison stars.

To get the photometry table for the chart code 13641BU you should enter the code 13731TCK.
It's better to use the more accurate photometry table values to reduce the data instead of the rounded chart values.

Cheers,
Sebastian

AAX
AAX's picture
Hi Sebastian, What do you

Hi Sebastian,

What do you think about this chart (attached). I select BSC magnitude stars with b-v < +1.0.

hasta la vista!

Alex!

STR
Alpha Com position angle

I've watched this beautiful close double off and on for the last 30 years.  The stars always seem to be equal brightness.  The position angle of the orbit is close to 11 deg.    --R. Stanton

Sebastian Otero
Sebastian Otero's picture
Spectral types

Hi Brad,

It depends on the source. Muterspaugh et al (2010) classify them as a pair of F5 dwarfs.

The SIMBAD qouted spectral type for alp Com B comes from a 2013 paper.

I wouldn't worry for such a difference in published spectral types, they are typical.

Cheers,
Sebastian

PMRuscitti
PMRuscitti's picture
Comparison star

Hello everyone,

I'd like to take some measures next month of Alfa Com. With my Newton and Atik 314L+ I cover a field of about 30 "x20" and with another Newton a field of approximately 50 "x35". There are no stars of comparable magnitude in these fields. How to proceed in these cases?

Thank you, Paolo

Tonisee
Hello Paolo, you can proceed

Hello Paolo,

you can proceed as PEP photometry manual describes: comparison - target - comparison - target - comparison etc. I have done that many years for a bright Be star X Per, where comparison star was ~1 degrees away. I used a 60cm telescope equipped with a CCD photometer 10'x10' FOV. That telescope is "electromechanical" - no fancy GoTo-s etc (pure manual power to overcome the inertia of a few hundred kg telescope), just setting circles and 0.5 degree FOV finderscope :-) One measurement of either comp or variable took usually 30-40 seconds (movement, locating star, centering on crosshair + 1-10 second exposure). Taking 5 variable measurements per filter bracketed by comp measurements usually gave pretty good photometry.

As with PEP photometry, you should watch for quality of the sky - most probably you don't get good photometry trhough variable cirrus etc. Just plotting comparison or random constant star (in target FOV) signal vs time shows those problematic atmospheric conditions very well.. Sometimes when transparency changes gradually, you can interpolate that changing extinction with good results.

Some links:

http://www.aavso.org/content/aavso-photoelectric-photometry-pep-program

http://www.hposoft.com/Astro/PEPManual/Manual.html

Hopefully that helps a bit :-)

PMRuscitti
PMRuscitti's picture
PEP Procedure

Thanks Tonis,

I understood the method. This is a new procedure to me, from experience because I have never worked with a photoelectric photometer.
I would say that with a control script of the equatorial mount, plate solving and synchronization of CCD, the procedure will be more comfortable.
When the weather will be better, I will start the tests.
Thanks again.

Paolo

HQA
HQA's picture
alpha Com

Hi Paolo,

What Tonis recommends is one good way to handle bright stars without nearby comparisons.  You still have to move a good distance with alpha Com in order to find something comparable, and the two drawbacks are that it takes more time to complete the measure and it requires photometric skies.

Another technique, which we are using with the Bright Star Monitors, is to offset the target far enough to pick up a handful of relatively bright stars (6-8mag), while keeping the target in the field of view.  I will have these calibrated shortly and then will use them as an ensemble to measure alpha Com.  Using multiple lower-signal stars as an ensemble effectively gives you one "master" comparison star that has enough signal/noise.  The other common technique is to take multiple short exposures, short enough to not saturate the target.  Then when you stack these exposures, you increase the dynamic range, and the fainter stars in the field now have enough signal/noise to be useful.

In general, though, this is a very bright star out of the galactic plane.  It is a good target for PEP (and we have a couple of SSP3 photometers that we are willing to loan), for DSLR, or for very wide field systems like a BSM.  Traditional 6-8-10 inch telescopes will have difficulty with this field, requiring very short exposures to not saturate alpha Com as well as having small fields of view.

Where there is a will, there is a way. smiley  I am sure that you will find a method that works for your system.

Arne

HQA
HQA's picture
BSM sites

Hi Phil,

Three of the sites are currently up and running: HQ, Hamren (HI), and South.  Two others will come on-line prior to the eclipse:  NM (most likely in a few days) and Berry (Perth, most likely in a few weeks).  There is an outside chance that Argentina will also be ready in January.  Remember that is is unlikely that all of these sites will be clear or functional on any given day; my guess is that we'll get daily coverage from one or two sites.

Thanks for raising your hand!

Arne

spp
spp's picture
BSM photometry

Arne,

Thanks for the clarifiation on the potential data stream from the BSM sites for the alpha Comae campaign.  I withdraw the "tentative" and volunteer with enthusiasm.

Phil 

WBY
WBY's picture
Reducing BSM Images of Alpha Com

This relates to Phil's posts of 12/9

I can help as well. What concerns me more than reducing the images is bandwidth needed to get them all  if we have to create master calibration frames for each night from individual bias, flats and darks. Unless we do all the work on Vphot with copies of the original images loaded on  the AAVSO server, or there are master calibration frames for the BSMs that we can download and use for calibration for multiple nights or even a single night for each telescope, you are talking about something on the order of a GByte of data per BSM per night for four-color photometry. That would be pretty tough for my DSL connection. 

Brad Walter, WBY

HBB
HBB's picture
DSLR

Having just completed the DSLR Photometry course, would this be a good project for DSLR Photometry?

Barbara

Mark Blackford
Mark Blackford's picture
DSLR

Hi Barbara,

as Arne mentioned, Alpha Com should be a good target for wide field DSLR photometry. Unfortunately it culminates at only 38 degrees here and I'm pretty sure it won't be visible at all from my observatory due to nearby bushes and garage. I might try using a portable mount elsewhere in my backyard.

Are you planning to use your 80mm refractor or a camera lens? Cheers,

Mark

HBB
HBB's picture
DSLR

Mark,

I considered using the 80 mm telescope but I don't think the FOV will give me enough comp stars so considering a camera lens.  

Barbara

Mark Blackford
Mark Blackford's picture
DSLR lens

Hi Barbara,

I was planning to use my 200mm lens which gives a 6.5 x 4.3 degree FoV with an APS-C sensor. A 100mm lens might be better. However, from Sydney alpha Com won't culminate until well after sunrise in mid to late January and even then it's only 38 degrees above the horizon. So I don't think I'll be able to contribute to this campaign. But I'll enjoy watching the light curve develope as you northerners submit observations. Good luck,

Mark

HQA
HQA's picture
BSM images

Hi Brad,

All AAVSOnet data is transferred from the telescope sites to AAVSO HQ, where it is dark subtracted and flatfielded.  The creation of the master calibration frames is done at HQ "behind the scenes."  Those processed images requested by researchers are then transferred to VPHOT and/or to our anonymous ftp server.  For alpha Comae Berenicis, we are looking at an average of two telescopes observing on any given night.  Each telescope will take a set of 33 images (5B,7V,9R,12I), each 1.5MB in size after compression.  Therefore, the transfer to the VPHOT server will typically be about 100MB, well within its capacity.  While it would be great to take time series data during the eclipse, each site will only get 4-5hrs of coverage during the expected eclipse date.  We'll do one dataset per night until near the eclipse date, then change to one dataset per hour, which will be a hit on VPHOT for only a few days.  It is possible to automate the stacking at HQ as part of the pipeline, before transferring the images to VPHOT, but the coding is complex enough that it won't happen before the expected eclipse.  Note that your DSL connection won't be impacted in any case, since all of the data before analysis is already on the VPHOT server.

A lot of the steps you have to take for your own images are taken care of for you with AAVSOnet!

Arne

 

HQA
HQA's picture
DSLR photometry

Hi Barbara,

Yes, this will be an excellent target for DSLR observers.  The two real advantages are that it is bright, so even simple techniques such as a fixed tripod will be adequate; and the field is sparse, so the wider coverage by a typical DSLR telephoto will help in finding comparison stars.  The other advantage in my mind is what I mentioned in my reply to Brad:  each site will only get a few hours of coverage, as this is an early morning object.  Having a few dozen DSLR observers spaced around the world would be an excellent way to cover the whole light curve!

Arne

WBY
WBY's picture
Preparing for the Campaign

Please escuse me if the following is a statement of the obvious for many of you.

If you are planning to do CCD photometry for the campaign and won't be using a small aperture telescope (say less than 150 mm), you might think about experimenting with aperture masks ahead of time that will allow you to do 10 second or longer exposures without exceeding your CCDs linear region. If you have a set of master flats that you use for multiple night's data you will need new ones with the mask in place. Stacking multiple shorter images can overcome scintillation, but if you have an astronomical CCD camera that has an iris shutter (I think that is essentially all  other than SBIG or "shutterless" frame transfer types) you will have vignetting issues due to the shutter reaction time for really short exposures.

I don't have a feel for the size of scintillation effects for telescopes in the amateur size ranges and different exposure times.  In the Dravins papers the autocovariance seem to be negligible for any telescope in the amature range at times well below 1 second. Hwever, his measurments were done at relatively high altitude at La Palma. It may take longer for the correlations to die out at sea level but I think exposures longer than a couple of seconds would bring scintillation noise well below other noise sources. 

Brad Walter, WBY

HQA
HQA's picture
scintillation

Hi Brad,

A good source for scintillation information is

http://astro.corlan.net/gcx/scint.txt

which will give you the scintillation noise for typical amateur apertures.  Scintillation is why there are so many frames being taken by the BSM systems for alpha Com (you decrease scintillation by stacking).

Another trick with bright objects and "big" telescopes is to use a neutral density filter.  However, the biggest problem with typical telescopes is the small field of view.  This target is in a very sparse region of sky, and you won't have much to work with, no matter how you decrease the amount of light through your system.

Arne

PMRuscitti
PMRuscitti's picture
wide-field photometry

Hi Arne,

you're right, you need to think of something for the observations. I'm preparing a setup consisting of CCD and lens at short focal length, and also I'm "dusting off" my Canon DSLR with telephoto. I will try a wide-field photometry really!

Thank you, 

Paolo

WBY
WBY's picture
Alpha Comae, Field of View and Comp Stars

Alpha Comae, Field of View and Comp Stars

There has been a lot of discussion about the lack of availability of comp star and huge fields of view that would be necessary to do Photometry for this object. However there hasn't been much quantitative information to date.

So I went to SeqPlot and downloaded a table of all the BSM North stars noting which ones were shown as variable (yellow). There are 21 stars of v mag 10.0 or less in an 83 minute (2500 sec FOV centered on Alpha Comae. This is the widest field that can be plotted in SeqPlot. I didn’t plot adjacent fields because I was interested in potential comps stars available in a  field of view available to fairly common small refractors. A 4" f/5 refractor will give you about 3.5 arcsec per pixel for 9 µm pixels and about 2.34 arcsec per pixel for 6 µm pixels. So if you have a camera with at least 2000 pixels in each direction you will probably be able to cover the above FOV with most CCD cameras and many DSL cameras. The telescope also cannot vignette or distort drastically over this sized FOV. Many wide field refractors in this size range can do this. 

The remaining question is which of these stars will provide useful SNRs.  Doing a bit of analysis, I found that there were 10 stars in V and 17 in R that appear to provide SNR greater than 40 with single exposures with max single pixel net count  of 25000 in the Alpha Comae stellar image. Using these stars in ensemble should provide effective SNR close to 100, and stacking 5 or 6 images should provide precision well beyond the 100 SNR level.   A spreadsheet is attached for anyone interested in the Seqplot comps. 

To make the calculation I made some assumptions. I assumed a radially symmetric Gaussian distribution with 2 pixel FWHM (diameter). Given the telescope parameters above, you will probably have to de-focus slightly with 6 µm pixels and you will definitely have to de-focus with 9 µm pixels to avoid under sampling. Therefore, a two pixel FWHM does not assume overly optimistic seeing conditions and is reasonable to use for calculations.  The Gaussian distribution assumption and the symmetry assumptions may not give highly accurate total flux values but they seem to provide 10% guidance for total flux. I would expect the ratios to be more precise. 2 Pixel radius FWHM equates to σ of approximately 0.849 pixels. The formula for integrating the total flux for a given star is then 

<see second file attachment. I can't find a way to insert an Word equation
or even an image of it in Drupal>

A scaling coefficient, “C”, of 5.05 was derived to go from the max counts of the central pixel to the total net counts for the star image. This was determined by taking the ratio of the value from the unbounded integral to the value of the integral evaluated to a radius of 0.5642 pixels (which gives an area of one pixel). This ratio is then multiplied by the Max Net counts in the central pixel entered in Cell AE1 and converted to flux using the GAIN in cell AG1. The spreadsheet is designed to allow different values to be entered for Max Net Counts and GAIN. The FWHM assumption of 2 pixels is fixed. The SNRs of the comp stars are calculated by multiplying the values for Alpha Comae by the square root of the flux ratios.

At first, this method seemed to give low SNRs. However when I compared them to some actual values measured at my site at similar max pixel counts (17,000 to 25,000) and adjusted by the square of the ratio of the FWHM values (FWHM with the f/10 OTA I normally use for photometry are most frequently in the range of 3.5 – 4.0 pixels). The actuals were within 10% of these rough theoretical calculations and the theoretical values tended to be slightly higher than the actuals.

So I think you can work effectively the FOV of a small refractor with stars that have been monitored by BSM North.    I know Arne is planning to put together a list of additional comps in a reasonable FOV. That should be an improvement over this list.

Brad Walter, WBY

 

WBY
WBY's picture
Alpha Com Table Update

I updated the table in my previous post by adding calculations of the separation distance and  position angle.I also weeded out duplicate stars which were confirmed as duplicates using Aladin images. This leaves 6 stars in the 85' field (The total field is actually 85' but The tick marks spanned 83' ) with SNR in V, R and I above 40 with  25000 single pixel max counts for Alpha Comae. If you can go to 30,000 counts without risking saturation, which many NABG cameras can do, the count increases to 11 stars. The SeqPlot chart showing BSM star locations is also enclosed

In addition to the BSM Stars SeqPlot shows an additional Tycho 2 star in the field. I have added it at the bottom of the list.However, at essentially 10.0 V magnitude you would have to increase the max pixel count for Alpha Comae to about 35000. to get SNR over 40.

Most of you know where your cameras start to saturate. You may want to verify linearity at these levels if you haven't already. I am probably stating the obvious, but just because a camera is NABG and has 16 bit digitization doesn't mean it saturates at around 65000 counts. For example, my ST7 saturates at around 41,000 counts and the non-linearity starts to become significant above 32,000. My STXL saturates at about 56,000 counts and maintains good linearity up to about 42,000 counts. Of course these are raw counts before image calibration, but with bright stars and short exposures you don't have to leave a lot of headroom between max net counts and max total counts. A few hundred counts is probably enough even if the moon is up. 

Brad Walter, WBY

chrismlt
Question about a double field photometry, and a mailing-list

Dear all,

 

My english is not very good, please don't worry about.

 

That's the first time I post a mail on this forum. I observe from south-east of France, and the Alpha Com eclipse is a long awaited one. For the last couple of weeks I have tried to federate french observers on this rare event. Our group comprise now about 10 observers (visual+DSLR+CCD), hoping for a cooperative weather in january, which is not assured ; autumn was severe here, as I did not even see any star for at least one month.

 

Our observations will be sent to the AFOEV (Association française des observateurs d'étoiles variables) and then, sent to AAVSO.

 

As many other, our main trouble is about the lack of reference stars for photometry in a medium sized field (say : one degree).

 

Asked about this subject, Mr Muterspaugh and Mr Henry where kind enough to answer and to point out those stars :

 

*

Quote :

 

HD 114300 (V=8.27, Hipparcos scatter of 0.015 mag)
HD 114265 (9.77, 0.037 scatter)
TYC 1451-117-1 (V=9.98)
HD 114219 (9.45)
BD+18 2691 (9.67)
BD+17 2592 (9.49)
HD 114762 (7.29, 0.0011 scatter; historically, this may be the first star around which an exoplanet was discovered, in 1989)
BD+18 2692 (V=9.63)
GJ 9428 (V=11.8, but bright for anyone with infrared equipment)
HR 4962 (V=5.87, 0.014 scatter)
BD+18 2699 (V=9.63)

I'm not sure how faint you can use reference stars without saturating the detector for the bright target star---it somewhat depends on the well depth of your CCD and its noise properties.  While the V=9-10 stars may thus not be useful, HD 114762 and HR 4962 probably provide sufficient references.

Gregory Henry : Unfortunately, there are not bright comparison stars within a one degree field of alpha Com A.  The closest & brightest within the field is HD 114300.  It's about 7 or 8 arc minutes to the southwest of alpha Com.  As far as I can determine, it is not known to be variable.  This will be a tough observation for CCD users.

*

About HR4962, I'm a little bit amazed, since this star is repertoried in the VSX. (Hipparcos scatter : 0.014 hpmag, VSX : V5.87-5.94).
 

So, I have a question that I'll leave to your sagacity, if you don't mind. I think this will be at least unorthodox, but still, I hope this not to be too severly wrong.

My mount is a steady one, but with no GoTo capabilities, so I will not be able for nights to move back and forth between Alpha Com and the comparison stars. Humm .. I can do that for a night in case of special alert, but as I have a day job, this will not be a practical solution for several nights.

I've tought about having two optics working conjointly : a telescope or a refractor, with a field ranging from 1 to 2 degrees for high precision photometry, with a 200 mm lens piggyback : field 5°+ for lower precision photometry.

The lens will survey a larger area that will comprise the two recommanded stars (HD114520 V=6.82 and HD 113848 V=5.99), and HR4962, and Alpha Com. The goal here is to be sure there is no significant decrease in the sky quality by passing clouds or other light gradient all night long.

Once I'm sure about the sky quality in the global area, I can check the star HR4962 for variability both in the lens field and the telescope field with other reference star in the field. If no variablity overnight of HR4962 is found, now I can compare Alpha Com with HR4962 in the telescope field for higher precision photometry, which could be better than comparing Alf Com against a 8.3 mag star, I mean. If any significant variability is found on the light curve of HR4962, I can still compare Alf Com with another star in the one degree filed, with a lower expected precision.

Can you advice on this, please ?

*

Finally, maybe of interest here : a mailing-list has been created on yahoogroups for last minute news, live alert and preliminary reports during the different phase of the eclipse.

This list can be found here :

https://groups.yahoo.com/neo/groups/Alf-Com-Eclipse/info

As this campaign will be an international one, the main language will be english. Some of our team members have already subscribed, and Mr Muterspaugh honored us by subscribing to this list too.

New subscribers are welcome.

 

Best regards,

Christophe.

 

HQA
HQA's picture
alpha Comae data from BSM

I kept wondering why Brad was finding BSM data in Seqplot for Alpha Com.  I had done a search for the star under its two primary names (alf Com and NSV 6116) and found no data.  Then I checked by coordinates, and see that I had entered the star in the BSM queue as "NSV_6166".  Typical typo from me.  Anyway, we have 2928 images from BSM over the period 2011-01-13 through 2012-06-20, comprising BVRI datasets on about 180 nights.  This dataset should be useful to look for variation of alf Com outside of eclipse, and for studying the 5.8mag HR 4962 (NSV 19637).  So I've done the differential photometry for both targets and submitted the BVRI observations (only rudimentary culling of bad data).  Anyone using VStar or similar can now have a dataset to play with.

Arne

HQA
HQA's picture
double field photometry

Hi Christophe,

I don't think that you gain anything by your use of two different fields of view.  Why not just use the 200mm system with its 5degree field of view?  I guess the advantage would be if the 200mm system was unfiltered or used a single filter, and the telescope system had a filter wheel so that you could take exposures with several filters.  Otherwise, you are always limited by the precision of the 200mm system in knowing the stability of HR 4962, so I'd also use it for measuring alf Com.

My Bright Star Monitor observing in 2011/2012 used an ensemble of nearby stars: 6 stars of 9th magnitude and one star of magnitude 7.3.  Combined, these give photometric errors in the 5-10mmag range per exposure, with better results when stacking images.  So using the brighter, but more distant, recommended comp stars may not be necessary.  If you offset alf Com in the field of view, you can pick up slightly brighter comp stars.

I would experiment.  You have a month before the critical exposures start, so spend a night or two with your two systems and see how well they perform and with what kind of photometric error on repeat measures.  That will tell you the right set of observations to make when the eclipse begins.

Arne

Tonisee
Ensemble of faint(er) comp stars

[quote=HQA]

My Bright Star Monitor observing in 2011/2012 used an ensemble of nearby stars: 6 stars of 9th magnitude and one star of magnitude 7.3.  Combined, these give photometric errors in the 5-10mmag range per exposure, with better results when stacking images.  So using the brighter, but more distant, recommended comp stars may not be necessary.  If you offset alf Com in the field of view, you can pick up slightly brighter comp stars.

[/quote]

Arne, how exactly you do that multiple (weaker) comp star combining into an ensemble? How do you treat individual uncertainties of comp stars?

Tõnis

WBY
WBY's picture
Ensemble of fainter stars

Tonis, 

I know you directed your question at Arne, but I was involved in exactly this discussion with him and a couple of others within the last month concerning the use of Transform Applier for ensemble photometry. 

If you are  using VPhot it will handle ensemble error calculation for you for untransformed values.

If you are transforming with Transform Applier you need to do a single comp measurement of the traget and check star individually with each comp and transform them individually in TA. Then you average the results from  a single image. You can use a straight average or weighted averages using 1/σi2/Σ(1/ σi2)  as weighting factors (Wi), where σis the error for individual transformed magnitudes using a single comp star. You use normal propagation of error calculations. For straight average that would be SQRT(Σσi2)/N where N is the number of magnitude measurements you are averaging. If you use the weighted average, the formula becomes just SQRT(Σ(Wiσi)2). the 1/N  is built into the weighting factors (ΣWi = 1). 

Do not use the magnitudes of the comp stars as a weighting factors. Use the error values you get from TA. 

If you really are a purist, then you may want to calculate fluxes equivalent to each of the transformed magnitudes, average them and then convert back to a magnitude. If you are at that level, then you know how to calculate error propagation and I leave the formula derivation to you. However, for the relative error sizes we are dealing with, the difference in the two methods is at least an order of magnitude smaller than the errors you report for your measurements and is, therefore, insignificant. 

For transformed values submitted to WebObs, you would report the transformed taget and check star magnitudes. Reporting the instrumental check star magnitude does no good for ensemble photometry since there is no instrumental magnitude reported for the comp. Also your check star must not be included in the ensemble of comparison stars.  I would report the ensemble stars in  the comments field for each measurement to which they apply. If you use the AAVSO labels (except where there are duplicates) you can fit "comps-"and up to 23 stars. I doubt you will have close to that number for this target. If non AAVSO comp stars are used in the ensemble I would report them and the comparison magnitudes used as well, space permitting. You are limited to 100 characters including punctuation and spaces. 

Brad Walter

weo
weo's picture
Alpha Com campaign cancelled

The observing campaign on alpha Com has been cancelled at the request of PI Dr. Matthew Muterspaugh. Please see AAVSO Special Notice #395 for details.

Many thanks, and good observing,

Elizabeth Waagen, AAVSO HQ

weo
weo's picture
Alpha Com campaign - no eclipse upcoming

Dr. Muterspaugh reports that the eclipse of alf Com, if it did occur, occurred about two months ago. He writes: "There is no need to continue observations.  The ones that have been taken in fact do give a useful assessment of circumstellar objects trailing the leading star in its orbit, so they are certainly valuable.

"I would appreciate it if you could include a very personal thank you and apology to the people who have been observing this system in response to the campaign announcement."

Many thanks, and good observing,

Elizabeth Waagen, AAVSO HQ

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