Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 06/01/2020 - 22:04

What are the options, criteria, and designs for selecting photometric filter set purchases? 

My primary interest at this point is in ExoPlanet observing (also interested in eclipsing binaries, CVs, cluster variables, pulsators, etc.).  Lots of questions;  ideas on best options and suggestions??

For general use, are the U,B,V,R,I choices adequate for general variable star observations using CCD cameras?  I see several manufacturers that make various filter designs like Bessell, Sloans, Cousins, etc.  Is any one type adequate for variable stars?  Some manufacturers seem to have discontinued their filter offerings and some offer regular vs. premium filter sets -- what are the differences?  Who offers what filter sets within the amateur price range?

I also see some kind of specialized filter intended for ExoPlanet observing.  What are they designed for that makes them preferable for exoplanets -- (instead of a V filter?)?

It seems some filters are dyed glass and some are interference coated glass.  Which are better and for what useage?  Are some filters better for faster f/ratio scopes, like f/6?  There are some suggestions that thinner glass filters work better at faster f/ratios.  If they are, how are they better?

NBL      Bob N.

Affiliation
American Association of Variable Star Observers (AAVSO)

I think some of the difference is due to the difference in the transmission curves for the TG filter of various brands. The Astronomik TG filter for example, follows the V-filter curve almost exactly to the left of the peak, then drops off more quickly to end at approx 625nm vs about 675 for the V-filter. The Astrodon Generation 2 E-Series TG filters transmission curve is much more rectangular in shape and cut off at a much lower wavelength (about 570nm) with no overlap in wavelength with the TR filter. The Astronomik TG has a good deal of overlap with the Astronomik TR.

This would be why the  UBVRI filters were standardised in the first place.

Affiliation
American Association of Variable Star Observers (AAVSO)

(1) Or TG-filter observers could try transforming to V passband. It should not be too difficult, and then star color would matter much less.

(2) If it's true that reported TG magnitudes differ from reported Vis magnitudes, it is not necessarily the TG measurements that are biased.

Affiliation
American Association of Variable Star Observers (AAVSO)

Eric,

Agreed on both counts.

This thread started with a question by Carolyn (registered 3 months ago), probably a beginner at photometry, so the idea of transforming tri-color green to V would probably only muddy the waters for someone early on the photometry learning curve.  But, now that you have brought this up....

Have you ever done that, transformed tri-color green to V?  If so, I'd be interested in your (or anyone else's) experience.  I'd be particularly interested in how well the tranformed V measurements work with red targets.

I have an ST 402me with the RGBC filter wheel.  The photometric wheel for this camera seems no longer to be available anywhere, even used, so I may need to try this.

Phil

Affiliation
American Association of Variable Star Observers (AAVSO)

A friend of mine had a nice astrophoto setup consisting of a telescope with SBIG STL-11000 monochrome camera with LRGB filter set (most probably from SBIG). I asked him to image M67 through RGB filters, measured preprocessed images using Sextractor, and compared results against Stetson M67 standards. Stetson's M67 (NGC 2682) photometry table doesnt have Rc filter measurements, so I decided to try Ic instead. I got following results:

 
Linear regression slope

Blue vs Johnson B
0.991 +- 0.004

Green vs Johnson V
0.991 +- 0.003

Blue - Green vs Johnson B-V
0.588 +- 0.014

Green - Red vs Johnson V-Ic
0.584 +- 0.010

Unfortunately it's not possible to add pictures here.. Those regressions look mostly very neat. Just blue-green vs B-V has noticable scatter. IMHO one could expect quite good transformed values from those measurements.

Best wishes,
Tõnis

Affiliation
Variable Stars South (VSS)

All of my TG measures are transfprmed. The ~0.2 mag difference from Vis for some LPVs persists.

It has occurred to me that a careful investigation of the difference could be interesting, but I haven't done that. I've had the impression that V mags for these stars also appear to be brighter than Vis, but some TG mags are wrongly reported as V so I'm not completely sure of this..

Brian Skiff has posted some  very red standard stars which would be appropriate for transformations for very red targets.

Affiliation
American Association of Variable Star Observers (AAVSO)

Let me be sure I understand what you do. You transform the green filtered magnitudes to V, then report them as TG?  Is that right? 

My understanding is that green filtered meaurements transformed to V should be reported as V.  Maybe this is where some of the confusion comes from.  Perhaps I misunderstood what you were saying.

I think  that comparing TG (untransformed) magnitudes and errors to true V (done with V filter) magnitudes and errors derived from the same system would be a interesting project, especially as the targets get redder. 

This might provide some guidance to TG observers as to how far into the red they can go and still get usable data.

Phil 

 

Affiliation
American Association of Variable Star Observers (AAVSO)

With the proviso that red stars are typically poorly transformed with B-V color index. V-I color index works well, possibly other red-side color indices (V-R, R-I etc) as well. The redder the star, the truer this is. B magnitudes of red stars vary widely, and they are scientifically interesting in their variety, but this is exactly what makes them a poor basis for transformation which wants consistent spectral behavior.

Yes, if an observer credibly transforms green-filtered magnitudes to V, then they are best-estimate V magnitudes and should be reported as V and compared to other V magnitudes. TG has nothing to do with it any more. If an observer's transformed-to-V magnitudes match other reported TG magnitudes better than reported V magnitudes, that observer's data may have bigger problems than transformation.

[Edit: in my view, most of these problems arise from the all-too-easy confusion between filters and passbands. A V filter (physical bit of glass) will render an approximation to V-passband (standardized spectral response) sensitivity, then transformation intends to estimate, to account for the difference. Very many confusions and mistakes in transformation arise from confusing filters and passbands, even it just for a moment.]

 

Affiliation
American Association of Variable Star Observers (AAVSO)

At the AAVSO fall meeting a couple of years ago I asked one of the professional presxenters about filters and he said he used Sloan so I bought a Sloan g'2 from Astrodon.  I know that the AAVSO has been using J/C since the dawn of time, but I feel that, since Sloan is the preferred one for professionals, the AAVSO should also give the comp stars in Sloan.  I will admit that I haven't done any photometric work as yet, being mostly a solar observer, but I hope to get into it.

Affiliation
American Association of Variable Star Observers (AAVSO)

Hi MLL--

I think the current answer is that are no Sloan filters available in the sizes typically used in amateur astronomer's filter wheels, and the situation is a bit dicey for J/C Bessell filters, too, with lengthy delivery times.
For the time being, J/C is the only choice. I'd suggest you begin work with a V filter and learn the ropes, then add Sloan filters when they become available to the amateur market.
Arne and others may wish to comment on the time and effort that will be required to integrate fully Sloan photometry into the AAVSO databases.

--Richard

Affiliation
American Association of Variable Star Observers (AAVSO)

Looks like Sloan is the preferred photometric system for the profesionals we collaborate with, although not sure since there are others. Will be very interesting to know if there are plans to add Sloan to the database in the short/ mid term. Interestingly AAVSO accepts SLOAN magnitudes contribution, right ?. Any insight on the subject is welcome so the amateur observer can invest properly as good filters are expensive.

--Jose Luis.

Affiliation
American Association of Variable Star Observers (AAVSO)

Jose Luis,

You may be interested the this forum discussion:

https://www.aavso.org/sequences-use-sloan-filters

As far as I am aware, the current situation remains as follows: 

APASS, which forms an important part of the data for making AAVSO comp sequences, collects images in B, V, and Sloan g, r, and i.  These APASS Sloan magnitudes are available but not via VSP, the Variable Star Plotter tool used for making AAVSO charts and comp star sequence tables used by observers.  Most AAVSO observers rely on VSP for AAVSO comp stars and sequences.

You can submit Sloan magnitudes to the AID (AAVSO International Databse); you can see Sloan magnitudes displayed in the AAVSO light curve generators; you just can't get AAVSO comp star sequences with Sloan magnitudes from the VSP.

Based on the discussion in the referenced forum, there is a long term plan, or at least an intention, to add Sloan magnitudes to the VSP.  Doing this probably requires a large investment of time and resouces which may currently not be available. 

Until Sloan comp stars are available from VSP, it doesn't seem to me that buying Sloan filters makes much sense for most AAVSO observers.

Phil

Affiliation
Variable Stars South (VSS)

I'm just catching up with this thread.

In post #58 Phil wrote:

"Let me be sure I understand what you do. You transform the green filtered magnitudes to V, then report them as TG?  Is that right? 

My understanding is that green filtered meaurements transformed to V should be reported as V.  Maybe this is where some of the confusion comes from.  Perhaps I misunderstood what you were saying."

The TG label is for DSLR or one-shot colour camera green channel photometry results. The Web Obs submission protocol allows for the result to be labelled as Transformed (to the Johnson V standard) or Non-transformed.

All of my (at least relatively recent) TG results are DSLR green channel data transformed to the Johnson V standard. For some long period variables, these results (and the TG results of other observers) are about 0.2 mag brighter than the reported Vis magnitudes.

Roy

Affiliation
Variable Stars South (VSS)

In view of some comments posted above, the following should be noted. The first 3 (TG, TB and TR) can be entries in the Filter field. The last one, TRANS is for the separate field indicating if the magnitude is transformed: YES if transformed, NO if not transformed Note that measurements through a Johnson V filter should be designated V, and that measurements through the green channel of DSLR sensor or colour CCD camera should be designated TG.

TG: Green Filter (or Tri-color green). This is commonly the "green-channel" in a DSLR or color CCD camera. These observations use V-band comp star magnitudes.

TB: Blue Filter (or Tri-color blue). This is commonly the "blue-channel" in a DSLR or color CCD camera. These observations use B-band comp star magnitudes.

TR: Red Filter (or Tri-color red). This is commonly the "red-channel" in a DSLR or color CCD camera. These observations use R-band comp star magnitudes.

TRANS: YES if transformed using the Landolt Standards or those fields that contain secondary standards, or NO if not. Document the method used to transform in the "NOTES" section.

Roy

Affiliation
None

My name has been mentioned a couple of times, so I thought I'd add a few comments.  I'm sure I've said much of this before.  And, as usual, these comments are mine alone and not AAVSO policies.

I currently have three filters on order from February, so I'm pretty aware of the current filter situation.  Phil and others have commented on the available filters.  I've been working with several vendors recently to produce new quality photometric filters, so the market should improve over the next few months.  I'm providing them with passband responses and will test the newly created filters with AAVSOnet.

You can submit Sloan magnitudes to the AID, as long as you know the Sloan magnitudes of the comp stars.  The direct APASS query, VizieR, databases such as Pan-STARRS, all give you such magnitudes.  However, since VSP won't display Sloan magnitudes in its photometry table, I highly recommend putting the comp star Sloan magnitudes in the "notes" section of your submission.  I'm not sure whether the Light Curve Generator can display Sloan magnitudes; I can't find any on my usual set of test variables.  So Sloan support by the AAVSO tools is not complete.  Hopefully, once the new website is released, more application development and improvement can be made.

Should a new observer purchase Johnson/Cousins or Sloan filters?  What should a seasoned observer who wishes to expand their filter set do?  Until the AAVSO tools fully support Sloan, I'd stick with Johnson/Cousins.  That certainly extends the light curves of AAVSO program stars.  Most researchers are happy with Johnson/Cousins photometry.  The exception is for those campaigns for which the researcher requests Sloan photometry.  As a side-note, AAVSOnet telescopes, especially the BSM systems, often have a fairly complete set of Sloan filters that can be requested for observations.

The filters to choose first when starting out have been mentioned in prior posts.  New observers coming from the deep-sky imaging direction usually have LRGB in their filter wheel, or are using one-shot-color or DSLR cameras, and for those observers, I recommend starting with TG (the Tricolor-Green) filter, using the V-band comp star values for your differential photometry, and submitting the results as TG.  It works, and at zero cost.   Side-note:  don't confuse the TG filter with the TG (Transform Generator) software application!  Roy gave a nice description of how to submit measures using RGB filters.

Arne

I greatly appreciate the clarifiication - and reiteration - of what I've read in AAVSO manuals and elsewhere. Although a newcomer to variable stars, I am a longtime amateur astronomer with some science background, and for me the current situation with filters - availabilty, non-standard transmission curves, J/C vs. Sloan, etc. - was a bit bewildering. 

Also, a big Thank You to Phil, who has been a huge help in getting me started on the right track. Now if skies would cooperate so I can get some testing done . . .

Affiliation
American Association of Variable Star Observers (AAVSO)

Peter,

Thanks for mentioning Omega.  I contacted Omega and received prompt and useful information about their Johnson-Cousins filters.

They make Bessell filters (colored glass, 5mm thick) to order.  They don't keep them in stock.  Availability is from 2 to 6 weeks depending on whether they have the required glass on hand.  The transmission curves shown on the web site are typical for J/C filters.  The filter edges are painted.

Their UBVRI fiters can be bought separately, or as a full set (with a lower price per filter).  All filters  can be ordered as unmounted round or square.

Prices:  25mm  $100      28mm   $125      32mm  $125     38mm  $150     50mm  $250

I asked if the smaller filters could be provided as 1.25 inch mounted filters.  Deb said that would require custom manufacturing "at custom prices".  I didn't ask for the custom price.  (Perhaps a group order would still make these filters less expensive than other brands.)

These US made, real Bessell filters have attractive prices, but they are unmounted.  For people in the market for 50mm unmounted filters, these would bear serious consideration, especially since you don't have to pay for a full set when you only want B and V, or B,V, and I.   (The Oplolong 2 inch J/C interference filters are cheaper per filter, but you must buy a set of five.  The optolongs are mounted.)

I would check with the maker of your filter wheel about clearance for 5mm thick filters.  This is the same thickness as the (soon to be discontinued) Baader filters.

Phil

Phil,

That's interesting. 

When I asked Deb for availability of 1.25" mounted B and V filters she responded "Thank you for your inquiry.  The XBSSL/V/32R is available currently but please allow 2 weeks to ship the XBSSL/B/32R.   These sell for 125.00 each.  Please let me know if you have any questions.

Best regards,

Deb"

She didn't mention anything about not offering mounted 1.25" as a standard...

Peter

Affiliation
American Association of Variable Star Observers (AAVSO)

Peter,

Strange. Perhaps their policy has changed.  When I asked about mounted 1.25 inch filters Deb responded, "We don't make them to fit directly into a ring mount. They have painted edges at 5mm thickness. We could make a custom option for the camera ring mount, but custom is going to have a custom price as well."

Perhaps, if more people ask Omega about this they might change their policy (again?) to make these available. 

Phil

 

Affiliation
None

A few years back, I got a set of colored-glass Chroma filters for evaluation.  They were fine, but used the Bessell 5mm prescriptions instead of the more common 4mm.  While they were mounted in 1.25" cells, the cells were not high enough for the normal retaining ring with these thick filters, so they glued them into the cell.  In two cases, the glue came loose and glass filters were rolling around in my filter wheel.

Be careful with the 5mm filters.  They may not fit into your 1.25" cells, and even if they do, they may be too high for your filter wheel (ZWO filter wheels are limited to 7.4mm height, for example).  I think the 5mm give better results, but they have limitations.  Most Astrodon interference filters are 3mm thick, so mixing and matching will require focus changes between filters.

The Omega filter diameters are not quite what filter wheel manufacturers are using these days.  Most wheels are 31mm or 36mm unmounted, while Phil indicates that they have 28mm, 32mm and 38mm diameters.  To mount in a 1.25" cell, filters need to be about 27mm diameter, so I'm not sure even the 28mm will fit.  Someone who has tried this might reply.  Their 50mm unmounted round filters might work in most wheels.  I do remember Don Goldman saying that his "50mm filters" were actually 49.7mm diameter as the filter wells in at least one filter wheel manufacturer was exactly 50mm and you couldn't get the filter into the well easily.  That might not be true any more.  I just checked the QHY wheel and it is 51mm well diameter with a 48mm lip.  The SBIG STL wheel was 51mm well diameter and 47mm lip.

Arne

I'll call Deb on Monday for a clarificatioon. Right now on ebay Omega has a clear (focusing) 1.25" filter and a photometric I filter for $69 and $79 respectively. The image of the clear shows a micrometer indicating 7.5mm thick, including threads. https://www.ebay.com/itm/Optical-Filter-Astronomical-Bessel-Clear-for-f…?

I use an SBIG CFW-10 and presently don't know what maximum height it takes.

Peter

Affiliation
American Association of Variable Star Observers (AAVSO)

These looks like they may have been left over from a custom order.  I'll be interested to hear what Deb says about them.

I also use and SBIG CWF10.  Currently I have Astrodons, Optolongs, and Chroma filters in the wheel.  These all work.  They are all intereference filters, so thinner than most true Bessell filters.

Phil

OK, it's as clear as mud, or maybe a degraded V filter!

I spoke with Deb just now. Omega Optical got purchased at the end of Aug by Artemis Capital Partners trying to stitch together a photonics company from small acquisitions. Everything is in flux.

The original owner "Bob" is now a 49% owner.

Deb told me the filters they sell in 1.25" are unthreaded. When I mentioned the ebay filters clearly having threads she said that is "Bob's website", not Omega's and suggested I reach out to him. I have and await his response She also told me that all the Photometric filters they manufcature are thinfilm, so no danger in degraded Schott glass.

Once I hear from Bob I'll report back. Hopefully his plans include buying the glass filters from Omega and mounting and selling on his own. But who knows??

Peter

 

Response from Bob Johnson was not encouraging:

"bjomejag is out of the office until 12/30/2020 and may not be able to respond to your message.

Message from bjomejag:
Check Omegafilter Build A Filter to get a very large selection of items."

Peter

This looks promising. Yes, I reponded by asking for the spectral transmission curve!

"Hi Peter,

We are shifting to the coated/thinfilm construction in the UBVRI filters, but we are in transition now and have both styles. They are $250 each with 1 week delivery on the colored glass style and 3-4 weeks on the thin film style. The colored glass filters are coated to protect them and they have a 2 year warranty. The thin film style have a 5 year warranty on deterioration.

To have this choice in style you would need to order directly from us, which you can do by email or telephone.

Best regards,
David Marcus, PhD
President

On 10/26/2020 1:14 PM, Peter Bealo wrote:

Hi,

I'm looking for a photometric B filter mounted in a standard 1.25" threaded cell for use in my old SBIG CFW-10 filter wheel.

Do you still offer these? If so, is it using colored glass or thinfilm? If colored glass, is it coated to protect the glass from humidity?

All the Best,
Peter Bealo"

 

They sent me the profiles: look very similar to Chroma. Nice rectangle functions except the U. I emailed the profiles of old vs new filters to Arne. Happy to send it to anyone else interested.

Don't vendors talk with customers anymore before coming up with new products?? I know I used to!

Peter

Affiliation
Variable Stars South (VSS)

A few weeks ago (September 1, 2020) on this Forum, during a discussion about DSLR TG and V magnitudes, Phil wrote the following in post #58:

"I think that comparing TG (untransformed) magnitudes and errors to true V (done with V filter) magnitudes and errors derived from the same system would be a interesting project, especially as the targets get redder. 

This might provide some guidance to TG observers as to how far into the red they can go and still get usable data."

I've done nearly the same thing: compared TG measures with transformed DSLR green channel measures (which should be reported on WebObs as V) of a set of E Region standards in the southern skies. They were bright stars of 5th to 7th magnitude. Photometry was performed with the camera fixed on a tripod, imaging through a 200mm tele lens at f/3.5. Results were averaged from 10 exposures each of 15 seconds duration.

Measures were obtained from 18 target/comparison pairs. The B-V of the stars ranged from 0.304 to 1.448. The error (target measured magnitude minus catalogue magnitude) was plotted against the B-V colour index difference for each target/comp pair. Both transformed (V) and non-transformed (TG) measures were plotted. Transformed magnitude errors were small, and (as expected) showed no correlation with the target/comp B-V colour index difference.

However, for TG measures there was, as expected, a strong correlation between TG, TV and TR errors and the target/comp B-V colour index difference. When the B-V colour index difference was zero, the expected errors for TB, TV and TR were 0.011, 0.002 and 0.007 mag units respectively. For every one mag unit B-V target/comp colour index difference, the expected errors for TB, TV and TR were 0.48, 0.12 and 0.29 mag units respectively.

Roy

Affiliation
American Association of Variable Star Observers (AAVSO)

Hello, I see more and more reporting brightness of SNs and QSOs in SG-filter (SI-filter). I'm planning to buy a Sloan G filter when it's available at Astrodon. My question: Which comparison stars shall be used? Still the AAVSO VSX V-stars as I use currently in reporting CV (unfiltered with V-ref.)? 
Regards Achim

Affiliation
American Association of Variable Star Observers (AAVSO)

Here is one way to find Sloan g magnitudes for (some) stars in AAVSO comp sequences. 

Load the VSP chart for your target, then look at the VSP photometry table.  Find the APASS stars.  These are usually identified in the table by the superscript 29.  Record the RA /Dec coordinates and the V mags of these stars

Go to the APASS search page:   https://www.aavso.org/download-apass-data .

Specify a field center and radius that includes the APASS stars you would like to use.  Download the CVS file, then open that with a spreadsheet.  You can identify the AAVSO sequence stars from the RA and Dec coordinates.  Looking at the V mags speeds up the identification process.  (The VSP mags may not exactly match the APASS mags in the spreadsheet.)

In my spreadsheet the Sloan g mags are in column K.

Phil