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Need lots of help on AAVSO reporting procedures for EB runs with DSLR

MKE's picture
MKE
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Joined: 2010-10-31

I need some help. So, Mike and whoever else can help, please help!  I told Mike some time back that I was devloping a set of questions.  Here's a batch.  Lots of questions.

 

I have just gotten started using a Canon T2i Rebel DSLR for Eclipsing Binary stars on a CPC800 at prime focus. I'm using AIP4Win 2.4 - specifically the Single Image Photometry Tool in AIP4Win. I have 7 runs total so far, observing 4 different stars and my results seem to be in very agreement with the Krakow Ephemeris. And for the stars I have two runs apiece on, the agreement with Krakow is consistently fast or slow by about the same amount of time. I have calculated my own TOM's to the ten-thousandth of a day which means a reliable accuracy of the first three decimals. The 7 runs I've done comprise about 1100 images taken in the last month and half.  I don't know if those TOM's I've generated will match what AAVSO produces for them, but they should be pretty darn close.

 

I've been in contact with Gary Billings, the Eclipsing Binary chair, about how to proceed. He advises me to send the data not to him (we used to send data directly to the EB chair in the good-ol' visual days) but rather to AAVSO. He suggested I should use the ENSEMBLE format. I had looked at that earlier but had not thought it was appropriate for what I was doing. So I have some questions.

 

Following is representative data from a single image I took the other night of FZ DEL (one of 73 images in this case). I haven't filled it out entirely because I have some questions which I hope one of you will answer. Here are the entries and my comments and questions.

 

STARID = FZ DEL

DATE = 2456200.63988

MAGNITUDE = 0.382 this is a differential magnitude! Produced by AIP4Win. See long note at bottom.

MAGERR. AIP4Win reports a sigma of = 0.0064. Is this what goes here? Usually the sigma isn't that large.

FILTER = This is a DSLR. AIP4Win has deBayerized the image and worked on the Green channel. Don't know what to put here.

TRANS = Don't know what this means. I would think NO is the answer but I don't want to guess.

MTYPE = DIFF I guess. But then AAVSO will kind of store it away and not look at it, if I read the website correctly. The whole purpose of this exercise is to get my data evalutated and used.

CNAME = I can use the AUID, no problem with that. But I am using multiple comps (up to 3 before AIP4Win blows up - otherwise I'd use more). Do you want a separate line for each comp, even though they would all have the same time? Or is this where I put ENSEMBLE and then where do I put the comps I used? I would think you would want to know that.

KNAME = na, I guess. Single Image Photometry in AIP4Win doesn't let me select a check star - unless I'm doing it wrong. Anyway, it would seem to reduce my comp stars to only 2, which would detract from the value of the reported differential magnitude.

AIRMASS = na.

GROUP = na, I guess, because this is actually time series data.

CHART = 8824PV which is from the photometry table published for FZ DEL. This is what you want, right? Not the finder chart ID?

NOTES = na.

 

Long note at bottom: I would just love to enter an actual magnitude, so that somebody would actually look at this data, but I get various magnitudes when I compare the differential magnitudes I get from AIP4Win on different comp stars. Presumably the difference is caused by star color - or something. But since I don't know the color of FZ DEL (or whatever star I'm doing), I can't get a reliable number anyway. What I've been doing is pretending I know the B-V of the variable. Whatever number I use produces the variable results I've been talking about. So I'm not at all confident about doing anything other than differential magnitude. Can anyone help with this?

 

Other items of interest:

I am doing automatic dark subtraction with each image. The Canon shoots a light, then a dark, and handles the subtraction internally before downloading the image.

I am not doing flats. I don't know how to do them. I have googled and googled about this but I can't find anything but the most general information - like: you should do them! Oh really? I would love to. Help!

I do not have a permanent facility - there are reasons for this which I won't bore you with right now - so I set up afresh and polar align every night. Not too bad. the CPC800 holds onto the field very well. I occasionally have to bump the telescope just slightly up/down, left/right with the paddle, but those events do not happen often. Over a five to eight hour run, maybe three times. Sometimes less. So, yes, there is drift, but not terribly much.

I use the computer time which is synched with the net. I also set the internal camera time but I also compare the info from several pictures with the computer time and during the course of the night the camera time does not vary. At any rate, if the camera time is different , even by a second, I adjust the reported time appropriately.

All of the images I've taken so far are 30 second shots at ISO 800 downloaded as RAW files (.CR2, really - which Canon's version of RAW).  AIP4Win extracts the green channel and I use that for my photometry. The resulting image isn't the snazziest image I've ever seen but it appears to be workable. I try to shoot an image once every two minutes.  I like to use AIP4Win while I'm downloading images.  Since I have to fire each image manually, doing photometry while I'm waiting helps pass the time and gives me a close to real-time idea of what's going on with the star.

Of course, I am archiving all my images.

Canon has an EOS Utility which allows me to control the camera from my computer.

 

If you want to look at the results I've gotten so far, you can go to www.bobmanske.com and click on the Eclipsing Binary page. Scroll down past the intro blurb and you'll come to the Canon section. All 7 runs are up there and a couple of comparison runs. You're welcome to browse all the Egyptology stuff, too, if you're interested.

 

Anything else you'd like to add, fine.  Any questions I will try to answer to the best of my ability.  Any and all criticisms are welcome.  Anything which I can implement to improve the quality of my work.

 

Help, please.

 

 

Various references and some answers
KTC's picture
KTC
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Joined: 2010-12-08

I'm using AIP4Win 2.4 - specifically the Single Image Photometry Tool

The Single Image Photometry Tool...I'm pretty sure that's obsolete, but continues as a relic/appendix for some manual-intensive work.

I recommend you learn to set up and use the Magnitude Measurement Tool (MMT).  Steep learning curve to configure it right...but powerful...will process hundreds of time-series images...and the output format is in AAVSO format if you so choose.

See

http://tech.groups.yahoo.com/group/AIP4Win-Photometry/  Join them and post AIP4WIN-specific questions there.

Also, I recommend you image some standard/calibration fields...and see how your instrumental magnitudes in AIP4WIN vary as B-V color index varies.  See this page:

http://www.citizensky.org/content/dslr-documentation-and-reduction

TRANS ... yes?  or no?  See

http://www.aavso.org/aavso-extended-file-format  for some explanation.

Flats - how?  AIP4WIN manual discusses various ways to do it...twilight, T-shirt, light box, etc.  Each has its strengths and weaknesses.  Some skill and mastery here is important.

It's good to start with manual processes (single star photometry tool...to see the various results, internal math, etc.)...but then think about ways to automate.  You mentioned the camera control software for your DSLR.  You can also automate other operations such as slewing the scope, plate solving and re-slewing to stay centered on the target...while you sleep.  Many ways to skin this cat...various software, various prices, some of it free.

I recommend taking data as fast as you can.  Why wait between images?  With automation...you'll take more data, and process it faster.

There is also software that others have written to calculate TOM's from EB curves, such as

http://members.shaw.ca/bob.nelson/software1.htm specifically

http://members.shaw.ca/bob.nelson/Software/Minima25c.zip

Lots to learn here.  Take it one step at a time.  I have not answered all your questions, but some of the above references will answer many of them...at least to get you started on concepts.

Keep the questions coming.

Flats
TCB168's picture
TCB168
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Joined: 2010-07-26

Flats are very important for accurate photometry. They are not hard to take with a DSLR. At the end of your exposure run aim the scope at a nice white wall without removing your camera. If you fon't have a wall then use a sheet of cardboard held a few metres in front of the scope. Illuminate the board with a light so that it is evenly illuminated. This is preferably done with 2 lights, one from each side.

Take an exposure with your camera on auto exposure. It should give you a frame with the histogram in the middle of the range of your camera. Hopefully the exposure is short enough that you don't need darks etc. Take a few of these and then process them using the flat function in AIP4WIN.

I have also used a flash on the camera aimed backwards and bounced of a wall to produce an even illumination. Just experiment.

cheers

Terry

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484