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Getting Started in Variable Star Photometry

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#26 Ed Wiley

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Posted 27 June 2022 - 08:39 PM

Degen1103,

Quite correct, there are several programs where precision is important and accuracy not so much such as exoplants and eclipsing binary minimum timing. The Meteor colorimetry paper was interesting. 

Ed


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#27 Degen1103

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Posted 19 August 2022 - 12:13 AM

I began to think about new mono camera ZWO ASI432MM for variables observation. I think, it should give high sensitivity, broad linear characteristic and comparely large FOV (about 30' with my Mac).

However, it gives scale 1"/pixel. Is it too much for stars photometry or the scale is reasonable?



#28 pbealo

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Posted 19 August 2022 - 06:13 AM

I began to think about new mono camera ZWO ASI432MM for variables observation. I think, it should give high sensitivity, broad linear characteristic and comparely large FOV (about 30' with my Mac).

However, it gives scale 1"/pixel. Is it too much for stars photometry or the scale is reasonable?

Do 2X2 or 3X3 binning.



#29 GaryShaw

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Posted 19 August 2022 - 09:44 AM

Do 2X2 or 3X3 binning.

Hi

Would that be a good idea even with the 13 micron pixels of the OPs camera? I didn’t even know there were amateur cameras with pixels that large. I’m sure there must be cases where such large pixels, native or binned, bring a benefit but I’m not sure what those situations would be.

 

I’ve used both an ASI294 with 4.63 micron pixels and an ASI178mm with 2.4 micron pixels and haven’t found a need for binning either one. Of course if you’re starting out and want to stick with brighter stars, larger pixels will help you avoid saturation.

 

If Ed Wiley is still monitoring this topic, he can sort all this for you.

cheers

Gary



#30 Xilman

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Posted 05 September 2022 - 05:12 AM

I have been gearing up to start doing variable star photometry.  I've been imaging a mixed assortment of variable stars in the eastern evening sky at my location.  I am starting out using a DSLR controlled by an ASIAir Pro.  For image analysis I'm using ASTAP (thanks Han).  I extract the green channel and work with that, reporting my observations as TG to the AAVSO.  I've only done 5 stars so far but my measurements seem in line with what others are getting.

 

I'm interested in your thoughts about how to generate a collection of stars to observe.  AAVSO says this is one common problem with beginners, but I didn't find their suggestions too helpful.  What is a good strategy for picking stars to measure other than looking at a list and picking something that is in my sky at the time I'm out.  I don't have access to the entire sky due to obstructions and currently my observing is limited to times before around 11 PM.

 

Thanks,

Bill

I and many other people have found the BAA Variable Star Section extremely helpful in sundry fields, including selection of targets, observational technique and data analysis. Something for everyone, from complete beginners to experts with decades of experience.

 

https://britastro.or.../variable-stars

 


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#31 GaryShaw

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Posted 05 September 2022 - 09:30 AM

Hi Bill

If you want your variable star observations to be relevant scientifically, you might want to subscribe to some of the AAVSO’s ‘Alerts’. These are requests from professional astronomers to the amateur variable star observing community for observations of specific stars that are the subject of current research. They usually are calling for observations with mono cameras and standardized filters, V, being the most common.

 

Often times observations in TG will also be of benefit so if you’re not ready yet for a mono camera and a Baader or Chroma V filter, your observations can still be of benefit in many of the professional requests. One recent example relates to researchers who have been awarded observing time on Hubble. Before making their observations on that expensive instrument, they often want amateurs to advise them when a star is at or approaching a minimum. For some research observations, the onboard instrumentation can be damaged if the observation is taken while a star is in outburst mode. Your observations in response to these ‘Alerts’ is of immediate and tangible value to the research community and I can’t think of a more valuable list of targets for variable star photometry.

cheers

Gary

 

 

 

PS: Of course, you could also consider getting involved with Exoplanet Transit observations which requires many of the same skills and amateur equipment needed for variable star observations. AAVSO has a great deal of information on getting involved with this - perhaps BAA as well.


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#32 Ed Wiley

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Posted 12 September 2022 - 01:16 PM

Bill,

I suggest two components to picking something.

 

1. Pick a class of variables that are within your skill range. Long period Variables (M, SR.) are usually good for beginners, especially the well observed ones. This is because there are lots of recent observations and you can compare your results with those results. You can easily find well-observed targets by consulting the LPV section and the LPV target list. As these vary a lot in magnitude over their periods they make for lots of learning in terms of things like exposure. 

 

Typical example: RU And, first pick on the list as I write. I select the name and it goes to the VSX and give me information. Down the VSX page I see "Extermal Links." From the External Links I see "AAVSO Light Curve." I select it and hit "go" This takes me to a "Enhanced LCG" page that shows the latest observation in light curve form. I see the latest observations at around 11.5 -12th magnitude. I adjust my exposure to cover that magnitude. I shoot four exposures. In the bag and ready to calibrate and measure.
 

2. While some projects like exoplanets are sexy as well as useful,  I would advise avoids advanced programs until you are confident in your skills and have moved to a cooled mono CMOS camera with at least one photometric filter. Exoplanets require milli-mag precision. That is within your reach, but you need to develop that skill.

 

Gaining confidence in your skills: Nothing like observing Landolt standard fields and measuring standard stars to see how close you come to the known magnitudes as training and confidence building. Use one standard star as target, another as comp and yet another as check. This also builds skills in picking comps and checks following the AAVSO manual. 

 

Ed


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#33 Xilman

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Posted 13 September 2022 - 05:32 AM

PS: Of course, you could also consider getting involved with Exoplanet Transit observations which requires many of the same skills and amateur equipment needed for variable star observations. AAVSO has a great deal of information on getting involved with this - perhaps BAA as well.

The BAA is definitely active the field of exoplanet transits. I am on their technical advisory group.

 

In one way you may find ET observations easier in that the important thing being measured is the timing of the eclipse, not brightness of the host star in any particular waveband.  You don't need filters, in other words. That said, it is often recommended to observe in V, R or Sloan r but that isn't strictly necessary to take scientifically valuable data.

 



#34 btschumy

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Posted 13 September 2022 - 02:19 PM

Thanks for all your comments.  My original post in this thread was back in February.  Since that time I've made a lot of progress.  I've moved on from using a DSLR and bought a ASI2600MP camera with a filter wheel with V & B filters.  I have been making transformed measurements using transformation coefficients averaged from two standard fields.

 

Many of my initial measurements were for long period variables just so I could compare them with what others are getting.  I'm now pretty comfortable that my data is at least as good as the more experienced AAVSO members and am starting to observe more challenging stars.

 

Just the other night I did my first time series analysis.  It worked out pretty well although there are still some a few hiccups to iron out.  I measured CY Aqr which has an approx 90 min period and got a nice curve.

 

I haven't looked at the BAA site but will do so soon.  I've been mainly using AAVSO.  

 

I have subscribed to the AAVSO's Alerts but haven't tried any yet.  My equipment is fairly modest and I can't go as deep as they generally want.

 

Bill


Edited by btschumy, 13 September 2022 - 02:20 PM.

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#35 pbealo

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Posted 13 September 2022 - 03:27 PM

Bill,

 

Kudos on the progress!! You've done a lot in a short time.

 

I am especially pleased you went the "transformed" route. I'm told only 1/3 of submitted data is transformed. Right now I'm doing some analysis on one star that has CCD data over ~8 years, and I am ONLY using transformed data from the database. I'm afraid untransformed data will have small magnitude shifts that would spoil my analysis, and I don't want to go there! 

 

Peter Bealo

AAVSO Instrument and Equipment Section Lead


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#36 S.Boerner

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Posted 13 September 2022 - 07:40 PM

waytogo.gif

 

Great going Bill!


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#37 Degen1103

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Posted 17 September 2022 - 01:38 AM

Could your please explain, how to use results got with unfiltered mono camera or with UV-IR cut filter on the mono camera?



#38 GaryShaw

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Posted 17 September 2022 - 09:01 AM

Hi

When I started photometry, with an AAVSO Mentor assisting me, I had only a OSC camera so he coached me on using a UVIR cut filter and then processing the images and splitting out the green channel for use with the AAVSO’s VPHOT program to analyze the photometry, determine the magnitude and submit the observation to AAVSO. I believe most of that process is covered in the AAVSO manual on doing photometry with a DSLR camera.

 

I’m doubtful that there is a method for doing this with a UVIR cut filter and mono camera but someone with a broader knowledge of this topic will likely chime in. If you’re really interested in variable star photometry, consider getting at least a photometric ‘V’ filter for your mono camera. The AAVSO has excellent manuals that explain the process and, if you are an AAVSO member, you can request a Mentor to help you get started on the right foot.

Good Luck.

Gary



#39 Photon Collecting

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Posted 21 September 2022 - 07:25 PM

Hey,

 

I'm feel I'm right at the entrance to photometry, but struggling to get through the door.

 

I read the book "the sky is your laboratory", and its got me feeling like I might be able to do this. I've got a OSC camera, but I'm hoping that with a uv/ir cut I might have some success with the green channel. What I'm really puzzled by is exposure time. How do I judge exposure length? I'm using an Asiair, so that limits my ability to analyze images on the spot. I can play with the histogram a bit, and see star size, but that's all. I'm also struggling to find information on the linearity of my camera. Facebook hasn't been too helpful. 

 

Any knowledge to help my start would be appreciated, I'm excited to get into this. 

 

Grant



#40 pbealo

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Posted 22 September 2022 - 07:22 AM

HI Grant,

 

For exposure time, you would want to try and get your variable star exposed well enough to provide an SNR of over 100 and not be saturated.

 

In practice that means shooting some tests at different exposures to characterize your system. You'll know you're near saturation when the ADU values of individual pixels stop increasing as exposure increases. For one instrument/camera setup I did a number of different exposures of a starfield and plotted ADU vs exposure time for a number of stars to come up with a curve showing acceptable exposure times for a given magnitude.

 

What's your camera?? I assume you are downloading images as FITS. JPGs don't work.

 

Peter


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#41 GaryShaw

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Posted 22 September 2022 - 08:29 AM

Plus 1 to Peter’s note.
 

Also, I don’t think you want to be mucking around with the histogram. You just want to use the right exposure, per Peter’s note, and set your gain to whatever is right for your camera - I use “0”.

 

When I started imaging and analyzing variables, I used an ASI294 MC uncooled. I did the linearity testing per the AAVSO CCD manual and found my camera was linear pretty much all the way up to 60000 adu. Even so, I’ve never needed exposures that took me to much over 40-50,000. Depending on your camera’s linearity range, you can always just pick targets that get to an SNR > 100 with exposures well below saturation. 

Good luck.

Gary


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#42 pbealo

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Posted 22 September 2022 - 11:15 AM

BTW: When I say SNR over 100, that's a GOAL. A good goal, but only a goal nonetheless.

 

For some variables I image repeatedly, I go for the SNR goal, or even a lot over the SNR goal, but if there are fainter known variables in the FOV that have less than 100 SNR, I'll report them as well IFF nobody else is presently reporting them or if the only recent reports are visual. I do this because I generally feel some data is better than no data, and if the only data is visual then error is probably no better than +/- 0.1 mag, so if I can submit data at +/-0.05 its better than just the visual.

 

Just my opinion, and I may be wrong!

Peter


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#43 Photon Collecting

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Posted 22 September 2022 - 03:51 PM

Thanks all,

 

I now think my best bet is to shoot FITS files of various stars (different magnitudes), and the check the SNR using a program on my pc. And then make a graph like Peter suggests. That or I'll have to download test exposures from the memory stick at the beginning of each run. I have a 183mc pro. Not sure if linearity is a manufacturer published stat, or will I need to determine this my self like Gary? I'm guessing it changes with gain. I'll need to improve my understanding of ADU, and probably quite a bit more related to my CMOS sensor. Its been too easy in astrophotography to accept the values others are using and call it close enough. But I'm feeling better about this, and ready to learn. 

 

Clear Skies!

 

Grant



#44 GaryShaw

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Posted 22 September 2022 - 04:54 PM

The testing process for determining your sensor's linearity is all spelled out in the AAVSO CCD Photometry manual. Pretty straightforward.

Gary


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#45 robin_astro

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Posted 23 September 2022 - 05:18 AM

Thanks all,

 

I now think my best bet is to shoot FITS files of various stars (different magnitudes), and the check the SNR using a program on my pc. 

You could play with Mike Richmond's calculator to give you an idea of the exposure you need for a given magnitude and SNR 

http://spiff.rit.edu...nd/signal.shtml

but in practise it depends on many factors, particularly the air mass (height in the sky) so can only be a rough guide. (A typical newbie error for example when taking a long series of measurements on a star which is rising is choosing an exposure at the start of the night and finding the images saturated later in the night.)

 

The key thing is to avoid getting near saturating any of the pixels. (Dont forget this includes not only the target but all of your comparison stars). This can vary for a given magnitude for example depending on the seeing (good seeing concentrates the same number of photons on fewer pixels so you saturate sooner) so it is useful to have a way to check the ADU values in the images in more or less real time. The advantage with CMOS though is the read noise is generally so low that you can afford to stay well away from saturation and combine several short exposures to collect enough photons in total for the SNR you want without a big read noise penalty.

 

Cheers

Robin


Edited by robin_astro, 23 September 2022 - 05:29 AM.

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