Advice on variable star photometry setup

[Roy Lemmon]

Hi,

I have recently joined the BAA without much prior experience, but I am particularly interested in variable star observing. At the moment I am learning the basics by observing visually with a pair of binoculars, which is already challenging but fun ! I will submit some measurements soon but do you have to join the VSS to do this ?

However, I am thinking ahead and my aim would be to build a setup for CCD/CMOS variable star photometry. I was wondering if you had any comments about the suitability  of a setup such as:

• good quality German equatorial mount
• 80-100mm APO refractor
• guide scope
• CMOS mono camera with filters

My main question is whether a good quality refractor of this size is suitable for variable star CCD/CMOS observing ? With the above setup, I could also do some visual and imaging work. I am aware that a small refractor will be much easier to track with and balance the imaging equipment on for a beginner.

Thanks for any help.

Cheers
Roy.

[Gary Poyner]

Hello Roy,

There is no formal way to join the VSS, as long as your interested in VS we’ll welcome you!  

To submit observations to the database, you will need an observer code and login details.  Have a read of this and it will advise how to go about it…

https://britastro.org/photdb/notes_submissions.php

I’ll leave it to one of the CCD observers to comment on your set-up, but it looks fine to me.

Don’t forget the VSS Circulars – free to everyone!  https://britastro.org/vss/VSSC_archive.htm

Gary

[Jeremy Shears]

Hello Roy,

following on from Gary’s reply, welcome to the Variable Star Section!

I’m not sure if you already have your kit or you are looking to buy. If you have some already I suggest you start with that.

I used a 100 mm refractor with a CCD camera for several years for photometry of variable stars. I typically took 60 sec exposures, which enabled photometry down to 15th mag, although I could detect stars much fainter than this. I was lucky enough to have a decent mount so unguided exposures were absolutely fine. It also had GOTO so I was able to find targets easily, but not essential.

There is some info about VS CCD targets here.

Most people are using CCD rather than CMOS. Latter can be used, but you have to be careful about calibration as the response of the chip is less linear.

Hope this helps. Happy to answer further q’s.

Jeremy Shears

VSS Director

[Jeremy Shears]

Hello Roy,

you might like to have a look at James Screech’s BAA Members pages too. For some of his photometry he uses an 80 mm refractor. Also uses an ASI1600MM camera.

Also have a look at Gary’s news article on OJ287. I followed this target with my 100 mm refractor.

Jeremy

[Roy Lemmon]

Hi,

Thanks very much for your comments and advice. I will have a look at those links.

No. I don’t yet have any of that kit. My aim is to build it up over the next couple of months starting with the mount and telescope, in particular a decent mount. If you have any recommendations, feel free to suggest.

Thanks

Roy.

[Dr Paul Leyland]

In my experience, size isn’t everything. it’s how you use it that counts. With a 100mm aperture you are not going to be making observations of 20th magnitude stars any more than I am of 23rd targets with my 400mm, which estimate is based on a roughly 3 magnitude difference in light grasp. I can perhaps manage useful observations (an accuracy of 0.1 m) down to mag 20 if I am prepared to spend enough time (hours!) on the exposure. You could manage perhaps mag 17 with a similar commitment but 15 to 16 should be entirely straightforward.

There are a hell of a lot of variables brighter than 16th magnitude.  Some are so bright that they will saturate your detector and so are effectively unmeasurable.

What you really need to do, IMAO, is to start observing, learn what your equipment can do and to learn how to analyse your data. Your equipment is capable of producing research quality data far beyond that of any amateur set-up as recently 50 years ago.

I would also recommend contacting the VSS, but I’m biased. I found it extremely helpful.

Go for it!

[Dr Paul Leyland]

“In my experience, size isn’t everything. it’s how you use it that counts” and  “There are a hell of a lot of variables brighter than 16th magnitude.”

Have you thought about monitoring stars for exoplanet transits? Some of their stars are quite bright and within range of a 100mm scope.

Of course, if you send the same amount of money on a reflector as on a refractor you could get a markedly larger aperture …  This assumes you haven’t already bought your equipment. If you have, there are still many things you can do with photometry on what you have.

[James Screech]

I’ve been using an ED70 scope with a finder guider for years, with both CCD and CMOS cameras. There is certainly no need for a large scope there are a lot of stars that can be followed by small portable systems.

James

[Dr Paul Leyland]

Hi James,

Could you report your practical limiting magnitude of your equipment please? For example, what is the faintest you can reliably measure to within 0.1 magnitudes after a 10 minute exposure?

I’m toying with getting a small equatorial mount for a Canon DSLR / telephoto lens combination and your experience will help set expectations for a roughly 50mm aperture.

There are limits to what can reasonably be extrapolated from something approaching two orders of magnitude greater light grasp!

[James Screech]

I’ve never used anything like 10 minute exposures, the longest I’ve used is 1 min, most of the time I use 10-20sec. 10sec will get me down to around mag 11 with errors about 0.03. I would only accept an error of 0.1 for secondary variables in the same field. Some fields I observe I can get photometry of 4 stars in the same field.

As for DSLR with my setup, a modified 500D with a 200mm f2.8 lens (about 70mm) I get good photometry to mag 8.3 with ISO400 and 20sec green channel only, unguided of an alt-az mount. I use a plan in APT that re-centres targets (with Goto++) about every 1 to 1.5 hours, as it does drift off centre without guiding on a low quality mount. 

Extrapolating isn’t easy as the magnitude will be a function of the camera pixel size, lens/telescope focal length and aperture.

[Dr Paul Leyland]

That is excellent data, thank you!  Just what I was looking for.

I recognize the difficulty of extrapolation but for present purposes any number which is good to within a factor of two is easily precise enough. For instance, 10s for mag 11 tells me that 30s should give about mag 12 because a factor of 3 in flux collection is close enough to the 2.5 times lower flux from a target one magnitude fainter.  The precision value you quote is also encouraging. I chose 0.1mag as an upper limit because that corresponds to the best which can be reasonably expected without a camera.

Last night an interesting observation of the moon with an iPhone and eyepiece projection suggests that using the 25cm Dob with the DSLR at prime focus might be feasible. It might not be, but it is certainly worth trying.

BTW, have you come across Russ Laher’s Aperture Photometry Tool (APT)? It runs on any OS with Java installed and can told to use elliptical apertures — just the thing for poorly tracked images.

[James Screech]

I’ve never come across Russ Laher’s Aperture Photometry Tool, I use Muniwin, although it can take a bit of experimentation to get some of the parameters correct. You could simply use larger apertures to overcome poor tracking if there are no other stars very near the variable or comparison stars.

[Dr Paul Leyland]

See https://en.wikipedia.org/wiki/Aperture_Photometry_Tool for an introduction and a download link.

You are quite correct in that a larger circular aperture can be used. Unfortunately that will include a larger amount of sky and increases the likelihood that the sky annulus will itself contain stars as you indicate.

Elliptical apertures really come into their own when measuring galaxies but there is nothing to stop one exploiting that feature in other circumstances.

The best way of doing photometry, in my view, is to use PSF fitting as implemented in DAOPHOT because then one is fairly sure that you are measuring only the absolute minimum of sky and that stars within the sky annulus are properly accounted for.  Unfortunately the only code I have yet found is labour-intensive and very hard to script for reliable use.

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