Equipment advice for photometry with club telescope

Technology Instrumentation & Equipment
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My astronomy club in New Jersey (the Sheep Hill Astronomical Association) has an 18" f/4.5 reflector on a permanent equatorial mount in our observatory (Bortle 6 skies, more or less). I’m looking into what sort of equipment we might get to start getting into variable star photometry with this scope and hoped I could get some advice from the experts. (I have been a visual variable observer for some time but have very little experience in imaging.) I don’t have an exact budget in mind but I’m hoping to bring the whole package (camera, photometric filters and filter wheel) in for around $2k if possible, or at least not too much more than that.

I’ve been playing around with the calculators on Astronomy Tools, especially the CCD Suitability calculator for matching a camera to a scope for optimal performance. Given the long-ish 2057mm focal length of this scope the calculator suggests that larger pixel size would be preferable, but as far as I can tell there don’t seem to be any lower-priced monochrome cameras with pixels large enough to get to the sweet spot of arc seconds per pixel that the tools call for. (Our seeing is probably average most of the time, below average sometimes, and better than average rarely.)

Is using binning on a camera with smaller pixels an appropriate way to get the desired result? For example, the ZWO ASI533MM has a pixel size of 3.76, which if binned x2 would give 0.76"/pixel, which is in the recommended range for average seeing. (In poor seeing x3 might be necessary.) Would a camera like this be suitable for photometry with this scope?

For filters, the calculator for filter size says that 1.25" filters would be large enough given the sensor size of the 533MM, so I was thinking a ZWO 1.25" filter wheel and, to start off with, 1.25" B and V filters. We can always expand to include R and I filters in the future.

Does my thinking above make sense? Any thoughts or advice are welcome…

Thanks,
Brian S.

P.S. - I should mention that although we don’t have a guide scope, the permanent EQ mount seems to track very well - I’ve seen exposures of up to 3 and even 5 minutes from this scope (using a DLSR camera) that seem to show no signs of star trailing.

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A $2000 budget is going to be tough to do. Binning is ok at your 2X factor mentioned with that camera.

Have you tried using your DSLR for photometry on the scope?

Digital Single Lens Reflex (DSLR) Camera Observing Manual | aavso

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If you could find yourself an old/used ST-8XME with filter wheel, I would recommend it highly. I am using one on a 8" scope of approximately that focal length, and it gives a good field of view for photometry, and the built-in autoguider is nice for when you need longer exposures. I paid approx US$300, last year, which was probably a particularly good deal. I am also using one on a scope with slightly longer focal length – still fine. Downside: it is an obsolete camera and can’t be repaired, but for that price… Odds are it will keep working for quite a while. Just two filters cost as much as the camera did! 9 micron pixels. Get a non-anti-blooming one, if you can.

Gary Billings

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I have started trying some photometry using the DSLR camera - so far I have not found an easy way to split the color FITS files it generates into separate color channels that I can then upload to VPhot and do photometry on. I can do one image at a time in ImagesPlus (which is very tedious) but have not found a way to do it in a batch from there.

I need to search around some more to try to find the right software to do this with, I think. (Hopefully a free one, but I’m willing to spend some money for something if I know it will do the job.)

Thanks,
Brian

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Brian:

IMHO, you have identified/searched for the correct camera parameters (e.g., binning to match seeing, and filter vs. chip field of view size). I assume you have done this carefully. They do sound reasonable to me.

If you have also carefully searched prices of the new hardware you mentioned and can do it for $2.000, I find no problem with your assessment. Which filter provider have you chosen for your starter filters (BV)? These are a good initial set.

IMHO, selecting a monochrome camera with photometric filters is a better/easier choice for '‘best’ photometry.

Ken

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Hi Ken,

Thanks for the feedback. As you say I was leaning toward starting with just B and V filters, thinking that I can always expand my filters later to include I and R. (I’ve heard that U is very rarely needed in practice.) But I see that High Point Scientific has a full UBVRI set of 1.25" Optolong filters on sale at the moment for under $600; the various other brand filters I’ve found for sale individually seem like they would mostly cost almost that much or more just for B and V filters as a pair.

Do you have an opinion on whether the Optolongs would be a good choice? Or is there a good reason why they’re less expensive :slight_smile:

Thanks also for the recent VPhot course, it was extremely informative - certainly one of the more challenging and rigorous of the CHOICE courses I’ve taken so far…

Brian

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Brian:

See following from an older post in old forum system. You could find it with a search on AAVSO web site.

<<Phil Sullivan loaned to me his set of 1.25" Optolong filters. I know that James Hamilton (Wisconsin) is going to scan a set with better equipment than I have, so I did not do any spectral scans other than to make sure the bandpass looked reasonable. I look forward to Dr. Hamilton’s report! The Optolong filters are mostly interference/dielectric and match the Johnson/Cousins bandpass central wavelength and shape. Since attachments are forbidden on AAVSO forums, I can’t include the normal images and plots that I would normally do.

The filters look nice. The cell is thin (5mm) and will not be a problem in thin wheels like the ZWO versions. I think the Rc and Ic filters start with colored glass; the others look like clear glass that is either dielectric or AR coated.

For on-sky testing, I always choose a very red star, usually a carbon star. For Fall observations, I choose T Cas, which has (B-Ic) of about 7.5. For spring observations, I choose R Leo, with a (B-Ic) around 8. These two stars are on Frank Schorr’s program, and so are monitored pretty often with several BSM sites. In this case, the observations were made on October 7 and used T Cas. For the 30-day period centered on the observation date, T Cas increased in brightness from B=10.65 to B=10.50. I looked at raw data from BSM_TX, BSM_NH and BSM_NM for T Cas. Over that period, there were 21 observations. Plotting the untransformed results gives a good idea of how various systems compare using the same comp stars. The untransformed scatter is about 0.05mags from the linear trend line. The check star, labeled 104_1 on a VPHOT chart, shows B=10.87 +/- 0.03 for all systems including the Optolong dataset, where the Optolong measure is about 0.03mag fainter than the trend line.

When plotting the Optolong B magnitude for T Cas, it is slightly brighter than the other systems (0.07mag), but pretty close. This deviation could be due to a slightly longer red tail on the Optolong B filter, or it could be due to a small amount of red leak. Certainly there is larger scatter between all systems for this red star, so small differences in the bandpass are probably present for other filters/telescopes. Perhaps James’ scans will give us more information. I’d say the B filter is perfectly usable for most projects, and might be fine even on T Cas when transformed.

For V,Rc,Ic, the plots all look good.

For U, there is definitely a red leak. This shows up both as a diffuse halo around T Cas, as well as an untransformed magnitude that is much brighter than it should be. The U filter might be ok for blue stars (say, a CV or a delta Scuti variable), but I would not use it for red stars.

I did take a dataset for NGC7790 that could be used to determine transformation coefficients, but I did not do this step.

I think the Optolong UBVRI filter set is a good one at a great price, with the caveat that you shouldn’t look at red stars with the U filter.

Arne>>

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Im new to this so hope another member corrects me if i am wrong, but i believe software binning used on a cmos does not have the same affect as on chip binning in a ccd. Just a note when looking for cameras

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Hi Ken,

Thanks for reposting this. Optolong has a good holiday sale going on and you can get UVBRI filters at a good price. I bought mine last year at a similar discount and while I’ve done very little photometry with them, I’ve got them for all future projects.

Thanks,
Andrew