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John C

Which scope for photometry?

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I would like to start photometry of variable stars and eventually would like to be able to contribute to the AAVSO . I am going to buy a CCD camera and need to match it to one of my two scopes. I understand that for photometry the FWHM of a star should cover 2-3 pixels. I’m not sure which scope is best for photometry. I have a Celestron 8 Edge HD and a Skywatcher 80 Equinox Pro. Which scope is likely to give a big enough field of view to pick up comparison stars yet also capture enough light to give scientifically useful data?  

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I think it's horses for courses here. For a given star the AAVSO maps will tell you what comparison stars you should be using, and you want to try to get at least 3 stars in the field. I would think the 80mm will be the easier to work with most of the time - a field of 1 or 2 degrees is usually fine. I haven't used that scope but would think with typical pixel sizes your FWHM would be about right too (don't worry about this so long as it's over 2 pixels or so - 5 or 6 isn't going to cause a catastrophe). For really faint stars you might go with the SCT, but so long as your SNR is over about 100 you'll be fine.

Billy.

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Thanks. That’s helpful. I’ll match the CCD to the refractor. I suppose I could always use binning 2x2 if the pixel size was too small for the SCT. 

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Don't bin it is far better to over sample for photometry as it give more accurate sky subtraction.

Regards Andrew 

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The Atik 460 EX gives a pixel scale of 1.9 arc secs and the 490 EX is 1.5 arc secs per pixel with my Equinox Pro 80 (focal length 500), so presumably the 490 would be a better match for photometry assuming average seeing is 4 arc secs.

However FLO says the 490 has a larger  dynamic range https://www.firstlightoptics.com/blog/atik-460ex-v-atik-490ex.html

Any advice? 

 

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Aperture is generally king for photometry. The larger the scope, the more photons you can catch and the better your signal to noise ratio will be. The only exception to this is if you have very bright targets and your exposure times get below about 15 seconds then scintillation noise can be a problem.

As far as scopes go, it doesn't really matter. Essentially you need at least two pixels per stellar FWHM but oversampling much more than that doesn't hugely help and as you stated you just cut down your field of view. In my experience if you have a FOV of 0.25 degrees then you can normally get enough comp stars from a catalogue like UCAC or CMC14. Half a degree is probably optimal. Much larger than this systematics across the field of view can start affecting your precision.

Another thing to consider is vignetting. A heavily vignetted system (like an SCT with an F3.3 focal reducer) can offer difficulties in terms of flat fielding.

I think that most prolific photometrists tend to either use an SCT (with a focal reducer or binning) or a fast-ish Newt. I'm just building up my kit again for photometry and have a C11. Previously I had a 10" Meade SCT with a QHY6pro CCD and F6.3 focal reducer. I haven't got a CCD for my new set-up yet (need a few more credits first) but i'm planning to get an Atik 414 and a Starizona F6.6 focal reducer.

Don't forget you will almost certainly need to guide too, and finding good guide stars in obscure fields is often much harder than finding comparison stars.

 

Cheers,

Darryl

 

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