What would be a recommendation for a scope dedicated to variable star observing. What mount would you use? I’m not asking for the ideal scope - probably can’t afford it. But I am looking to acquire a second scope that I would dedicate to variable star observing. I currently have a CDK12.5 on a L350 mount but can’t afford to duplicate that system.
I was thinking of a refractor based system. Since exposures are normally less than a minute, could I get away with an altaz mount? I assume not really but thought I would ask.
What is the most useful apeture and f ratio?
I know there are a lot of variables and no pefect answer, but just trying to narrow down the options a bit.
I’ve thought about this too, but have not done anything other than short trials. An advantage would be the possibility of long time series through the night with no need to plan for meridian flips. One issue is field rotation, which in itself is OK, as long as rotation is around the centre of the field. If you used an off-axis guider rotation would be around a star outside the FOV which would be unacceptable. However if you could use an on-axis guider and the guide star was at / very close to the centre of the field, it seems to me such a system could work for most of the sky. On-axis guiders are somewhat more expensive than the off-axis equivalent.
Alternatively, if a guide 'scope were used, provided it could be aligned accurately to guide on a star at / near the centre of the field, rotation would again be around the guide star and perhaps could work. Of course there is still differential flexure with a guide 'scope. Just to emphasize: I’ve never seriously tried any of this, so the above is just thinking through the issues.
Nowadays some observers use smart telescopes and avoid the above.
A while back I used a 120mm f/7.5 ED refractor on an equatorial mount for photometry, obtained nice sharp images and enjoyed the experience. The 'scope and the mount were consumer grade so therefore not very expensive. Pointing was thus a little off, but didn’t really worry me because I did time series throughout the night on short period variables and had just one target per night. Astrometry to find the field centre, synching the mount to the sky position then re-pointing usually did the trick fairly quickly. The only problem was the meridian flip: I never trusted the mount enough to automate the process so had to wake up in the middle of the night to look after it.
I’ll focus on my experience. I concentrate on LPVs, a few YSOs, and the occasional nova that are within the range of my setup. I have a backyard observatory housing a 102mm f/7 ED refractor (William Optics), filter wheel with VBRI filters and a vintage Atik 314L+ ccd camera. For guiding a piggyback 80mm short focus doublet refractor is suitable. I utilize multiple exposures of 5 seconds to 600 seconds as needed for stacking to create a useful SNR. The V magnitude range of this setup is 8-14. The mount for this scope is a Losmandy G811G, handles it easily and has goto functions. If a variable becomes brighter than 8th magnitude in V, I have another Atik 314L+ also on the mount with a Nikon 200mm focal length lens stopped to f/8 with V and B filters for wider field, brighter star photometry.
Kind of an overview of what I utilize. I try to gather data on variables within one or two nearby constellations in an observing run so it is not an all night affair. For me that is enjoyable and productive.
I think the primary thing to consider is matching your potential setup with your typical seeing. For example, if your telescope and camera end up giving you 0.1" per pixel and your seeing is usually over 2" you are wasting time (on each exposure) and money (obviously you could bin your images but you’ve over spent on the system).
Alternatively, with short refractors you may end up with over 1" per pixel which may lead to contamination of your target from other sources.
If you are planning on observing single, or a few, targets per night over several hours your main concern for a mount would be tracking. It doesn’t really matter how quickly you can move to a new target. And with some kind of guiding system and decent software your guiding should be good enough anyway. For photometry you don’t need perfectly focused as-small-as-possible sources, an image that would look terrible for astrophotography might (or likely) be just fine for photometry.
I should say that the 2 examples I gave do have applications - if you want millimag precision from single images (say for exoplanet transits) then having the light spread across 100+ pixels is a benefit. Or if you want to quickly image a large field and are mainly interested in positions of objects (say during a total solar eclipse) then fast optics and big pixels helps.
So you should also consider the objects and timescales you want to observe since what is good for one thing often isn’t as good for others.
You can do photometry with almost any telescope. Yes, it is ideal to match your system to your seeing, but if you already have a camera I wouldn’t worry about this until you have some experience and know if you want to invest in a new camera or telescope to optimize the system. I’d suggest for starters use what you have.
You already have the most important part of a photometry system, a high quality mount. I’d suggest mounting a small telescope piggyback on your main scope.
I use a C5 mounted piggyback on my C14 when I need a wider FOV. The C14 gives a FOV of ~23 min; the C5 (with a focal reducer) gives a FOV of ~60 min. The C5 works very well for photometry.
The main obstacle in setting this up was flexure in the piggyback mounting, even when I used high quality adjustable rings mounts. At the end of a slew the C5 and C14 would end up pointing in slightly different directions. I finally solved this by using an Optec “Libra” adjustable mount for the C5.
If you have a 5-6" SCT or a 3-4" short focus Apo refractor you already have what you need for this. If you already have a camera you plan to use on the photometry scope, just use that. If you don’t already have a camera, now is the time to consider optimizing the system.
In my case, I have a Planewave CDK 12.5 but wanted to free it up to do other observing. I’ve got several 100K worth of observations in the AAVSO now. After all the comments, I’m leaning towards just getting a better scope for DSOs Thanks for the reply.
If you want another scope “dedicated to variables” you must decide what you want to do as a program and then design a system that will fit in your budget and program goals. Advantages over the “surveys and satellites” is hard to achieve but not impossible. Fast time-series on individual stars is probably something worth doing. Brighter stars than say 12th are probably more valuable today for a normal once a night observation program. The surveys sweep everything fainter but the amateur fast time-series still has an advantage in most cases. I have had great success with an iOptron CEM40-EC and an Esprit 100 (f/5.5) triplet and BVRcIc filters observing portable. Something like that could be set up at a permanent site. Remote use and automation is possible with work and the right software controlling everything.