I am a newbie trying to get my way into visual estimation of VS. I have been thinking hard on my choices and what it would be the best instruments given my situation. I live in a large light polluted city (Bortle sky 8.5-9) and even though I go out to better skies time to time (Bortle 3-4), I plan, of course, to mostly do my observing sessions from the rooftop of the building of my city apartment. I have selected some bright stars and I am observing them with my very good 10x50 Fujis + tripod and during clear nights I can go as deep as mag 8-8.5. I would like to add another instrument that permits go a bit deeper, say 1- 2 magnitude deeper so I can enjoy a bit more the cycle of some stars I have already have in my list and even to add some more. My other scopes being a 4" F11 and a 4" F13 refractors, I guess I need to get some more aperture (and larger TOFV).
Then I have just recently a 5.1"F5 Newtonian to experiment and see if Newtonians are a good choice for me knowing they have their own idiosyncrasies, namely collimation, field illumination and coma. I have read that to optimize a Newtonian for getting a proper and even illuminating of the field of view, both the collimation -specially proper placement of the secondary mirror in the focuser axis- and the size of secondary itself have to be carefully considered.
On the other hand, coma is really intrusive to my eyes and I find difficult to do any estimation if stars are even slightly comatic. So next I ordered a coma corrector (the Explore Scientific variety) so I can correct as much as possible the off-axis distorted stars letting me compare point-like stars across the field of view . However I have just read somewhere in this forum something like “coma correctors and visual estimation of variable stars do not go well along” which left me wondering why it is so. Even though I heard somewhere that some coma correctors may induce some vignetting (as small secondary mirrors do) I wonder how people do to overcome dealing with coma in F5 reflectors (or faster) that are the common ones from 10” and larger when doing visual VS estimation? I am waiting for my coma corrector to arrive and certainly I will try to do estimations with it but I wanted to know what are the problems I would expect to have? By the way, as previously mentioned I bought the small reflector to learn the quirks of Newtons as I was planing going later with a 10” F5 Dob (+ coma corrector) that would let me go deeper by +3 magnitudes compare to my 10x50.
As a final thought, I also have been considering staying in binoculars-land by complementing my 10x50 Fujis with some quality 20x80’s gaining nearly 1 magnitude and be content with that. That would be a very neat rig with minimal hassle and very transportable indeed, and would let me cover down to ~mag 10 in my city and even deeper from my dark site.
All your comments and opinions would be greatly appreciated,
I can’t say whether coma correctors are a problem. Vignetting is certainly a possibility.
You have very nice gear as it is, for almost all variable types. I say just use that, until you know what observing targets you like best.
I use a 4” f7 refractor and binos from 5x25 to 10x50 in a light polluted city. I’m focused on all variable types I can observe for their full period. There are hundreds visible from Europe with a minimum ≥7.5 and an amplitude of ≥0.2. Bino observers can also observe more variables per session, which is great if you want to contribute a lot of data.
If you become infatuated with one type in particular, such as CVs (which are often fainter) you can always re-evaluate a larger aperture.
Hello! I've been looking at a wider field system than my 8" LX200 classic and ST-402. An excellent system, but even with 0.6 focal reduction, it comes to abokut 15x20 arc-minutes.
I've been looking at fast newtonians and coma correctors as you have. Folks with more experiece can provide additional guidance since my ATM days were several years ago. From what I could remember/read of secondaries, the edge of field illumination is about the same regardless of size of the secondary. What changes is the rate of drop off from center to edge. Small secondaries have a much quicker drop off.
With CCD, that can be accommodated with flat fields. I believe that for visual work, the target and the comps should have the same illumination. Different illuminations may yield inaccurate results.The coma corrector itself probably does not have an effect as long as the target and comps are in the same fully illuminated field..
What is the coma like in your 5.1" f5 newtonian at the edges of the fully illuminated field?
For example, with the f5 system, using a Plossl 25mm eye piece with AFOV of 50 degrees, the true field of view is about 2 degrees. If you have a largish secondary, it is possible that only the central third is fully illuminated. What is the coma like 1/3 out from center? If that coma is not objectionable, perhaps you would not need a coma corrector for visual work? Best regards.
Something else... each element in the optical chain absorbs some light, and from the sound of what you say, you can't afford to lose any more! Like you I observe purely visually (14" newt on a Dob, rural conditions) and the trick is simply not to use comp stars at the edge of the field. So just give your scope a little nudge/pull etc., to bring them closer to the centre. You don't need extra tech, and you can spend the money you would have spent on more aperture instead! Sorted.
I don't see any issues here. As long as your system is collimated, and there is nothing to cause the vignetting or optical distortions to vary around the circumference, as long as you put the variable and the comp at the same distance from the center of your field, there won't be any problems with accuracy of estimates.