Hi all and happy holidays ,
My current visual observing kit, for my 6" F/8 Newtonian , includes a 24mm, 16mm , and 8mm set of eyepieces . I was wondering if any of you use a 24mm to 8mm zoom eyepiece for your variable star observing? (There seems to several versions of this eyepiece on the market that may be the same design, just rebranded)
My main consern is if this style of eyepiece suffers from any issues that would have a negative impact like vignetting on VSOing?
Thanks in advance.
Steve Toothman (TST)
First off, I would bet that the eyepieces are not all of the same design. Having said that, let me express my perspective as a visual observer:
1. It is difficult to assess the optical quality of the lenses from what I was able to find on the internet. But one thing that stands out about all of the zoom lenses is their relatively narrow apparent fields of view. At minimum focal length, they are all in the respectable 65 70 degree range, but at the other end, they are terrible! Typically 40 degrees - 10 degrees worse than the lowly Plössl! For me, field of view is a high priority - I want to be able to see my target and several comparison stars in the same FOV. Thus I have gradually upgraded my set of four eyepieces over the years to include only ones with apparent FOVs of 68 to 82 degrees. For me, the inferior FOV of the leading zoom eyepieces would rule them out for variable star observing.
2. For my photography semi-profession, on the other hand, I have a single lens for my main camera, a Canon L-series 24 - 105 mm zoom. Particularly at higher f/ numbers, its performance is very close to those of single focal length lenses in the same (premium) quality range and so I am skeptical as to whether the early prejudice against zoom lenses' image quality still has any validity, at least in the case of the high end lenses.
I use a Celestron 8-24 mm eyepiece for most of my visual variable star observing, and I think that it's great. The outstanding feature is the ability to zoom in to an appropriate field width that is the ideal compromise between darker sky background and the right number of comparison stars visible, appropriate for the particular variable star and whatever brightness it is, all without needing to remove the eyepiece and risk moving the field of view, and without paying any attention to the resulting magnification, focal length or width of the field. I simply adjust it until the field of view has the stars that I need for comparison. The field of view is definitely a bit narrower than my other Plossl eyepieces, and sometimes I use a 32 mm eyepiece to help find the target star. (I use setting circles and starhopping to locate objects.) The zoom eyepiece is great with my 10" Newtonian (which is my most often used scope for VSO), 6" Newtonian, and 5" SCT scope. The zoom eyepiuece is also very hany for deep sky observing, and I only switch to a non-zoom eyepiece for a sharper view when viewing star clusters and the larger planets, when seeing conditions are really good. But for VSO, I find the zoom eyepiece to be great.
Again thanks for the information.
Measuring my scope components and doing the math, it appears the scope can provide an unvignetted field of 50'. So I'll give the Baader 24-8 a try, since its FoV seems to be the best match match to my scope.
Have a great new year .
Steve Toothman (TST)