2-inch eyepieces with a C8

[Alex_S]

I have some 2-inch eyepieces I currently use with a Tak TOA130 refractor. Unfortunately I need to sell soon but will be replacing with a C8 SCT.

What kind of issues (if any) am I likely to run into using the 2-inchers with a C8 aside from a loss of field of view?

[Camelopardalis]

Alex, I have several 2 inch eyepieces I’ve used with my C8 in the past.

You shouldn’t have any issues with them, besides the long focal length of the scope making the FOV smaller - but you know that already.

The baffle tube of the scope is around 40mm if I recall, so you might notice some light drop off towards the edge if the field stop is much larger than this. I’ve personally not noticed it.

[MortonH]

I never noticed any problems using 2" eyepieces in a C8.

[mental4astro]

What "loss of field of view" are you talking about?

There is an urban myth that using long focal 2" eyepieces in an 8" SCT will result in vignetting. People who say this ALL have NEVER tried this combo & are only repeating what they have heard.

I have used long focal length 2" eyepieces with not only 8" SCTs but also a 127mm Mak which has an even smaller bore in its primary mirror. I have gone out of my way to look for this vignetting & have never seen it. I have used all manner of ling eyepieces, including 30mm 82°, 38mm 70° & 55mm 45°, at night with stars & the Moon & with daytime viewing to make sure I wasn't missing anything & have never been able to see any vignetting.

HOWEVER, the image will vignette ONLY if you use an f/6.3 focal reducer-corrector with these long focal length eyepieces. But neat, without a reducer, no vignetting.

But please, tell us what this "loss of field of view" is that you mentioned. Using 2" eyepieces INCREASES the true field of view, not decreases it, with long fl eyepieces. By "loss of field of view", do you mean the AFOV getting smaller beyond a certain focal length? Or are you talking about some reduction in the TFOV? Because the two are not the same thing.

This is all with visual use of these scopes. I am not discussing imaging as your post is about visual.

Alex.

[Tinderboxsky]

I agree.

I used a 42mm LVW42 eyepiece successfully with a Mak127 with it’s small bore for many years. I could not detect any vigneting despite trying hard to detect it using various targets.

[Don Pensack]

Obviously, some people are much less sensitive to a reduction in illumination than others. I could see vignetting in the 8" SCT with any eyepiece having a 38mm field stop of larger. It didn't bother me that much, but I could always see it.

The C8 was designed to fully illuminate a 1° field, according to the white paper, and anything larger would result in more vignetting than the 50% illumination at the edge that was designed into the scope. I found I could tolerate vignetting up to about 1.2°, but not larger. A 46mm field stop yields 1.3°, and vignetting was definitely noticeable 100% of the time.

I could also see it in my 127 Maksutov when the field stop was larger than about 25mm. I never used 2" eyepieces with that scope because vignetting would have been severe.

Some use no coma corrector at f/4. I found coma horrible at f/5 and bought a coma corrector the next day after getting the scope.
We all have different sensitivities.

[mental4astro]

Interesting you should say this. As I said, i have gone out of my way to look for vignetting, observing under as many conditions as possible to make sure I wasn't missing anything. I have never seen it in these scopes. You are the first person I have encountered who has said they have while no one else has who has also made a point of looking for it. Not questioning your observing skill which for me is never in doubt.

Different sensitivities is also very nuanced. There can also be many reasons for why different people see things differently from others, such as eye health, gender, age, genetics and others. I have noticed changes in my own eyes over the years and I wonder just how many people are honest enough with themselves let alone with others. As for what different people are able to tolerate or not in what they see, that's different again.

[Don Pensack]

A lot of eyepieces have intrinsic vignetting as well, which can easily be seen by pointing the scope at a daytime sky with a scope that doesn't seriously vignette the field.

[Argonavis]

You seem to misunderstand.

Vignetting is the stopping down of the FOV by the field stop in the eyepiece and size of the focuser.

The size of the 100% illuminated field and the light loss drop off towards the edge of the FOV is different, and depends on the size of the diagonal and the shape of the light cone.

[Don Pensack]

Field stop vignetting is 100% vignetting and is not usually referred to as vignetting.
Light loss at the field edge that is noticeable as darkening is what is usually referred as vignetting.

Almost all scopes have some vignetting at the edge of the field due to the size of the diagonal or secondary mirror or the inside diameter of the focuser.

[Alex_S]

I simply meant a loss of field of view using the 2” eyepieces with a C8 as compared to with the refractor… as a result of the increased focal length.

[lochiel]

I am thinking of buying a 2" diagonal for my C8SE.
Mr google informs me as follows but would be interested in peoples view ?
Obviously plenty of discussion around vignetting below but my eyepiece range tops out at at a Hyperion 31mm anyway.

Pros of a 2" Diagonal on a C8:

Wider apparent field of view:
2" eyepieces offer larger apparent fields of view than 1.25" eyepieces, allowing you to see more of the sky at once.

Potential for more comfortable viewing:
Some find the larger field of view of 2" eyepieces more comfortable to use, especially for extended viewing sessions.

Cons of a 2" Diagonal on a C8:

Vignetting:
The C8's baffle tube is not designed to fully illuminate the field of view of the widest 2" eyepieces, leading to vignetting (darkening of the edges of the field) with some eyepieces.

Slightly increased focal length and potential for spherical aberration:
The longer light path introduced by a 2" diagonal can slightly increase the focal length of the telescope and potentially introduce a small amount of spherical aberration, though this is often minor.

Increased back focus:
The primary mirror may need to be adjusted to compensate for the longer light path, potentially affecting the optimal focal length and introducing spherical aberration.

Thanks
Ewen

[Don Pensack]

All of that is true--vignetting is very minor, however.
The lengthening of the focal length can be calculated--for every 1mm you add at the back, you increase the scope's focal length by 3.1mm.

The main issue with the 8SE, however, is not having enough clearance for the 2" diagonal to pass through the bottom.
This can be avoided by, say, using a Baader Dielectric diagonal, whose nosepiece can be removed from the diagonal, leaving a thread in the body of the diagonal that can be screwed directly onto the back of the scope.
You need a small ring Baader sells to go between the diagonal and the scope so you can lock the diagonal in place. You add the least back length that way, and add the least focal length to the scope.

Also, the optical tube can be slid forward in the cradle to give more clearance for the diagonal on the back. For many 2" diagonals on the 8SE, you have to use a Dremel tool to cut away a bit of the plastic covering the fork arm to allow the scope to slide forward more so the 2" diagonal will clear the battery department cover when it swings through.