2" eyepieces adaptable to C5?

[amillego]

Is it possible to adapt a 2" eyepiece to a C5. If so, what is the possible downside of doing so?

[Mr Spock]

The problem with SCTs like the C5 is the size of the internal baffle. This will cause vignetting on most 2" eyepieces.

[brianb]

Is it possible to adapt a 2" eyepiece to a C5.

Yes, you can get a 2" visual back which will screw on in place of the 1.25" one.

There is no point whatsoever in doing so. The field of view is restricted by the internal baffle to a bit less than is available in an "optimum" 1.25" format eyepiece (e.g. a 32mm 60 deg AFOV Plossl). The extra optical length of a 2" diagonal will result in the focal length being increased (if focus is still obtainable, which it probably will be), this will reduce the actual field of view. The weight of the 2" diagonal & eyepiece will result in the centre of gravity being even further back than it is normally, making it hard or impossible to balance the tube (without a counterweight system).

A 2" visual back is worthwhile with SCTs of 8" aperture and above but not for smaller scopes.

[E621Keith]

Like others said, it is possible to fit 2" visual back, but there would be limited benefit doing so due to the internal baffle of the smaller SCT. I do it on my C6 because my 2" dielectric diagonal is much better than the stock 1.25". However, I've only got 1.25" eyepieces and don't see the need for 2" with my current set up.

[amillego]

Thank you, all, for the advice. I appreciate you taking the time. I'll stick with what I have and keep on enjoying the view. Cheers.

[John]

If you would like a lower power, wider view, the F/6.3 focal reducers work well with the C5's.

[brianb]

the F/6.3 focal reducers work well with the C5's.

But the field is actually limited by the baffle tube ....

Using a focal reducer with a SCT for visual work is pretty pointless, just more glass to soften the contrast ... when imaging with a small chip, a focal reducer makes a lot of sense!

[John]

But the field is actually limited by the baffle tube ....

Using a focal reducer with a SCT for visual work is pretty pointless, just more glass to soften the contrast ... when imaging with a small chip, a focal reducer makes a lot of sense!

Fair enough Brian, but I found it worked pretty well for me - which is why I posted the suggestion.

Different eyes, different conditions I guess

[michael.h.f.wilkinson]

I used to use the 6.3 focal reducer visually on my C8, when I only had 1.25" EPs, since I got the 2" back and 2" EPs the reducer is only meaningful in imaging.

[John]

I used to use the 6.3 focal reducer visually on my C8, when I only had 1.25" EPs, since I got the 2" back and 2" EPs the reducer is only meaningful in imaging.

Absolutely

But as we have advised the OP that the 2" route is not feasible with the C5 I thought it would be good to mention a viable alternative to obtaining lower power, wider views with 1.25" accessories.

The Japanese made F/6.3 reducers are nice optics and comparable to a fine barlow lens but with the opposite effect of course. I found the views quite satisfying when I had a C5

[michael.h.f.wilkinson]

Absolutely

But as we have advised the OP that the 2" route is not feasible with the C5 I thought it would be good to mention a viable alternative to obtaining lower power, wider views with 1.25" accessories.

The Japanese made F/6.3 reducers are nice optics and comparable to a fine barlow lens but with the opposite effect of course. I found the views quite satisfying when I had a C5

I do not doubt it. Do not forget the eye is generally very insensitive to vignetting (unlike cameras:eek:). Even a 30-50% loss at the edge of the FOV might not be troublesome, especially at night, when the background is dark anyway

[Mr Spock]

It doesn't matter what accessory you use, it won't change the size of the baffle tube. It is that limiting the field of view not the focal length of the scope or the type of eyepiece

[michael.h.f.wilkinson]

It doesn't matter what accessory you use, it won't change the size of the baffle tube. It is that limiting the field of view not the focal length of the scope or the type of eyepiece

Absolutely, but the F/6.3 reducer can help you make the most of that field.

[CGolder]

I am not sure on the exact size of the internal baffle when compared to the C6 that i have but i can happily use my 36mm 72 Degree hyperion in my C6 with the 2" diagonal and have no ill effects, i also sometimes use a focal reducer but it doesn't always improve the experience and i do notice some visual problems introduced, it does however let me view m45 through my C6 so does have its uses.

[michael.h.f.wilkinson]

The C6 is a much newer design, better prepared for 2" optics.

[E621Keith]

The C6 is a much newer design, better prepared for 2" optics.

I don't agree. I just measured the opening at the back of my C6 and it's only 1 inch in diameter. SCT is not a wide field instrument anyway, so the losses in light collection for 2" compatibility far out weights the gains for wide field performance.

Lets put this in perspective, 2" is 33% diameter for C6 and 40% diameter for C5. The secondary required to support them will be even larger. In the current configuration, the secondary on a C6 takes up 38% diameter and 36% on a C5. Personally I rather have a narrower field 1.25" SCT than to sacrifice more larger light gathering power for a slightly wider field. If one wants wide field performance, a Newtonian will do a better job for a lot less.

A 24mm Hyperion will yield a 1 degree field on a C6. If I want wider field, I might well just pick a ED80 or a 150p.

[michael.h.f.wilkinson]

Given the size you measured, you are certainly stuck on the C6 with 1.25" eps. However, your remark on the secondary does not take into account its magnification (5x on the C5, C6, C8 and C11). This means a modest increase in secondary size is needed. Besides, 40% central obstruction equates to 16% light loss (0.4 squared). On my C8 2" is readily usable, and I get 1.4 deg FOV at an exit pupil of 4mm. For widefield, my little frac has the SCT beaten by a mile.