Widefield Eyepiece for a Celestron C5 SCT.

[Guest]

I have some nice widefield 1 1/4 inch eyepieces that I use with my C5. The Eyepiece with the longest focal length is a 16mm Skywatcher Nirvana. I was contemplating the Explore Scientific 24 mm 68 degree. But I am also looking at the ES 26 mm 62 degree. Whilst the sct is not a wide field instrument. There is no harm in trying to garner every last minute of field of view from the telescope. Both will give approx 1.3 degrees field of view. 

Both eyepieces are 1 1/4 inch  fitting. I would prefer to use this size with the C5. 

Any thoughts would be welcome. 

[John]

You should be looking for any 1.25" eyepiece that has the maximum field stop diameter that the barrel will support which is around 27/28mm. The 24 / 68 will have this as I believe does the ES 26 / 62. A 32mm plossl of decent quality (eg: GSO) will also have the max field stop so, in a sense, you can pick the magnification and apparent view that you prefer to make your choice. I tend to prefer the wider AFoV / higher magnification approach because my observing is usually done under moderately light polluted skies. In an SCT the exit pupil genereated will be a reasonable size whatever focal length you go for.

The next question is can the rear port diameter of the SCT allow an eyepiece with a 27/28mm field stop to be fully illuminated right across and I've seen varying opinions on that. If there is light loss my guess is that it's minimal.

 

 

[Louis D]

You could also get a 0.63x reducer/corrector to thread onto the rear port to widen your field of view.

[michael.h.f.wilkinson]

4 hours ago, Louis D said:

You could also get a 0.63x reducer/corrector to thread onto the rear port to widen your field of view.

I am not sure that would help for the C5 (it does for my C8), due to the fact that the FOV of the C5 is limited by a much narrower baffle tube than the C8. The image circle of the C5 is about 28mm I gather, so the reducer will not fit more into the maximum possible field stop of a 1.25" EP

[Louis D]

6 hours ago, michael.h.f.wilkinson said:

I am not sure that would help for the C5 (it does for my C8), due to the fact that the FOV of the C5 is limited by a much narrower baffle tube than the C8. The image circle of the C5 is about 28mm I gather, so the reducer will not fit more into the maximum possible field stop of a 1.25" EP

I use widest field 2" eyepieces on my 127mm Mak with no noticeable visual vignetting despite it having a 27mm rear baffle which is about the same as the C5.  Bright stars and planets do flare into ovals when they pass the edge of the baffle, but visually, dimmer objects don't seem to extinguish like you'd think they would.  For $70 used, it might be worth a try for the OP to pick up an SCT reducer/corrector and see if it works for him/her.  If it doesn't, it can be sold on for the same price and just a loss of shipping costs.

[Stu]

18 hours ago, John said:

In an SCT the exit pupil genereated will be a reasonable size whatever focal length you go for.

I think SCTs and Maks, with their longer focal ratios are some of the examples of when 32mm and 40mm plossls make sense, even though you dont gain extra field of view. Particularly for filtered views, the 40mm gives a 4mm exit pupil vs say 2.4 for a 24mm in an SCT, a very useful increase to add brightness to the view.

[Guest]

3 hours ago, Stu said:

I think SCTs and Maks, with their longer focal ratios are some of the examples of when 32mm and 40mm plossls make sense, even though you dont gain extra field of view. Particularly for filtered views, the 40mm gives a 4mm exit pupil vs say 2.4 for a 24mm in an SCT, a very useful increase to add brightness to the view.

Interesting. Another slant. What would be a good 30 mm and 40 mm Plossl's to buy. I have a 40mm Plossl and you have to hold your eye quite far away to see anything. If you move your eye closer, you just see the inside of the eye piece barrel. 

[Stu]

23 minutes ago, Grumpy Martian said:

Interesting. Another slant. What would be good 30 m and 40 mm Plossl's to buy. I have a 40mm Plossl and you have to hold your eye quite far away to see anything. If you move your eye closer, you just see the inside of the eye piece barrel. 

I have a pair of Celestron Omni 40mm Plossls for Ha solar and they seem quite acceptable. Plossls by definition have eye relief related to their focal length, so yes, the 40mm does have plenty to spare! I made up some eyeguards from thin modelling foam and this helps with eye placement and reduces glare and reflections too. The alternative is of course a Televue Plossl with two very expensive eyeguard extenders stacked on top of each other....