?Longest focal length EP for an 8" SCT?

[mental4astro]

Hi all,

I'm aware of the maximum exit pupil that should be allowed for should lie in the range of 6mm to 7mm. For an f/10 SCT that would mean an EP range of 60mm to 70mm. Are there design limitations within the SCT that would preclude the use of such eyepieces?

I ask as I've come across this eyepiece (http://cgi.ebay.com/2-inch-60mm-Super-Wide-Field-RKE-Eyepiece-/110678663060?pt=LH_DefaultDomain_0&hash=item19c4f65f94) on ebay from Russell Optics.

Does anyone have experience with this eyepiece too? Such an eyepiece would give me close to 2 degrees AFOV.

Cheers,

Mental.

[Wavytone]

Alex, the practical limitation with an 8" SCT isn't the eyepiece focal length - it's the aperture out the back of the mirror.

The longest eyepiece I've used on a C8 was a 2" Unitron 60mm Kellner (I know the owner, could probably borrow this beast if you want to try). It has a modest field stop - perhaps 45mm diameter.

But there are a few 2" eyepieces that basically have no field stop - ie the inside diameter of the eyepiece barrel is effectively the field stop, at about 48mm. An example is the TMB Paragon 40mm (you can try mine, next time) and I am sure there is at least 1 Nagler like this.

But the real snag with a C8 is that the aperture of the back at the mirror cell is significantly smaller and it will cause vignetting in a big 2" eyepiece.

I'll have the TMB 40, TMB30, LV50 and NLVW30 for sure next time at Katoomba and could probably arrange the 60mm Kellner if I can find an adapter for it (it has an odd-size barrel). Of these eyepieces, the Vixen LV50 is still my pick on my 180mm Maksutov and I expect you will conclude the same on a C8. I'll bring my my set of LV's as well if you want to try comparing colour/contrast using say the LV25mm vs the RKE28mm, or the LV15 vs RKE15, or LV7 vs RKE8mm (seeing permitting for a C8).

[mental4astro]

Thanks Wavy,

Looks like the old orange tube C8 is going to get to stretch her wings at a dark site for the first time in a long time this New Moon. Will make for a very interesting battery of scopes on show.

A 60mm Kellner, man...

[The_bluester]

I was actually about to ask a similar question but about my CPC925. The EP that comes with it is a 40mm Plossl in 1.25" and I have wondered on more than a few occasions how low power and how wide TFOV I could successfully go. I have suspectded a couple of times on full moons that I can see a shadow from the secondary with this EP, but given what the light level would do to pupil diameter (Not so sure about this EP but my 11mm Nagler is very sensitive to this, it can be prone to kydney beans and blackouts and the problem is far worse during daylight or on a full moon when pupil diameter would be smaller, it is more or less a frustration unless the eye is well dark adapted)

For some extended objects I have thought I would love to have a greater TFOV than the 40mm Plossl gives me but I just have not been sure how far I could push it. At least the 9.25 has a 2" thread for the visual back for larger format EP's.

[Wavytone]

Paul, you would definitely get a bigger field from a larger 2" eyepiece. It might not fill a stop-less eyepiece like my TMB40 to the edge, but it's well worth trying to see which eyepiece it can fill.

Kidney-bean and blackouts (commonly seen in some Naglers) is a result of spherical aberration at the exit pupil of the eyepiece; some particular Naglers were particularly well-known for this. There are other UWA eyepieces that don't suffer from it so badly.

The true field of view is determined by the diameter of the field stop at the bottom of the eyepiece. If you lay all your eyepieces on their side, and look into the barrels the one with the biggest hole has the largest TFOV. See the photo at top centre here http://web.me.com/wx88/Site/Optical.html

The technically-minded can reach for their calipers and measure, to be sure of this.

[The_bluester]

I suppose my biggest concern is that by pushing for the largest true field I can get I may expose a weakness in the scope itself. No point adding to the TFOV if it ends up perpetually distorted or out of focus.

I suppose in my case, the question for the floor would be to owners of 9.25 Celestrons, what wide field EP have you got and how does it perform?

Taking the obvious "Wow" object, it would be great to frame M42 a bit wider to be able to take it all in in a single field of view.

Celestron do not list field stop diameters but there is no obvious stop inside the barrel of the 40mm so it probably gives about as large a TFOV as can be had in a 1.25" format. Even taking the same TFOV and spreading it around a larger AFOV would probably increase the wow factor.

I do like my Nagler, but it is a touch finicky about eyeball position. Wonder how the 31mm nag would go!

[casstony]

The C8 has a 38mm clear aperture at the rear of the scope. A practical low power eyepiece that avoids vignetting is the 35mm Panoptic which has a 38mm field stop. A 31mm Nagler would be useable but with some vignetting since it has a 42mm field stop.

I tried a 50mm GSO superview with a C8 and while the eyepiece is decent, I didn't keep using it due to sky brightness in my light polluted yard - it would be fine at a dark site.

[The_bluester]

If you mean the ID of the baffle tube which the visual back screws to, on the 9.25" the inside diameter is about 44mm. I am pretty sure from memory last time I was in Bintel that the 8" is smaller. The visual back on the 9.25 is much smaller internally than the threads it is screwed to.

[casstony]

In that case you won't have a problem with vignetting with any low power eyepiece. Even if there was vignetting with a 41 Panoptic for example (46mm field stop) it would be so minor you wouldn't notice it.

The C9.25 has a slightly flatter field than the other schmidt cassegrains so the coma/field curvature will be less evident in the outer field of a low power eyepiece. A 31mm Nagler would be very nice and tend to show less outer field aberrations than lesser wide fields.

If you've yet to buy a 2" diagonal Paul avoid William Optics since they have a 40mm clear aperture.

[Steffen]

I've got a 55mm Televue Plössl that I'm mostly using with the 1800mm f/12 Intes MK67, for 33x magnification and an exit pupil of 4.6mm. Next time the clouds part I'll check whether I'm getting the theoretical 1.46 degrees TFOV or whether there is some vignetting.

Cheers
Steffen.

[mental4astro]

Tony & Paul,

I've come across this Celeston spec's page (http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2429) which I feel will help in the discussion.

The bottom of the page has the dimensions of the rear hole in the various Celestron SCT models.

This is all making sense to me now. A year ago, a used a 35mm EP with an eyepiece fitting focal reducer in my C5. The new FOV through the EP was TINY. About half of what the EP delivers neat. Now I understand why.

I haven't had a chance to revisit my C5 with other EP+focal reducer combinations. I'll try to over the next few day, and post my observations.

Here's a link to Russell Optics. (http://www.russell-optics.com/two_inch.html)

[mikerr]

Tony, I also use an Orange Tube C8 and would be interested to know your opinion of the GSO 50mm Superview eyepiece when you get the chance to give it good try. At $49.00 from Andrews it looks interesting!

Here is some edited info from the AOE site attached in pdf.

97736

Michael

[casstony]

The 50mm SV has been sitting in the back of the cupboard forgotten for a few years - I've been meaning to try it with the C11 and nebula filters - next clear night I'll give it a go.

From memory it shows a good image, plossl sized field of view and very long eye relief; I never liked it in town with the C8 though (with or without a narrowband/UHC filter) due to light pollution causing lack of contrast.

It's seems to me that the extreme contrast robbing effect of light pollution below about 50x is still apparent when using filters.

[mental4astro]

I too am considering the 50mm SV. Looks like a better option than the 60 RKE. I've used my 30mm SV in my C8, & it is a very good match. With my 30mm, the 40mm SV isn't much of a gain. But, if there is such an issue with contrast in town use, maybe the 40mm is a better option if you don't have a 2" Superview.

Anyone have a 50mm SV they no longer want?

[The_bluester]

Luckily I have not bought one yet, it will be a little while before I can look at a widefield EP. Plenty of time to do the homework. What would be really handy would be a report from (Or even better, a look though) a 9.25 with an EP such as the Nagler 31 etc. Plenty of time before I am likely to be buying again so hopefully I can manage one of those two first.