2" Eyepiece for a C6

[Ags]

I have not dabbled in 2" eyepieces because I strongly dislike big eyepieces, but I quite like the low(ish) power views with my C6 and want to explore that a bit more. The 2 eyepieces I have in mind are the Panaview 32 mm and the Aero 35 mm, as I am looking at exit pupils of 3-4 mm. I understand the Aero is better at faster focal ratios, which is a paper advantage as you need to be at f7 at least for an exit pupil of 5mm or less. I do prefer the Aero as it is a bit lighter (360g vs ~420g).  

Are there other eyepieces to consider? Is a field stop of around 42mm too much for a C6?

[badhex]

I have the Aero 35mm and I really like it. I picked up the 40mm as well after a long search but it definitely doesn't perform as well in shorter FL, although at F8 neither should really be a problem. Both suffer from aberrations in the outer ~20% (depending on FR) , but are great for search and survey type usage or just taking in large fields. And they are pretty reasonably priced.

I think the biggest issue is the small size of the rear baffle in the C6, which would necessarily mean some vignetting. I found a site where someone did some experiments with 2" EPs, I'll see if I can dig it out. 

 

[Louis D]

You could try using a 0.63x focal reducer/corrector.  That way, you get to roughly the same widest field by using a widest field 1.25" eyepiece with it without having to invest in a 2" diagonal, 2" visual back, and 2" eyepieces.

[Ags]

I do have a .63 reducer and do use it, but I want to try the 2" route.

[badhex]

Okay, a bunch of links. 

Rear baffle/port opening sizes, and some tips on 2" EPs according to Celestron:

https://www.celestron.com/blogs/knowledgebase/i-want-to-use-2-in-eyepieces-with-my-sct-and-or-mak-will-there-be-vignetting-from-the-ota-itself

An in depth look at using 2" accessories with a C5, with practical testing, measurements, photos etc. Note that although this is for the C5, the rear baffle size very similar in a C6 (27mm) vs either 27mm or 25mm for the C5 depending whether you listen to celestron or real world measurements:

http://www.waloszek.de/astro_ce_c5_2z_e.php

There also a bunch of other stuff about adding finder shoes and other bits. 

There's a key takeaway from the above which may be of interest, namely whether using 2" provides any benefit over using the reducer + a max field 1.25" EP:

Quote

You will, however, not achieve a larger field of view this way! Using the f/6.3 reducer/corrector with the "maximum" focal lengths for 1.25" is more or less equivalent to using 2" maximum focal lengths without the reducer/corrector.

Obviously this is one person's experience but I felt that it was quite well documented and tested, and so am inclined to believe the findings. 

Edited July 5, 2022 by badhex

[badhex]

Just another thought, I just re-read what I wrote and it might have come across that I'm saying it's not worth bothering; for avoidance of doubt, what I actually meant to infer was that the overall outcome of the testing link comes to no specific recommendation of one solution over another, and that I'm inclined to believe his general outcomes as he seems to have done a very good job collecting evidence. 

[Don Pensack]

Some notes.

The first link lists the rear port aperture on different sizes of SCTs and Maks.

What is generally not known is that the field illumination at the edge of this port is only about 50%.

We are really poor with seeing vignetting, so no one typically comments they can see the vignetting with an eyepiece that has a field stop of this diameter.

But if you add to that vignetting, at some point you will see it, so it's probably desirable to not go larger than the port using eyepieces with larger field stops.

 

It can be photographed with a full-size chip camera (say, a 38mm chip) at full aperture, with short images.

 

Experimentation, if you can afford it, will teach you where you can see vignetting.

For example: on an 8" SCT of 2032mm focal length, I could see vignetting easily when the field size exceeded 1.2°, even though an eyepiece with a 37mm field stop yields 1.04°.

So I could go a bit larger than the 50% illuminated field and still get away with it.

 

About focal reducers: they do two things: reduce the size of the 50% illuminated field and reduce the focal length more the farther you are back from the lens.

An SCT with an f/6.3 focal reduce and a 2" diagonal is actually operating at f/5-f/5.5 depending on the length of the 2" diagonal and visual back.

It is typically only f/6.3 with the 1.25" visual back and diagonal that comes with the scope.

So, the 50% illuminated field of 37mm is reduced to a 50% illuminated field of 23.3mm when the f/6.3 reducer is used.

Though a 32mm Plössl yields a true field of 1.21° on the 8" SCT when used with the 1.25" diagonal, visual back, and f/6.3 reducer, it has a reduced brightness at the edge.

That may be OK for a low power (40x) on the C8, but you might be able to see the vignetting.  It's easy to see when pointed at a daytime sky to look for edge illumination.

 

The above does point out, though, that focal reducers and 2" eyepieces don't go together.  Achieving larger fields of view is either a 1.25" + focal reducer thing, OR a 2" thing.

 

The measurements provided by Celestron also point out that the C6 and smaller catadioptric scopes are 1.25" scopes.  The only reason one would use a 2" visual back and diagonal

on such a scope is to support large heavy 1.25" eyepieces, like a 13mm  APM XWA, i.e. large eyepieces with field stops small enough for then to be 1.25" eyepieces.

Edited July 5, 2022 by Don Pensack

[Ags]

I have used a .63 reducer with 1.25" eyepieces and haven't been troubled by vigneting thus far (not that I have gone looking for it). I am thinking of getting the same effect with less glass with a 2" eyepiece (and no reducer of course). Also, I have read that the .63 reducer crops the effective aperture of a C6 to something like 135 mm.

[Don Pensack]

The standard SCT reducer/corrector on a C6 shouldn't reduce the aperture at all because its clear aperture exceeds the diameter of the rear port.

The "choke points" are the rear port I.D. and the I.D. of the front tube on the 1.25" diagonal (and any internal stops in that diagonal).

What it does do is reduce the field illumination diameter by the ratio it reduces the f/ratio of the scope.

So if the rear port allows a 50% illumination at the edge of a 27mm  field, then with the focal reducer that same illumination is at the edge of a 17mm field.

Note that a 17mm field stop at f/6.3 yields the same true field as a 27mm field stop at f/10.

The white papers released many years ago point to the SCT scopes being designed to illuminate a 1° field (plus a little), so technically we are exceeding the original design intent to see wider true fields.

As I mentioned, though, I personally found a 1.2° field was fine, and that is a field stop of 19.8mm at f/6.3 in the C6.

 

All these discussions of trying to get wide fields out of the SCT scopes just indicates a misunderstanding of the scopes.  SCTs are narrow field, high power scopes that can be used for most large DSOs at low powers.

Think about it.  How many objects are really bigger than a degree?  And of those, how many are going to be viewed in a C6?

 

As I see it, an SCT is a really nice general use transportable scope and for the occasions when you really need a much wider field of view, a refractor makes a nice companion.

An 8" SCT and an 80mm refractor make a really nice pairing and can often be used on the same mount.  A 60mm refractor could even be piggy-backed on an 8" SCT.

You don't use a hammer to install a screw, and you don't try to squeak a 1.6° field out of an SCT.

 

[Ags]

I do find the combination of a 30mm plossl with  the .63 reducer is not ideal, I can't recall the details but I vaguely recall seeing the shadow of the secondary.  But I might have been experimenting (ok, messing about) by day, so a small entry pupil could have been half the problem. If it stays clear I will try tonight. 

[Greymouser]

13 hours ago, badhex said:

http://www.waloszek.de/astro_ce_c5_2z_e.php

Thank you, I find that interesting and will give it some proper attention when I find the time.

[Ags]

I had a quick peek at my local oak tree (I swear it moves around to interpose itself between me and any astronomical phenomenon!) with the 30 mm plossl and the view seemed fine in my deliberately unscientific assessment. The view seemed a little easier to hold with a 20 mm 68 degree eyepiece.

Edited July 5, 2022 by Ags

[Mr Spock]

That baffle tube field size of 27mm is your limiting factor. There's not really much point in going to 2" eyepieces unless it's to squeeze the last couple of mm out of it.

I used to have a C9.25. That had a baffle tube size of 46mm but I would still see some hard vignetting with a 42mm LVW. Only a couple of degrees, maybe reducing from 72° to 69-70°, but it was there.

[Ags]

The baffle tube is before the focal plane, so it doesn't absolutely dictate the width if the image circle. At least viewing a tree with a 30mm plossl and a reducer seems to give an ok view. Certainly no hard vignetting. I think by the way that the greater focal length of the C9.25 might make the vignetting harder? Its a longer focal length but the same absolute distance from baffle to focal plane.

[Ags]

9 hours ago, Don Pensack said:

The standard SCT reducer/corrector on a C6 shouldn't reduce the aperture at all because its clear aperture exceeds the diameter of the rear port.

I think the stated reason was the reducer moves the focal plane, which in turn means the primary mirror is moved towards the secondary to compensate, so the secondary is too far into the primary's light cone, so light from the edge of the primary is lost.

Edited July 6, 2022 by Ags

[Ags]

Just had a super short test with the SCT,  0.63 Reducer, and an  ES 24/68° and NPL 30/50°. There is a lot of astigmatism off axis - bright stars are concentric arcs toward the edge of the plossl field and to a lesser extent the ES. I think this is coming from the SCT as this isn't visible with the same eyepieces and an f6 refractor. It is also less of an issue with the ES because it is a widefield and the distorted edges are just further away! Given the severe distortions, I would say the NPL30 is too widefield for the scope/reducer, but the ES gives a lovely view (~1.4° true field). I turned the scope to the Double Cluster and it was nicely framed in the ES eyepiece with the red stars showing their color well. I moved the clusters to the edge of the field and didn't see any dimming. Also, the astigmatism wasn't really troubling with the fainter stars of the clusters.

Nah.... I don't need a 2" diagonal etc...

Edited July 5, 2022 by Ags

[Louis D]

Try the experiment again without the reducer in place.  See if the level of astigmatism gets better or worse at the edge.  You may also notice increased field curvature or possibly coma (IIRC) without the reducer/corrector.

[Ags]

I can't repeat the experiment without the reducer, as the widest I can get with 1.25" at 1500 mm focal length is 1.0°. However, I'm happy with about 50x power and 1.4° field with the reducer and the ES 20/68°. That should work for my style of DSO hunting and the views are aesthetically pleasing.

[Louis D]

You could repeat the experiment at a narrower field of view with the eyepiece alone to see if the astigmatism was R/C induced.  If it goes away or greatly reduces, it was the likely culprit.  If there is no change, it's likely due to the eyepiece.  That's all I was suggesting to help narrow down the source of the astigmatism you were seeing.

[Ags]

30 minutes ago, Louis D said:

You could repeat the experiment at a narrower field of view with the eyepiece alone to see if the astigmatism was R/C induced.  If it goes away or greatly reduces, it was the likely culprit.  If there is no change, it's likely due to the eyepiece.  That's all I was suggesting to help narrow down the source of the astigmatism you were seeing.

I see - that makes sense.

[Don Pensack]

On 05/07/2022 at 15:04, Ags said:

I think the stated reason was the reducer moves the focal plane, which in turn means the primary mirror is moved towards the secondary to compensate, so the secondary is too far into the primary's light cone, so light from the edge of the primary is lost.

Nice theory, but the focal reducer requires less than an inch of back focus adjustment, and many eyepieces can fall into that category, too.

I don't think a reduction in aperture will be as important as a big increase in spherical aberration.

[Louis D]

28 minutes ago, Don Pensack said:

I don't think a reduction in aperture will be as important as a big increase in spherical aberration.

Does the R/C induce SA?

[Ags]

Well, I have read that the R/C changes the primary-secondary spacing and this increases spherical aberration.

It seems there is a lot written about the R/C and what it does or doesn't do - but not a lot of agreement. Without getting too scientific or name-checking any aberration types, I can only say that the views through the R/C are just plain better than without. It helps that the magnification is lower of course, but the R/C seems to give cleaner images at matching levels of magnification*. I like it.

@Don Pensack - 1.4° field of view is more than necessary for most DSOs, but it is not just a matter of framing. A wider field helps with me finding the objects, and given that I use a manual and pretty basic AZ mount, a wider field makes life far easier. Also, aesthetically, the field of view and the quality of stars delivered by the C6 + R/C + ES20/68° are very nice - I think the field flattening of the R/C helps with the widefield eyepiece, and if there is any added SA, it's not visible at 50x power.

Having said that, I have tried the R/C for planetary viewing (a couple of years ago) and I recall I had enjoyable views... So it seems to work at high powers too. 

*Regarding matching magnification, I have the perfect R/C tester eyepieces: my Speers WALER 10 mm and 13.4 mm 82° eyepieces. They come with an extension tube that gives 1.6x zoom - which almost perfectly compensates for the 0.63 unzoom of the R/C. 1.6 x 0.63 = 1.008.

Edited July 7, 2022 by Ags

[Don Pensack]

I recall from reading about the SCT design that if the spacing between the primary and secondary mirrors differs by more than 3mm from the design spacing, it adds 1/4 wave of spherical aberration.

Whether you could see that at low power is a different issue.

 

On my older 8" SCT, the focal reducer flattened the field and reduced edge of field astigmatism in the scope as well.

I thought it improved the image quality, so I used it 95% of the time.

 

As for having a large field to find things, a good 8x50 finder, piggy-backed on the scope, would give you a nice big field for star hopping.

At a dark site a huge number of DSOs are visible in the finder as well.

[Ags]

On 08/07/2022 at 23:25, Don Pensack said:

As for having a large field to find things, a good 8x50 finder, piggy-backed on the scope, would give you a nice big field for star hopping.

Actually, I have both an RDF and a 6° RACI finder. But I like to leave nothing to chance 🤣