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What eyepieces are suitable for Cassegrains taking into account Cassegrain designs?

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#1 25585

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Posted 19 September 2021 - 11:44 AM

Which eyepieces are suitable for Cassegrains, all types?

 

In the context of vignetted losing AFOV, expensive faster optical qualities for faster scopes (unless with a reducer maybe), too high magnification & too small exit pupils for certain focal lengths. Also impractically small FoVs.

 

Cassegrain owners, what do you use? Is your scope guided or manual?  

 

 

EDIT: TITLE & TEXT CHANGES  

 

   


Edited by 25585, 19 September 2021 - 05:09 PM.


#2 markb

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Posted 19 September 2021 - 12:02 PM

I think you'll need to specify cat type and aperture at least. They run from f15 90mm maks to C14s at f10, with weird ones thrown in the mix.

 

Baffles tubes on SCTs really vary in size affecting vignetting, at the least.

 

On the C8 and 11 I run from a max of 2" 30mm 72 to 82 degree UFFs and Widescan at f10, the same with an Alan Gee TC at about f5 in the 11, down to a Nagler 4.8 and (not the home seeing is ever good enough to use them) to N7 or N4.8 with a 2.8 Klee Barlow. Theoretically. Seeing hasn't cooperated yet.

 

Mounts are Nexstar or T head alt az depending on mood.


Edited by markb, 19 September 2021 - 12:14 PM.

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#3 deepwoods1

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Posted 19 September 2021 - 12:08 PM

Hummmm, I have to say that I use all my eyepieces at some point, if conditions allow. Old school C8. Under that criteria, the short focal lengths are used the least, but only because of "seeing" conditions. Mostly manual, but not always. Ultimas from 35mm (widest I can achieve with a .63 reducer, Approx. 1.5*) to 5mm. Orthos from 25mm to 4mm. These two sets have tight FOV's but are sharp to the edges and have very good to excellent ghost control. I use them for the Moon and planets. I also use the old Axiom line with a 2" 50 (don't see any vignetting} , 34, and 19mm. Round out with the ES/82 8.8. 6.7, and 4.7. While the ES's are wider FOV, they have ghosting issues on bright objects like the Moon or Jupiter, so I use them for DSO and Globulars. Really, it all depends on what my targets are. Tonight with the Moon near full and a "forecast" of excellent seeing, will be an Ortho and Ultima night. 14 days from now might be different. Clear and steady skies....


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#4 jimhoward999

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Posted 19 September 2021 - 12:25 PM

Almost every eyepiece is great in a Cassegrain provided the focal length is long enough to support F#>10 which means longer then 8mm or so and short enough to deliver the full AFOV given the format.  That is one of the great things about having a big aperture F/10 in a compact size.   No need to worry about if your mid-priced eyepiece will support F/4 or F/5.  At F/10 it is a non issue.  At the same time high end eyepieces aren't wasted because of slightly improved contrast.   SCT owners don't mind spending a few hundred an eyepiece giving only small improvement over generic because their 8" scope only cost $1500. 


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#5 rkelley8493

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Posted 19 September 2021 - 12:25 PM

So far with the New Mu [210], the widest field/longest focal length eyepiece I've used is the 22T4 Nagler. The clear aperture of the scope [visual back] is about the same clear aperture as the Baader T2 Zeiss Prism [34mm]. I don't see myself using the 40 XW or 31T5 with this scope, but then again it wasn't designed for viewing large patches of the sky. I have my refractors for that niche.


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#6 MarMax

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Posted 19 September 2021 - 12:45 PM

I use the full kit of EPs with the C11, so a 41mm Pan to a 6mm Radian. The wide field EPs are always nice and as conditions allow I step up the magnification. The 12T4 is my telltale for conditions. About 70% of the time I need to stay below 150x but on the right night the 6mm Radian is amazing and provided my best views of Mars.

 

Not that you asked but the C11 with an f/3.3 reducer for EAA is a hoot. A $1,500 scope, $100 reducer, $300 camera and a $500 laptop . . . a grand total of $2,400 to see things in my light polluted back yard that I thought were not possible.

 

2014 CPC 1100 Alt-Az w/ StarSense . . . does not get any easier to use than this. waytogo.gif


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#7 Echolight

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Posted 19 September 2021 - 12:55 PM

I’m planning to try the XW40 and 0.63 reducer on a C8 for visual on a manual alt/az mount. I’ve read reports that the vignetting is not overly objectionable for visual. I figure at worst I’ll be able to use the 28-82 behind the reducer to get above 1.5 degrees. But I think I might need a Baader Maxbright with a SCT lock ring to keep the light path/focal length as short as possible. We shall see.

 

I’d imagine that extremely small exit pupils wouldn’t be all that great due to the large central obstruction. So I’m thinking 0.75 to 1mm might be a good high power range. That ought to get me up to 265x, or a little more, depending on final focal length. Maybe my 8.8 in a 2x barlow at .69mm exit pupil for 290x. 

 

This is all speculative for now. Never used a reducer before. And not sure how well this will replace my 6 inch f/8 achromat as a manually mounted visual suburban/rural deep space and casual planetary option with much shorter eyepiece articulation. But I’ll never know until I try.


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#8 markb

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Posted 19 September 2021 - 01:41 PM

Echolight, look at the Baader Alan Gee TC, f5.9 spaced per Baader. 

 

There is a made to purpose Maxbright version, the regular version goes in the baffle secured by a T2 thread diagonal. I've made mine convertible but it takes a weird setup. Consider them not interchangable.

 

The AG + Maxbright BV on a C11 is just astonishing. Really.

 

BVs don't give low powers on SCTs normally, due to the 1.25 barrel limitation.

 

A true 2" diagonal with SCT body threads can work with adapters but the Baader T2 is the norm. With the long light path on a specially adapted AG I get about f5 on a C11. A 82 degree 30mm has no visual vignetting I've noticed.

 

It blew away the older 6.3 Celestron and Meade TCs. No comparison.

 

The C11 native at f10, the AG 2" C11 at f5 and my old Jaegers 6" achromat are a wonderful set of choices to use.


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#9 RAKing

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Posted 19 September 2021 - 01:44 PM

The XW40 will work fine on your C8 with the f/6.3 F/RC.  I also use the XW 40 without the F/RC, but I have noticed some "shading" at the edges, so I usually just stick with a 35mm Panoptic as my "wide field" eyepiece and I don't use the FR/C.  It has been almost twenty years since I had a Yard Cannon, but it would be fun to see how the views compare. cool.gif

 

If you run Ethos eyepieces in a regular (non-Edge or ACF) SCT, you might notice coma or light fall off at the extreme edges of the FOV.  The 31T5 Nagler had a lot of field curvature and I could not handle it, but I have used the shorter Naglers for years with no problems.  Nowadays, I prefer Delos eyepieces.  These eyepieces are more comfortable for me to use, excellent across the field, and I don't notice any FC or edge issues in my regular cats.

 

Cheers,

 

Ron


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#10 Echolight

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Posted 19 September 2021 - 02:45 PM

Echolight, look at the Baader Alan Gee TC, f5.9 spaced per Baader. 

 

There is a made to purpose Maxbright version, the regular version goes in the baffle secured by a T2 thread diagonal. I've made mine convertible but it takes a weird setup. Consider them not interchangable.

 

The AG + Maxbright BV on a C11 is just astonishing. Really.

 

BVs don't give low powers on SCTs normally, due to the 1.25 barrel limitation.

 

A true 2" diagonal with SCT body threads can work with adapters but the Baader T2 is the norm. With the long light path on a specially adapted AG I get about f5 on a C11. A 82 degree 30mm has no visual vignetting I've noticed.

 

It blew away the older 6.3 Celestron and Meade TCs. No comparison.

 

The C11 native at f10, the AG 2" C11 at f5 and my old Jaegers 6" achromat are a wonderful set of choices to use.

Thanks for the great info!

 

The 2 inch diagonal with SCT body threads and lock ring was the idea. I don’t have the eyepiece collection for bino viewers.

 

I casually investigated the Alan Gee. But figured, that since the one I was looking at fit inside the baffle, it would reduce the clear aperture. Maybe this is irrelevant.

 

Plus, although not an SCT, I want to try a reducer on my old F11 C90, to try and get around 2.6 degrees with a 24-68. In which case I’d probably use a 32mm T2 with SCT adapter. The helical focuser on the 1.25 inch eyepiece holder would come in handy for fine tuning, as the scopes helical focuser is a little fast and hard to get pinpoint. I tried a 40 Plossl, for a not bad 1.7 degrees, and there was minimal shading at the edge in the daytime.

 

But maybe also with a C5 eventually.... where I’d try a 2 inch eyepiece holder on the T2 along with a 28-68 or similar for around 2.4 degrees potentially.

I might end up with both styles of reducers.


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#11 Echolight

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Posted 19 September 2021 - 02:51 PM

The XW40 will work fine on your C8 with the f/6.3 F/RC.  I also use the XW 40 without the F/RC, but I have noticed some "shading" at the edges, so I usually just stick with a 35mm Panoptic as my "wide field" eyepiece and I don't use the FR/C.  It has been almost twenty years since I had a Yard Cannon, but it would be fun to see how the views compare. cool.gif

 

If you run Ethos eyepieces in a regular (non-Edge or ACF) SCT, you might notice coma or light fall off at the extreme edges of the FOV.  The 31T5 Nagler had a lot of field curvature and I could not handle it, but I have used the shorter Naglers for years with no problems.  Nowadays, I prefer Delos eyepieces.  These eyepieces are more comfortable for me to use, excellent across the field, and I don't notice any FC or edge issues in my regular cats.

 

Cheers,

 

Ron

This is good to know that there are several people who weren’t offended by the view with a widest field 2 inch eyepiece behind a reducer in a C8.

The common theme that is often proclaimed as gospel is that it can’t be done successfully. I always have to know for myself.


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#12 markb

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Posted 19 September 2021 - 04:20 PM

The c90 and C5 can only use the simple .63  reducers, the baffles are too small for the AG.

 

The baffle AG version is like the Edge HD, in that it puts the optics inside the baffle, and I believe that's what makes it so comfortable to the eye in use. The focal length of the TC is,  IIRC, longer than the .63 types, giving that much more freedom of locating the eyepiece. Baader has a range, and my full 2" setup is at the longer part of that range, but still produces great images.

 

The clear aperture is much greater than my M or C .63 TCs. Badder makes a big deal of their difficult to make super thin AG tubes. Vignetting? None visually noticable to me in a maximum true field of view 2" ep (30mm 82 degree). Even when tested on a light color cement wall.

 

I bought my .63 Meade and Celestron decades ago and never use them since I do find the vignetting awful in a wide 2" setup.

 

Only AG in baffle tube or the Maxbright + AG version have satisfied me, but my market sample has been tiny, only 1990s M and C versions. But I cannot see newer ones of similar size working any better, for me.

 

So, personally, I'd not go to the conventional .63 with a 2", for visual, unless there is an oversize one made specifically for the task.

 

The body SCT thread I mentioned was a diagonal with actual SCT threads cut into the main body for the normal smooth nosepiece. Using the ring style like you have will add at least 20mm of back spacing and may not work at all.

 

Getting a Baader T2 diagonal, whether the 'big' one or the 32mm prism 1.25, is the best way to do it. Even with a body threaded 2", I use a 3 part adapter stack. No recollection now but it wasn't cheap.

 

The T2 32mm will also allow use of the Maxbright on a C5, my orange tube has limited focus range, but there really not enough aperture for me, personally, to enjoy. But who knows if I change my mind over time. I could not reach focus without the T2 diagonal plus Maxbtright.

 

Going back to the core topic, the long SCT and Mak f ratios normally seen are pretty tolerant of almost any ep. I have ones that I like better than others, but everything does at least decently.  Not like those short dobs!


Edited by markb, 19 September 2021 - 04:26 PM.

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#13 Echolight

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Posted 19 September 2021 - 04:58 PM

The c90 and C5 can only use the simple .63  reducers, the baffles are too small for the AG.

 

The baffle AG version is like the Edge HD, in that it puts the optics inside the baffle, and I believe that's what makes it so comfortable to the eye in use. The focal length of the TC is,  IIRC, longer than the .63 types, giving that much more freedom of locating the eyepiece. Baader has a range, and my full 2" setup is at the longer part of that range, but still produces great images.

 

The clear aperture is much greater than my M or C .63 TCs. Badder makes a big deal of their difficult to make super thin AG tubes. Vignetting? None visually noticable to me in a maximum true field of view 2" ep (30mm 82 degree). Even when tested on a light color cement wall.

 

I bought my .63 Meade and Celestron decades ago and never use them since I do find the vignetting awful in a wide 2" setup.

 

Only AG in baffle tube or the Maxbright + AG version have satisfied me, but my market sample has been tiny, only 1990s M and C versions. But I cannot see newer ones of similar size working any better, for me.

 

So, personally, I'd not go to the conventional .63 with a 2", for visual, unless there is an oversize one made specifically for the task.

 

The body SCT thread I mentioned was a diagonal with actual SCT threads cut into the main body for the normal smooth nosepiece. Using the ring style like you have will add at least 20mm of back spacing and may not work at all.

 

Getting a Baader T2 diagonal, whether the 'big' one or the 32mm prism 1.25, is the best way to do it. Even with a body threaded 2", I use a 3 part adapter stack. No recollection now but it wasn't cheap.

 

The T2 32mm will also allow use of the Maxbright on a C5, my orange tube has limited focus range, but there really not enough aperture for me, personally, to enjoy. But who knows if I change my mind over time. I could not reach focus without the T2 diagonal plus Maxbtright.

 

Going back to the core topic, the long SCT and Mak f ratios normally seen are pretty tolerant of almost any ep. I have ones that I like better than others, but everything does at least decently.  Not like those short dobs!

Thanks for the very informative post for anyone looking to go outside of what many say are the design parameters of an SCT.

 

The Maxbright that I was referring to was the 2 inch Clicklock diagonal with SCT threads. Possibly incorrectly on my part. But I had seen it advertised as, Baader MaxBright 2" Clicklock Mirror Diagonal”.

1532A2A3-A15B-4B13-BD66-D93094DC9680.jpeg

 

The lock ring just enables the diagonal to be tightened at the correct orientation when used without the nosepiece and without a visual back. Therefore giving the shortest possible light path with a 2 inch diagonal, and keeping closest to the native focal length.

https://agenaastro.c...nal-to-sct.html

https://agenaastro.c...nal-to-sct.html

 

I’m always a little unsure when people refer to backspacing. Are you saying that using the ring and the SCT threaded body diagonal will put the eyepiece too close? And than using a visual back along with the refractor nosepiece is the way to go?
If that’s the case, then all I would need is the AG. Because I already have a 2 inch visual back and refractor style diagonal.

 

I’ll have to look on Baader’s site and see how the AG attaches. And proper backspace.


Edited by Echolight, 19 September 2021 - 05:25 PM.

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#14 sanbai

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Posted 19 September 2021 - 04:58 PM

Everything in my signature down to 5mm works in my C8edge. I don't miss anything longer than the 55mm/50° AFOV. Despite (or because of) the smaller exit pupil, the 31T5/ 82° covers 100% of unfiltered views and most of nebula-filtered ones for such TFVO. Limitation for eyepieces is on the sky conditions, not the scope.
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#15 25585

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Posted 19 September 2021 - 05:00 PM

This is an interesting counterpoint to the "SCTs are the Debil" thread still burning. 'Fracers trashing CATs as optically inferior, which is patently false, is the norm on CN. But now us CATers are stupid for buying nice optics?

 

I don't own any eyepieces that are "wasteful". I enjoy what I have, and I buy what I will enjoy. I'd like to see a post about why 'fracers have to continuously name-drop and buy for "glory" and not practicality. I'd also like to see a post dealing with the obvious Napoleonism Newt-ites and 'Fracers have against CATers. I am getting sick of continuously explaining myself to telescope snobs. Post something helpful, positive, or funny. There is enough scope-shaming already without your help.

 

Excuse me while I enjoy my rig and my kit, and the images taken from other CATs like the Hubble, JWST, anything on ISS, and the design of choice for professionals worldwide. Or, call Al Nagler. He uses his eps on an old C8 regularly. 

I own a classic orange C8 which came on a GEM. 

 

The C8 can use eyepieces with satisfactory views, but which perform poorly with my F5 & F6 Dobs. Had I only the C8, I would not have needed to buy more expensive eyepieces. That is what I meant by "wasted", quality not required because it's not needed as much. 


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#16 Echolight

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Posted 19 September 2021 - 05:22 PM

I own a classic orange C8 which came on a GEM. 

 

The C8 can use eyepieces with satisfactory views, but which perform poorly with my F5 & F6 Dobs. Had I only the C8, I would not have needed to buy more expensive eyepieces. That is what I meant by "wasted", quality not required because it's not needed as much. 

I guess I misunderstood.

 

I’ve seen lots of people say that eyepieces like the WO SWAN 40mm or Agena 38mm SWA work great at F/8 or F/10. But I saw a bit of astigmatism at F/8. And had already obtained an XW40 before I switched to 2 inch accessories with my C8. So I never tried the 40mm SWAN in the C8.

 

All of my eyepieces were bought with the idea that I might use them in faster scopes as well though. Maybe an F5 achromat or similarly fast newtonian. I figure some day I will have a slightly customized 10 inch f4 newt on a dob base...maybe., or maybe not.



#17 25585

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Posted 19 September 2021 - 05:24 PM

I changed the title and some text of my OP. No slight intended against Cassegrains at all.


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#18 markb

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Posted 19 September 2021 - 05:49 PM

Echolight, hardly anyone has one of those great full aperture diagonals,, so I was translating the description to the common 2" diagonal with a stub nose and a locking ring ( the ones that suddenly loosen and spin, dropping out loose eps).

 

Rather than  hijack the thread I'll follow with a PM.

 

Anyone wanting to continue this line, PM me so I'll make a fresh thread or just send the exchanges.

 

 

Back to the thread...

 

I agree with sanbai on the 31 Nagler, it is essentially the maximum true field available in 2" setups. Anything wider, has to be longer FL (commonly 40 to 50mm)  and much narrower fov (not to my liking either).

 

The UO (KK) 30mm Widescan, and most clones of it, hit about the same FOV but not as cleanly. The 30mm APM UFF is close and very sharp. And 1/2 or so the price for us bottom feeders.

 

As to the unexpected tiff, I had assumed what the OP 25585 has just posted in clarification, the question being whether SCTs are as picky as fast dobs on eyepieces...generally an SCT can use almost anything due to the long f ratio. I posted my favorites earlier. All but the UO WS also work well in my f5 6" Jaegers achro. The curved field kills the WS, but the UFF tames it nicely, giving an awesome RFT scope.


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#19 MarMax

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Posted 19 September 2021 - 06:40 PM

The other relatively unknown piece of kit that is very versatile and inexpensive is the older Celestron #93519 2" diagonal. The great part about these diagonals is the body is SCT threaded. So you can remove everything and adapt to just about anything. When you do this the light path through the diagonal is only 70mm. I have three of them.

 

I like to use it directly connected to an f/6.3 reducer but you need to shim it a bit so the reducer does not contact the diagonal mirror. This is the shortest, reasonably priced light path possible. You can usually pick them up for $60 shipped.


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#20 luxo II

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Posted 19 September 2021 - 08:01 PM

Cassegrain owners, what do you use? Is your scope guided or manual?  

 

I'm using 3 tiny custom-made huygens eyepieces with a 50mm MikroMak (f/12), they are quite nice to use, though the AFoV is about 35-40 degrees.

 

In my bigger maks (f/12 and f/15):

- For high power, MB supermonos 6, 7, 8 and 10mm;

- Midrange: Ultraflat 10,15, 24 and 30mm;

- Lowest power: SWA 38mm 70 degree.

 

Scopes are guided.


Edited by luxo II, 19 September 2021 - 08:18 PM.

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#21 Notdarkenough

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Posted 19 September 2021 - 08:03 PM

I use a SCT-threaded 2" Female Clicklock Clamp directly on the rear cell, and then insert the 2" nose of the Maxbright ClickLock Diagonal into that clamp. This is almost as short at the Locking Ring (which didn't quite fit with the Celestron Focus Motor), prevents diagonal/mount strike at zenith, allows for any diagonal orientation, and preserves the ability to thread a 2" filter to the front (nose) of the diagonal to make ep changes easier when using a filter. The difference is a couple of cm at most, and an extra $50 (Locking Ring was ~$50 and the Female Clamp was ~$100).


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#22 Jon Isaacs

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Posted 20 September 2021 - 04:00 AM

The XW40 will work fine on your C8 with the f/6.3 F/RC.  I also use the XW 40 without the F/RC, but I have noticed some "shading" at the edges, so I usually just stick with a 35mm Panoptic as my "wide field" eyepiece and I don't use the FR/C.

 

 

So, you are seeing "shading" at the edges without a reducer corrector, that's telling me that it's vignetting already.  With the F/6.3 reducer corrector, substantially more vignetting would be expected. How much do you see?

 

The 35mm Panoptic is a good fit for a C-8 since the 38 mm rear port nicely matches the 38.7 mm field stop of the 35mm Panoptic.

 

Jon


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#23 25585

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Posted 20 September 2021 - 06:23 AM

Using manual guiding I start with a Meade 40mm SWA 5000. It has a sharp field stop and is lighter(!) than my Axiom LX 31mm or Rodenstock 40mm. 

 

I agree the 35mm Panoptic is ideal (with any scope). A Meade 34mm SWA 5000 works well too.

 

My SCT was my 2nd reflector after an 8" F8 Newtonian I wish I had kept. Its one reason I have more 20mm> FL eyepieces than others. 



#24 cst4

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Posted 20 September 2021 - 09:42 AM

My C6 shows some pretty noticeable coma and field curvature.  And at F/10, eyepiece aberrations are less prevalent then the aberrations inherent to the scope.  So I find my preferred eyepieces are ones that mask the field curvature in the C6 the best.  These are not always the same EP's as what I prefer in my F/7 4" apo.  I've discovered I like to keep the apparent FOV below about 70 degrees so less of these aberrations are noticeable.

 

I use a 2" diagonal, no reducer.  My two favorite wide fields lately have been a 40mm TMB Paragon clone and 20mm Orion LHD.  The 40mm does vignette a decent bit but it's a subtle fade so it is not bothersome to me.  These two EP's appear flatter and sharper across the field to my eye than other wide field eyepieces in this range that I own.  My assumption is that they have some slight opposite field curvature cancelling that native to the scope.  My 12.5mm and 9mm Morpheus eyepieces work great in this scope, like every other scope I've put them in.  

 

Since I finally got some binoviewers I've been using my C6 quite a bit more, especially on the moon and planets.  Binos work great in this scope and I love using 2 eyes on these bright targets.  Absolutely wonderful views with my 25mm Celestron volcano top kellners, 18mm Russel Optics konigs, and 14mm Starbase orthos.  I've used the binos with my 4" refractor as well but the views just aren't as good.  They work much better in the C6 for some reason.


Edited by cst4, 20 September 2021 - 09:45 AM.

  • markb and 25585 like this


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