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simon gale

2 inch visual back on SCT

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If I put a 2 inch visual back on a 6 inch SCT and use a 2 inch eyepiece, will I make full use of the e/piece, or will I lose some of the area around the edge of the field of view? Or is it better just to use a focal reducer with 1 1/4 inch e/pieces?

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It will depend on the field stop diameter of the eyepiece, if the field stop is larger than the aperture through the internal shade tube of the SCT then the field of the eyepiece will be vignetted.  :icon_biggrin:

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Thank you. I presume the internal shade tube will be 1 1/4 inches. And I assume those eyepieces which fit both sizes of drawtube will suffer similarly.

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With 1.25" eyepieces the max field stop is limited to around 27mm so as long as the internal baffle / light shade has a diameter of that or more, there should not be any vignetting. 2" eyepieces can have field stops up to 42mm or so.

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I was wondering same for my 9.25 a while back and found this on the Celestron site, the 'bottom line' is at the bottom !

(I'll quote a small bit of it here cos trying to find things again on their site can be ummm, frustrating )!

Quote ---->  By definition, a 2 in eyepiece is only 50 mm across at its base. In reality, field stops in these oculars are smaller, sometimes much smaller than the 2 in barrel diameter. A longer focal length, wide-angle design like Celestron’s 32 mm Axiom LX has a field lens 46 mm in diameter. A standard Plössl or Kellner 2 in design of the same focal length might have a 40 mm field lens. Shorter focal lengths of any design will have even smaller field lenses, sometimes as small as only a few millimeters.

Here are the approximate diameters of the back of the baffle tube for Celestron SCTs. Two Maks are also shown for comparison. 

OTA                            Baffle opening

C14 

54 mm

C11

54 mm

C925

46 mm

C8

37 mm

C6

27 mm

C5

27 mm

C127 Mak

27 mm

C90 Mak

15 mm

 C14s through C11s should have no problems with vignetting. Vignetting will probably occur for the widest angle, longest focal length 2 in oculars with the C8’s 37 mm opening. The smaller OTAs will have vignetting with all but the shortest focal length 2 in eyepieces.

Updated 12/27/13

Edited by SilverAstro

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It's not the field lens but the field stop that defines the apparent field of the eyepiece. The largest field stop that can be used in a 2" eyepiece is 46mm-47mm.

I'd be interested to see a ray trace of the various SCT optical designs to see where vignetting might occur. I've tended to assume that the internal light baffle and rear port diameter would be the limiting factor but that might not be the case :icon_scratch:

Another question that occurs is, if vignetting occurs, how does it manifest itself ?. Again I've tended to assume that the edges of the field of view will be dimmed / suffer from light loss but is that the way it happens or is there a slight dimming across the whole field :icon_scratch:

I'm not sure that I know the answers to the above. I thought I might but I'm questioning my understanding :undecided:

 

Edited by John

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I've used my 40mm Meade SWA 5000 with about a 46mm field stop in a 2" visual back on my 127mm Mak.  It sure is nice to get that wider field of view, but bright objects cast an odd, distorted oval reflection on the opposite side of the field of view when the object passes the 27mm field stop limit of the baffle.  It seems to be some sort of unsuppressed reflection because I'm using the scope beyond it's intended field stop.  Try as I might, I couldn't detect any obvious vignetting like I get with my 22mm clear aperture binoviewers when using 32mm plossls.  If you've got a 2" diagonal already, I say go for it for about $25 to $35.  It will allow you to use 2" only eyepieces with sub-27mm field stops like many 92 and 100 degree eyepieces.

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Hi there. There is a lot of info about visual back. I own a 9.25sct, i swapped out my VB for a 2inch WOptics SD. I use wide field eyepieces, and use both 2inch and 1.25. At the end of the day an SCT is a longer focal length , usually around F10. So I would say use a 2inch SD, and use both 2inch eyepieces, and 1.25 (with the adapter).

I recommend the 2inch VB, what you lose you gain (if that makes sense)

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Sorry that did answer your question really. I would say I use 2inch VB and compare to my friends 14inch f4 dob. Well it compares okay, but with the faster focal length of a dob the field is bound to be wider. I have been tempted with a focal reducer, but I don't see the point really (would like to hear if its a worthwhile purchase).

https://astronomy.tools/calculators/field_of_view/

Try this Simon?

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I believe that the focal reducers (the F/6.3 ones) are designed for use with 1.25" eyepieces and do vignette when used with 2" ones ?

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11 hours ago, SilverAstro said:

OTA                            Baffle opening

C14 

54 mm

C11

54 mm

C925

46 mm

C8

37 mm

C6

27 mm

C5

27 mm

C127 Mak

27 mm

C90 Mak

15 mm

According to this, there is some point in putting a 2" visual back on a C8, as you gain some field width, but not the full 2". But on a C6 it appears to be a pointless and costly exercise. 

I haven't bothered with my C8, as I have wider field telescopes that I could deploy (though it would be good to have the wider field available without erecting another instrument).

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Thank you all for some interesting points. It seems simplest to stick with 1 1/4  inch eyepieces and use a focal reducer.

 

Simon.

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On ‎8‎/‎26‎/‎2017 at 04:23, John said:

I believe that the focal reducers (the F/6.3 ones) are designed for use with 1.25" eyepieces and do vignette when used with 2" ones ?

From my own experience with a C6 and installed 6.3 reducer, 2" Baader visual back, 2" diagonal, with or without a 2" Barlow, and a DSLR at prime focus, I get no vignetting on my photos (crop sensor). With an extension tube and an inserted eyepiece for eyepiece projection photography, I don't get vignetting. With the 2" back, diagonal, Barlow and a 1.25 eyepiece in the diagonal instead of the camera, at 25mm (Plossl) there is no vignetting, nor at 15mm(Plossl).

Not sure on other combinations, those are the ones I've used. I have no 2" eyepieces (yet)

Edited by Luna-tic

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