The truth about long f/l scopes?

[The Warthog]

I often hear about 'vignetting' and inability to do deep space stuff with Maks and SCTs, which typically have f-numbers of 10 - 13. The more I think about it, the more it sounds like rubbish to me.

Any f/10 scope has an angular opening, from any point on the objective, of about 5.7 degrees. If you have a 6" f/10 scope, and are using a 30mm Plossl, you will have a magnification of 50, and a FOV of about 1 degree. This is the same magnification and FOV you get with a 12", f/5 scope. Put in a widefield ep of about 40mm, and you will enjoy a magnification of 37.5, and a luxurious FOV of 1.75 degrees. You aren't going to bump into the sides of the OTA, which is three times that size.

Any restriction of FOV in a SCT or Mak, or my f/9.5 refractor, is a matter of focal length and magnification, not f/number. I have a smaller FOV in my 32mm ep in the frac than in the Newt because the frac has a 1000mm focal length, and the Newt a 750mm focal length. That's all. If I put my 32mm ep in an 8", f/5 scope, I will have exactly the same field of view as I have in my refractor. The View will be brighter, and have more resolution, but it will be in no wise restricted because of the high f/number.

You can, of course, get a focal reducer to shorten the effective f/l of the OTA, and give yourself a similar performance to a Newt with a catadioptric, and there's no reason you shouldn't. That won't cause you to bump into the sides of the OTA, either. As long as you aren't expecting a 6 degree FOV, you can do everything with a catadioptric or long tube refractor that you can with a Newt or short tube refractor.

There are lots of other good reasons for buying a Newt, including manageability and cost per inch, the short tube of the catadioptric is equally manageabe, so please stop telling us that we will somehow lose FOV in a catadioptic, OK? It just ain't so.

[GazOC]

Those examples may be fine if you stick to ~30mm Plossls but a lot of Cat designs will have vignetting when you get to the low power 2" eyepieces that have a large AFOV.

[The Warthog]

Well, in my second example, I was assuming a 40mm Erfle with a AFOV of 66 degrees. How low and how wide do you have to get?

Your daughters are very sweet in that pic, BTW. I'd post a picture of mine, but they'd get mad at me.

[GazOC]

From Mapug:

"Field Stop Illumination Limits Caused by the Telescope

The above analysis helps the user select an eyepiece that has a large enough field stop so as to attain an actual field of view that is desired. Another closely related issue is that of the ability of the telescope to fill the field stop with a real image of the sky that is both sharp over the field stop and also uniformly illuminated. For refractor telescopes this is usually not a problem since the objective lens throws a cone of light directly down the tube and is limited only by the size of the eyepiece holder and focuser mechanism. If the telescope will take a 2" eyepiece, the focusing mechanism will generally be open enough to accept the necessary cone of light.

The problem is not so simply with a more complex, folded optical system like the SCT. In the SCT, there is a secondary which projects a cone of light down a rather restrictive tube to the final opening in the back plate of the telescope. It is easily possible for this tube and the size of the opening in the back plate to limit the cone of light. With smaller SCTs it is almost certain that the cone of light will show some vignetting at diameters will under 2". Thus, with the larger diameter eyepieces, the field stop may not be fully illuminated. For example, a 2" 56 mm eyepiece has a field stop diameter of 49 mm. That is as large as a field stop can possible be in a 2" tube. Note however, that if the 2" eyepiece is connected to the back plate of a SCT with an adapter of the type that carries a Schmidt thread, the inside diameter of the opening in the adapter will be only 38 mm. It is clear that this type of adapter will vignette the cone of illumination at the edges of the field stop in a 2" eyepiece. In order to get full illumination with a field stop of 49 mm one should use an adapter with a full 2" diameter opening. This size is provided, for example, by the JMI focuser, which has a clear opening of 50.6 mm when it is used with the special adapter plate that they provide. I must recommend that when a 2" eyepiece is used, care be given to opening the connecting tube to the full 2" with appropriately large adapter tubes. JMI, Optec, and Lumicon provide adapters which take these matters into consideration in their designs. (Remember that the Schmidt Thread was designed for small SCTs and is quite marginally sized for 10" and larger telescopes.)

For a discussion of back plate opening sizes, see elsewhere on this website:Telescope Back Plate Aperture Sizes

This comment and warning is given because it is too bad to spend a large amount of money for a 2" eyepiece when the field stop cannot be fully illuminated by the telescope. The use of a field reducer is especially problematical since they compress the cone of illumination even more. The un-vignetted light cone for the 0.63 reducer is only about 26 mm. This circle of illumination is suitable for a 1-1/4 " eyepiece only. In such a case, the longer focal length eyepiece will decrease the magnification without increasing the actual field of view."

A link to more detail.

http://www.mapug-astronomy.net/ragreiner/arpert.html#Top

Bear in mind that the back thread on a 10" SCT is a lot wider than on the small Maks so the problem would be even more pronounced for those scopes.

[Macavity]

[Climbs on Hobby Horse] I think many people repeat "what they've heard" on forums - especially re. MAKs. As ever the "limited field of view", comes up, rather than the observation that these scopes merely have a long-ish focal lengths (FWIW, for a MAK 127, little more than yer average Donsonian!). But then many folks never experiment, and scopes are e.g. supplied with 1.25" diagonals by default?

To be fair though, understanding the vignetting of a catadiopteric ain't easy?!? Certainly VISUALLY it's possible to accommodate a 2" diagonal (and eyepieces) with a MAK 127 - With some (better baffled) SCTs, this seems NOT to be the case! But who knows? Each case seems to be different! As a (albeit lay!) MAK advocate, I've kind-of given up on saying "yes, but..." every time... :mrgreen:

P.S. I think the main (negative) MAK-thing is the "cooling time". But again, one has to think/experience this?

[The Warthog]

Ah HAH! It's the second tube that is the problem. How does this play out in your experience, however? I have seen M81 and 82 in the same field in a 8" SCT, with no problem getting everything in, and have ne'er heard a SCT owner shout "Damn this restricted FOV! I can't see it all!"

These are vital questions to me after all; I'm considering a 8" SCT for my retirement, and an 8" Newt is starting to make some good arguments.

[MartinB]

W/h with my 8" SCT I couldn't get a full FOV from the supplied 1.25 40mm plossl. A Moonfish UWA 30mm works brilliantly giving an respectable FOV and certainly taking in M81 and 82.

[GazOC]

Ah HAH! It's the second tube that is the problem. How does this play out in your experience, however? I have seen M81 and 82 in the same field in a 8" SCT, with no problem getting everything in, and have ne'er heard a SCT owner shout "Damn this restricted FOV! I can't see it all!"

These are vital questions to me after all; I'm considering a 8" SCT for my retirement, and an 8" Newt is starting to make some good arguments.

I can't really comment on SCTs users anecdotes you've claimed never to have heard.... but I use a 55mm Plossl on my 10" SCT and I don't get the full 'paper' FOV I should but its still good enough for me and can take in all the M42, M43, NCG 1976 area in nicely.

I'd guess the problem would be a lot more noticable in my Maks but TBH I've never tried anything wider than a 21mm Hyperion on either of them.

[The Warthog]

Well, thanks for that explanation.

[EA2007]

I think that someone is in somewhat of a grouch.

The FOV is wholly dependant on the eyepiece, type and manufacturer of a scope, even within the realm of maksutovs and schmidts then the focal length and ratio can change by quite a margin. I know for one that I have complained about this issue on imaging with a mak. HOWEVER it was made because I have somewhat of a bad mount and also a bad camera, both of which do not complement either the a) great optics small size or c) light-weight that maks/schmidts can offer.

I have no problem with either a schmidt of mak, providing the accompanying tools are upto the job. Its kinda like wanting to make a nice sandwich but the only bread you can get is five years old and lidl's own brand!

[Astroman]

Vignetting on my C8 is severe with an EP larger than 40mm fl, and a 2" diagonal is very restrictive. I've always believed it was due to the central tube, rather than EP limitations.

JMHO, a C8 is a wonderful retirement scope. Portable, versatile and generally great optically. I've seen all 110 Messier objects through one, split Antares with one, seen and estimated variable stars down to 14.3 mag with one, and imaged, (or, more often failed to image), a few GRB optical candidates with one. It's a great all around scope!

[Macavity]

Vignetting on my C8 is severe with an EP larger than 40mm fl, and a 2" diagonal is very restrictive. I've always believed it was due to the central tube, rather than EP limitations.

I have come to believe things like that - In the cases of budget MAK OTAs too. Though I suppose indirectly the "exit hole" (Fnar) of common belief. Ultimately baffling? [sheesh] Visually, there seem to be both hard and "soft" limits to such things. But always worthy of experiment, despite the theory...