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Short optical path diagonal


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I'm new to astronomy and have a celestron c8. I fitted a feather touch focuser 2", followed by a televue diagonal. All works well until a fit a celestron reducer and then I run out of focus. If I fit the original celestron diagonal then I can just about reach focus. I've been looking for short optical path diagonals, but find it difficult to find their optical path length. The Williams Optics 2" quartz seems good and has a short optical path. But the Revelation quartz is apparently better value, but I cannot find its optical path length . Does anyone have one and can tell me. Or is there a better solution to my problem.

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Hi Chris

I use a C8 and I have the Revelation 2" SCT quartz diagonal. I do not know the optical path, yet unlike the televue diagonal it does not require the addition of a visual back, as it connects directly to the optical drawtube, so therefore is quite short.

I replaced the stock focuser with a feather touch micro focuser, rather than the 2" focuser that you have, that attaches to the diagonal. In this circumstance, when I use the celeston reducer, I have experienced no problems in achieving focus.

I expect that others will comment more specifically to your focuser type.

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Hi all

Thanks Iain that's useful information.

Yes James it is for visual use as for photography I mount the camera on the focuser and don't use the diagonal.

One thing I forgot to mention is that my current Televue diagonal is 1 1/4" with a 2" - 1 1/4" adapter. All my eyepieces can be 2" or 1 1/4", so I thought it would be best to get a 2" diagonal for the better FOV.

Chris

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Hi

Thanks, I did have a look this but was undecided. It doesn't give the same FOV as a true 2" and I couldn't find a UK supplier of the necessary adapters . I will have another look and add it to my short list, no pun intended.

Chris

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If you are using 2" diagonal, I assume you will be getting 2" eyepiece. In which case don't use the reducer because it is designed for imaging and 1.25" eyepiece. It will vignette with 2" wide field eyepieces.

A 40mm Skywatcher Aero ED will show as much as a 2" barrel will show. Adding a reducer will not show more sky, just lowers the magnification and reduce the AFOV

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The reducer does NOT increase the aperture. It makes the light cone steeper (f-ratio, important for photography), and flattens the field. As it has been mentioned, it is designed for imaging, not visual use.

The maximum field is defined by the field stop of the inner parts of the telescope. I think it is around 46 mm for a C8, but it will probably vignette significantly towards the last mm at the edges.

The reducer actually limits the field that can be seen. Do not use the reducer for visual use.

A 2" diagonal will by the big mirror and big connectors always have a very long optical path. Very long optical paths can cause loss of aperture on Cassegrains, though this is mostly an issue with binoviewers. However a feather touch focuser (though a nice piece of equipment) and a 2" diagonal may actually start to cause loss of aparture. For a C8 loss of aperture will probably start at around 150 mm optical path. Give or take 10-20 mm.

The recommended Baader T2 diagonal is also what I recommend for a diagonal with shorter optical path. It has a 35 mm field stop, though you can probably go to 36-37 mm field stop on your eyepieces without noticing. A 1 1/4" diagonal will have a field stop of around 27 mm. A 2" around 46 mm.

You cannot "adapt" a 2" eyepiece to a 1 1/4" without making the field of view narrower. This assumes that the eyepiece has a field stop over 27 mm though, but almost all 2" eyepieces has this, as that is the main reason for making them 2". Some Nagler and Ethoses are exempt as they are physically large and needs the support that 2" gives.

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If you really wants to cut down the optical path, use a Baader T2 prism instead of T2 mirror

The optical path lengths of various Baader diagonals are as follow

T2 prism - 38mm

T2 Zeiss prism - 41mm

T2 Amici prism - 48mm

T2 Maxbright Mirror - 54mm

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The F6.3 celestron Corrector / reducer can be usful for visual under certain circumstances. I find mine most useful when viewing some larger globular and open clusters - at a dark location and at high power using 1.25" ep's. The flatter wider field can enhance the view a little. I think that this is probably the only occasion I use it though.

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Hi All

I now have a good understanding of my problem, thanks to everyone's response. I now have to decide on the best way forward for me. As a newbie to astronomy and this forum I very much appreciate all your help and advice.

Thanks to everyone

Chris

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