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Thread: Orion 180mm Mak-Cas FOV

  1. #1
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    Default Orion 180mm Mak-Cas FOV



    Hi everyone,

    I'm considering to put my hands on an Orion 180mm Maksutov-Cassegrain telescope for mainly visual observation. I read many excellent reviews of this scope and it seems that it is a fine instrument. I think I am aware of the pros and cons of a Mak-Cas system, but I have doubts about one thing, and it seems that there is also quite a bit of confusion on the various forums about it, which is the following.

    What is the largest useful true FOV that one can reach with this scope? Older versions came with a 1.25" back, which clearly limited the use of eyepieces with long focal length and large apparent FOV because of vignetting. Newer versions sold today have a 2" back, apparently. Now, I suppose that these have a matching larger exit aperture stop, otherwise I think the 2" back would not really make sense.

    If I use an eyepiece such as the Orion Q70 SWA 2" 38mm (or something similar) with this scope, the math says I could obtain a TFOV of almost 1 degree. This would be a decent TFOV, more than enough for all DSOs that I would be interested in observing with this scope.
    My question is: will the image from the above setting be free of noticeable vignetting?


    Cheers,
    stewe

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    Default Re: Orion 180mm Mak-Cas FOV

    Hello stewe, I have not looked through one of these but from what I have been told the new model has a 2" back on it but the baffle tube size is still the same as the one with the 1.25" back.
    The size of the baffle will be your restriction to eyepiece size/ max FOV.
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    Default Re: Orion 180mm Mak-Cas FOV

    I too have heard very good things about that particular scope. I read a comment by one person on another site that said he could not go any wider than a 35mm Panoptic (68° AFOV) in his without seeing some vignetting. That EP yields a TFOV of 0.9°, so you are close with the Q70 38mm. Of course, I cannot validate what that person stated, but see no reason for him to be lying about it. Hope that helps.
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    Default Re: Orion 180mm Mak-Cas FOV

    I just checked, and the baffle opening on my SW 180 is ~31.25mm
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  5. The Following 2 Users Say Thank You to SpyderwerX For This Useful Post:

    Gabby76 (01-20-2019),stewe (01-20-2019)

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    Default Re: Orion 180mm Mak-Cas FOV

    Thanks SpyderwerX, that is useful.
    Can I ask what is lowest mag / largest FOV you use with your MAK and if you notice any vignetting?

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    Default Re: Orion 180mm Mak-Cas FOV

    Quote Originally Posted by stewe View Post
    Thanks SpyderwerX, that is useful.
    Can I ask what is lowest mag / largest FOV you use with your MAK and if you notice any vignetting?
    This scope always stays set up with a T2 size diagonal with 1.25 eyepiece collet. The widest I typically use is a 24mm/65° widefield, which only gives a FOV of 0.58° in this scope. So in that setup I've naturally noticed no vignetting. I haven't experimented with lower power 2" EPs as I would turn to another scope for a wider view.
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    Default Re: Orion 180mm Mak-Cas FOV

    A quick update.
    After inquiring at various vendors, I finally got a reply from astroshop.eu (on a Sunday -- seems they have a 24/7 customer support).
    They told me that the current versions of the SW and Orion MCTs are identical, they come with a standard SCT thread with 2" visual backs, and they both have a baffle tube opening of 30mm. That is very similar to what SpyderwerX wrote.

    Therefore, the maximum fully illuminated true field of view is ~0.7 degrees, and can be reached with eyepieces such as:
    40mm Plossl (43 deg AFOV)
    27mm Erfle (68 deg AFOV)
    22mm Nagler (82 deg AFOV)

    These will all have the same true FOV, but will obviously look very different.

    I guess one could push it larger, perhaps to 0.8-0.9 degrees without vignetting to be noticeable.
    Any thoughts about that?

    Now what I am not sure about is whether using a 2" diagonal (and perhaps a Crayford focuser) would make any difference in terms of field illumination, since these would require a long backfocus.
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    Default Re: Orion 180mm Mak-Cas FOV

    Indeed, Sky Watcher, Orion and even Celestron Maks are all produced by Suzhou Synta Optical Technology in China. So it really comes down to availability, accessories, or even paint color which guides the decision of brand purchase. I like the black/white color scheme of the SW units.

    I employ one of these on the rear of my 180 SW: https://www.firstlightoptics.com/sky...elescopes.html The stock focuser is serviceable, but not a super precision unit. The Crayford does add just a bit to the length of the light path, hence probably a very slight reduction in total FOV.

    Additionally, the full size 2" diagonal will add a slight bit more to the focal length. I opted for a T2 size diagonal as a match for this scope (which I can also use in any other scope I own via a 2" nose piece), as the full aperture of the T2 pretty much matches the baffle tube diameter, and it's a much shorter light path than the 2" version. Of course all of this possibly adds a bit more total cost in the end, but it's a lifetime investment.
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