6 replies to this topic

[archer1960]

I have been searching for this info for a while, but can't seem to locate any reliable data. I just got a used Orion 180 Mak, and am working on setting it up for planetary and lunar photography and visual. What is the "nominal" back focus for this scope? I know that with the moving mirror focus, I have a huge range of usable back-focus, but where is it expected to be the best, with the fewest aberrations? The idea is that I want to get extension tubes to get the camera close to that point, to get the best images possible. Eventually, I'm likely to put a 3rd party focuser so I don't have to move the mirror; this scope doesn't have much image shift when reversing focus direction, but there is a little, and I'd like to eliminate that if possible.

[Astrojensen]

I don't think it's wrong to assume that the optimized position the makers have designed for, is for an 1.25" eyepiece in an 1.25" diagonal, so about 115mm/4.5" from the rear threads. 

 

 

Clear skies!

Thomas, Denmark

[luxo II]

The usual 120mm or so, it’s not critical.

[archer1960]

Thanks, guys. I knew it wasn't really critical, but figured if I have to get extension tubes anyway, I might as well try to optimize it at the same time.

[Eddgie]

Initially the Orion 180 was shipped with a different rear interface and was equipped with a 1.25" visual back and had a smaller baffle and secondary, which made the scope more optimized for high power (planetary) use.

 

You can see a review of the original version here:

 

https://www.scoperev...om/page1am.html

 

I believe that as they marketing people saw that most people wanted to use 2" diagonals for the wider fields, that they altered the design to use 2" diagonal, and the rear interface was changed and the baffle and secondary were made a bit larger. 

 

I don't remember if this scope came with a diagonal but if the scope was designed well, they designer would likely have specified a 1.25" prism diagonal due to the shorter light path than would be achievable with a mirror diagonal.

 

 

 

Anyway, if it is an early version it is probably optimized for about 96mm of back focus. If it is the later scope, probably about 125mm. 

Edited by Eddgie, 30 November 2023 - 10:18 PM.

[planedriver]

What you can also do is to set up your scope with the supplied visual back, diagonal and an eyepiece and then count the number of focuser turns from the end stop when in focus( I always go CCW ).
That number of turns corresponds to the design setup and most probably the design focal length as these scope's advertised focal lenght is achieved with a visual back and diagonal. Yes, there can be variations in the exact values as Eddgie pointed out.

(this is not only useful to keep the optimal mirror position but also to avoid aperture loss when adding too much into the imaging train and also to check if your GPC or Barlow gives you enough correction in this case)
Andras

[archer1960]

It is one of the newer ones, with a 2" connection on the back, so I guess the consensus is 115 - 125mm. Thanks!