Does the standard (classic) SCT without a reducer/flattener, have an optimum back-focus distance? I've been seeing a lot of conflicting information on this. To my mind it would seem that there can only be one optimum primary to secondary distance that will minimize aberrations and field illumination.
Standard SCT back focus
Well Written Engaging
I think this is for regular, non edge SCTs https://www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts
As far as I know, the optimal backfocus is 105 mm from the rear cell 2" thread.
Bear in mind that SCTs were designed as visual telescopes and in the film era.
The correct spacing between the mirrors is achieved when using the 31.8 visual back, the standard prism diagonal and the standard 26mm eyepiece.
Using a camera, the spacing is correct when using the T-mount for direct focus photography (50 mm long) and the standard assembly T2 ring + camera (55 mm long).
So you have 105 mm.
Same if you attach to the rear cell the 0.63x reducer-corrector.
Hope this helps.
Ciao.
Lorenzo
Bear in mind that SCTs were designed as visual telescopes and in the film era.
The correct spacing between the mirrors is achieved when using the 31.8 visual back, the standard prism diagonal and the standard 26mm eyepiece.
Using a camera, the spacing is correct when using the T-mount for direct focus photography (50 mm long) and the standard assembly T2 ring + camera (55 mm long).
So you have 105 mm.
Same if you attach to the rear cell the 0.63x reducer-corrector.
Hope this helps.
Ciao.
Lorenzo
Helpful Insightful Respectful Concise
Himanshu Pandey:
I think this is for regular, non edge SCTs https://www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts
I saw that but you have to measure from the baffle tube lock ring. Is that even accessible?
Lorenzo Siciliano:
As far as I know, the optimal backfocus is 105 mm from the rear cell 2" thread.
Bear in mind that SCTs were designed as visual telescopes and in the film era.
The correct spacing between the mirrors is achieved when using the 31.8 visual back, the standard prism diagonal and the standard 26mm eyepiece.
Using a camera, the spacing is correct when using the T-mount for direct focus photography (50 mm long) and the standard assembly T2 ring + camera (55 mm long).
So you have 105 mm.
Same if you attach to the rear cell the 0.63x reducer-corrector.
Hope this helps.
Ciao.
Lorenzo
I think it will. I did a test image with a small 585 sensor with a back focus of only 50mm and the coma was very noticeable.
Tony Gondola:Himanshu Pandey:
I think this is for regular, non edge SCTs https://www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts
I saw that but you have to measure from the baffle tube lock ring. Is that even accessible?
I'm not sure what you mean but yes, its from the back of the SCT. I have an edge SCT that requires 133.5 or 105.5 depending for F10 or F7 with a reducer. The distances are different between the regular and edge but the measuring point is the same. I have not had a problem achieving either distance.