RC10 Backfocus has moved since mirror cleaning and collimation

Equipment Ritchey-Chrétien Collimation
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Tim Ray avatar
Tim Ray avatar
Hello eveyone,

I have an Orion Steal Tube RC10 F/8. Last weekend I removed and cleaned the primary mirror. I reinstalled and collimated the mirror and snuck in a quick imaging session on Wednesday.  I have modified the secondary mirror holder and it no longer has the ability to move towards the mirror to change the separation distance between primary and secondary mirror. 

Before the cleaning the subs were plate solving at 2035mm of FL. Now the subs are plate solving at 2060mm. The camera does focus further from the rear of the mirror cell. Currently the focus distance is 282mm. I know both of these measurements fall long as they relate to specifications of the OTA. Do I need the increase the separation distance between the two mirrors to decrease the focus point behind the cell and bring the overall focal length closer to the 2000mm spec? I have a good collimation, I would hate to redo it if I don't need to. Is the 2060mm focal length still within the "sweet spot" of these GSO RC optics?…

Thanks for everyone's commits in advance!

CS Tim
Helpful Engaging
andrea tasselli avatar
andrea tasselli avatar
If the original position (secondary - primary) was the one to minimize spherical aberration at a given BFL then any change in that distance will result in an increase of said aberration. Do you have a number for the original BFL? Without actual numbers (re: optical layout) is hard to come up with a meaningful number.
Concise
Tim Ray avatar
Tim Ray avatar
Andrea,   The spec back focus should be 233mm. It is currently 282mm for a 50mm delta… I was at 240mm before and thought I was close enough. Now with the gap being  50mm instead of 7mm I thought that might be to much.
andrea tasselli avatar
andrea tasselli avatar
Hi Tim,

This is what I get with your data before you changed the spacing:

Note that you were a tad slower than nominal @ f/8.14

Now the present situation:

Note the difference in the secondary conic constant b.2. Since this was fixed at the time of building (and figuring the mirrors) it cannot be modified and that change in spacing will result in some (extra) spherical aberration and coma. 

N.B.: it was my educated guess that the primary F/# is 2.
Insightful
andrea tasselli avatar
andrea tasselli avatar
After performing the calculation that amount of despacing will result in the diameter of the aberrated PSF (100% geometrical energy) to be equal to 10.4", e.g. about 7 pixels with your typical sensor. On Axis. Coma is probably worse.

P.S.: I stand corrected (large secondary, fast system, low sensitivity), coma is only about half as much at 30' from the optical axis.

P.S.S.: I corrected the diameter in pixels as I was given the radius instead (late night…)
Tim Ray avatar
Tim Ray avatar
Thanks for the datasheet and calculations. I guess I am moving the main mirror and collimating. Thanks again for the math.

CS Tim
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