How to adjust the focus on Celestron HD after changing the filter?

Equipment Reflectors Focuser
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Olaf Fritsche avatar
Olaf Fritsche avatar
Hello, 
for me the time has come to complement my refractor with a SCT with a longer focal length. I am thinking of a Celestron Edge HD1100. 

I want to operate a monochrome camera with different filters in an automatic filter wheel and I have a question: 
When focusing, I move the primary mirror. When the focus point is reached, the mirror is fixed to prevent displacement. With fixed mirror I may and can no longer focus. 
How does this work if I change the filter? With my refractor, this works automatically and the software readjusts the focus. With the Edge, do I have to manually loosen and retighten the locking screws? 

Thanks for all experiences, tips and advice!
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Bruce Donzanti avatar
Bruce Donzanti avatar
Hi Olaf

I have been using a C11" EdgeHD for many years.  There are multiple ways to do this:

1- There is no need to "lock" the primary mirror.  You can use a filter wheel or filter drawer and manually adjust for focus or use an autofocuser attached to the scope focus knob with software to do it for you. (note: the mirror is not really locked with the screws- they just stabilize it better from big shifts).  So, it can work essentially like the refractor.

2- You can "lock" the primary mirror and use a crayford  autofocuser design which works more like like focusing with a refractor.  

3- You can "lock" the primary mirror and use the Optec secondary mirror FastFocus system  to autofocus with its software.
 
Option 1 is the least expensive but you can get mirror shifting and flopping which can vary from scope to scope.

Option 2 adds cost and will take up significant backfocus but it is a great design and works very well.

Option 3 is what use now.  It also adds some cost but it takes up no backfocus as focusing is solely done by the small secondary mirror.  To me, this is the ideal way to focus with the larger SCTs.

And of course, as you know, it always helps to use parfocal filters or set filter offsets. 

Hope this helps.

Regards,
Bruce
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Olaf Fritsche avatar
Olaf Fritsche avatar
Hi, Bruce, 

This helps a lot! Thank you very much!
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