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Hi,
I want to place a CCD camera where the secondary mirror is, remove the sec. mirror completely.  It,s a home made F5 14 inch. Do I have to use a coma corrector or/and a field flattener? Can I use a Barlow with a field flattener or coma corrector, can I use eye piece projection with a field flattener or coma corrector?
Many thanks,
Markus

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Hello Markus, welcome to this forum.

What you ask certainly can be done, I got close to finishing a similar project using an 18" F3.6 mirror. However, during a period of doing other things I read enough negative comments about this procedure that the project is now on hold pending a rethink. The main problems stated were the mounting of the camera, the effects of cabling, potential difficulties with focusing and heat from the camera. I was proposing to use an integrating video camera for near real time imaging to a monitor, it has a very small footprint and small CCD chip so off axis image distortion would have been minimal. The main advantage of the system seems to be not having to invest in a secodary mirror.   :icon_biggrin:

 

 

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Hi Peter,

the footprint of my CCD is smaller then my secondary, the cabling I would have tried to integrate in the spider vanes, basically constructing new cabling unless the cabling would be out of focus if exiting the telescope a distance ahead of the spider. The CCD is cooled to -40 degrees but I do not know what would happen outside the camera regarding temperature effects, this might be a project killer. My main question is regarding the field flattener/corrector setup, if it would be needed. I have seen field flattener/correctors that would do the job online. In regards to mounting the CCD and Barlow/eye piece projectin train I am not too worried, I have build very stable spiders in the past.

regs,
Markus

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Very interesting project.... How would you focus the camera?  I guess that the faststar system in SCTs uses a camera in front of the OTA and the cabling/thermals don't seem to present a problem there so there is no real reason why you can't do this with a newt?  I wonder if there is a way of using the spider vanes as a heatsink for the camera?!

Edited by CraigT82

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Here are a few thoughts:

1) Construct/modify some ribbon cables if you cant route them via the vanes.

2) You will need to figure in a coma corrector somewhere

3) You will need some sort of fine focus option, most likely a non rotating helical focuser - to which you would bolt your spider vanes (it replaces your secondary holder)

4) The camera position will be further forward than you might first think - it will be the same distance as if it were from the secondary mirror to the focal plane.

5) Your spider vanes must be very strong (no twist or flex).. remember, you will be asking your vanes to support anywhere up to 1kg in the form of your imaging train (including corrector, helical, and camera)

6) Figure out a way to get filters in there

 

Youre not the first to have considered this, but its a case of having the time to get it all set up with no certainty that it will work.

 

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I looked at this idea some while back. For me the difficulty of collimation and focusing made me stick with a secondary mirror. It can and has been done before.

Regards Andrew

Edited by andrew s

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Hi guys,

Thank you all for your worthwhile contributions. I liked the idea with the ribbon cable and using the spider as a heat sink. In the past I discovered some ultra thin very rigid metals and as long as the spider vanes are straight, it can have the depth to support the weight. I was thinking of an electronic solution in regards to the focusing, perhaps a conversion of an existing electronic non slip focuser. One question, is there any difference in the optical setup between a setup with secondary mirror and one without?
Normally no corrector is needed, when attaching a CCD with an eyepiece to a focuser in a standard Newton telescope with a secondary mirror, why can I not use the same setup but skip the secondary flat mirror. I am catching the focused beam of the primary just in the same way and at the same point, only the focused beam does not get folded 90 degrees. I would understand if one would say "the use of a correcter or flattener enhances both setups", but why does it always come into play with the one mirror setup?

Thank you,
Markus

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