Scope making

[Simon67]

Would it be possible to have a 350mm primary mirror f4.7 which has a hard to handle focal length . 1/ instead of putting a secondary mirror and make something to place ccd in its place that you could focus.. 2/ same primary mirror but have a second parabolic say 150mm to send light down the ota to a flat mirror just in front of primary to bend the light at a right angle to a focused. I would appreciate any help

[ronin]

First one should be easy, the tube/truss's would be the focal length plua a bit, so a bit longer. Aligning would have to be performed via the camera system and a good finder of one form or another as you could not use an eyepiece. You may need to place a flatener or coma corrector in front of the camera for best results. Actually for imagiung I have been a bit surprised that this design is not more common. Suppose that people just expect an eyepiece and so manufacturers make allowance for one.

Go build a 150mm f/6 version. If truss tubes these may need to be a bit bigger in diameter

Second is going to be possible but not easy, not sure why a second parabolic but I suppose the SCT design uses similar - Mak's are all spherical if I recall. Sure I have seen the design implimented somewhere but it seemed to just not work well. The secondary would be bigger then usual so you lose light and add diffration as the obstruction is bigger. You end up with a "softer" image. Search telescope designs and you will find one somewhere.

Most of the professional scopes will be the first design as they do not look through a scope these days to collect data. So easy to place the sensor at the focal plane on a substantial spider. So in that espect they are likely much more common then people realise. All it is is the same arrangement as a refractor with a camera. Just you are swapping the lens for a mirror. Again go search for images of observatories as if there is a reflector with big truss tubes the odds are that at the top is camera and not a secondary in sight to divert the light.

[furrysocks2]

6 hours ago, Simon67 said:

... instead of putting a secondary mirror and make something to place ccd in its place that you could focus..

I'm no expert, but re: camera at prime focus...

 

There has been a build thread on CN over the last year... https://www.cloudynights.com/topic/560250-purpose-built-eaa-telescope/

I tried to start a build, but it's wood and steel (heavy), and far from finished... https://stargazerslounge.com/topic/283277-prime-focus-camera-newt/

Might not help a build with a 14" mirror which I assume you already have and want to build with, but worth knowing about: https://starizona.com/acb/hyperstar/whatis.aspx

 

A couple of things to watch out for perhaps:

• size/shape of camera and top-end parts (obstruction)
• cable routing
• how substantial does the camera support/spider need to be
• all the above can affect diffraction
• possibly also the heat generated by a camera being directly in front of the mirror, which is not present when a secondary is used.

Then there's the ability to focus, collimate/align, and repeat these operations within some tolerance. Also consider designing your top end components early for weight/balance, or make sure you design in the ability to re-balance.

 

These aren't the only links to folk with similar ideas, but it's an appealing idea. Personally, unless you're already a mirror wizard with access to the right test kit, I'd avoid complex optical arrangements - additional mirrors are only going to reduce the amount of light collected due to greater obstruction and <100% reflectivity, and introducie compound errors. I guess that's one of the attractions of a single mirror with a camera at prime focus.

[Peter Drew]

The first system, as mentioned, is a relatively straightforward project, favoured by EAA specialists who normally use small dimension and lightweight cameras.

For the second possibility, I dont think double parabolas would be the ideal optical solution, twin mirror systems other than Newtonians tend to have complex optics. One of the main problems is getting the focal plane down to the lower end of the tube. This usually depends on the use of an amplifying component as in a Cassegrain, this would increase the effective focal ratio beyond the desirable. To achieve this without amplification would require a circular flat mirror but this would have to be relatively huge causing other problems.