Coma Correctors and Focal Reducers

[Rocket_Guy]

So this is a question for all you optics gurus out there.

I have an f/4.9, 8" Newtonian reflector, a Baader Multipurpose Coma Corrector (MPCC), an Apertura 2", 0.5X Focal Reducer, a ZWO EFW, and a ZWO ASI1600MM camera. My question pertains to whether there any way to make these different optical components play nice together through the use of spacers, etc.

There are reducer corrector combos out there for sale so I know it must be possible to design optical trains which operate correctly. It would be nice to know if there some theory or rule of thumb out there to use I could use for guidance in getting things set up properly with my equipment. I have done some searching on the web, but no one seems to be able to give a definitive answer on the possibilities. Anybody have any knowledge which would be helpful?

Thanks everybody, Bill

[Jean-Francois]

Hello Bill,

If you calculate backwards ... detector = 17.7 mm x 13.4 mm, reducer will take a large image and reduce on the detector.
Means you need an image of 35.4 mm x 26.8 mm before the reducer, also at the output of the MPCC.
What is the quality of the stack ? ... is the MPCC with your telescope so good in the corner (full frame size) ?

Now with the assumption that the MPCC produces a perfect plane image, what is with the 0.5x reducer ?
If the reducer is calculated as reducer and flattener for a telescope lens, then it will not work together.
If it is calculated for a Newton telescope ... the same it will not work (because it corrects the coma, that is already corrected by the MPCC).

For the distances .. MPCC to reducer and reducer to detector ... that is not easy to find the best distances.

With only one component MPCC + camera, then you have only one parameter ... you can change the distance with a variable extension tube.
Take pictures at different distance and compare.
Now with two components MPCC + reducer + camera, then you do not double ... you square the possibilities.
If you try at 4 distances, with only the MPCC you will have 4 images, with MPCC + reducer you will have 16 images.
That will take much more time to set.

Regards,
Jean-Francois

[Rocket_Guy]

Hello Jean-Francois,

Thanks for your response. You have hit on the essence of the problem. Using the MPCC alone gives great images with no coma out to the corners of the image at least with my ASI1600. The reducer gives me the wider field that I like, but at the expense of having a good deal of coma at the corners. My hope was that I could put the two optical pieces together and get a wide field with little coma.

The MPCC requires a certain (known) spacing to eliminate the coma which I set when I first got the piece, however, I feel certain that the reducer will screw up that spacing. I was, therefore, hoping to find some rational for determining the spacing needed between all the pieces. The amount of reduction also depends upon the spacing. Trial and error will work ... maybe ... eventually, but I was hoping to avoid doing that if possible. I am also not sure what should come first, the reducer or MPCC. I can make a rationale either way.

I haven't really attempted doing anything yet before I had a plan of action and a little more knowledge.

[admin]

Hi,

my guess would be (and this is just an educated guess) that you may need to reduce the gap between the MPCC and the sensor if you introduce the focal reducer. That's because the focal reducer will make the light converge more sharply than it normally would at f/4.9, so everything in the optical path beyond that point is likely to happen in less distance.

Whether you are going to get much value out of it is a different question entirely. I suspect that the MPCC was designed to work nicely at about f/5 to f/6 or thereabouts. Quite what will happen when you use it in a system that is f/2.45 is unknown!

cheers,

Robin

[Rocket_Guy]

The MPCC specs say it is good down to f/3.5 so I'm wondering if I adjust the spacing on the reducer so that it gives me a 0.7X reduction whether that would be the thing to do. A 0.7X makes the scope approximately f/3.5. Just wondering. Thanks.

Best, Bill

[Jean-Francois]

Hello Bill,

Like Robin said ... first MPCC and after the reducer ... you will have an image, but it is not sure that you will like it.

If you have all the details of the optics (radius, thickness, glass) of both, then I can calculate something with Zemax.
If not ... then you need to start some tests.
For simplifying the tests ... you can do it with an artificial star and the telescope on the mount.
With the mount you can move quickly the artificial star over the detector (rotate the camera until the star moves in the corner).
Set some distances, set the focus for the center and then observe the star in the corner.

For the FD number and the "problems" ... it is not the same if you change from 5.0 to 4.0 or from 3.5 to 2.5.
A difference from "1" at FD 10 has no effect, but it is a different story if you are in the 3 or 2 FD numbers.

The other point to have in mind ... you have not only a correction of the coma. You can find an optimum correction of the coma between
both optics, but at the same time, what is the correction of the spherical aberration and the field curvature ??

Good luck
Jean-Francois

[Rocket_Guy]

Ok. Thanks all. I guess I have a bit of testing to do.

Best, Bill