Precise Daytime Collimation of a Makutov-Cassegrain (projection method)

[alex_stars]

Greetings fellow stargazers,

At the moment I am tinkering a lot with my Mak (OMC-140) and therefore I am constantly re-collimating the Mak. Yesterday I worked on the real star again, but today I would like to introduce you to a possibly "new" method. At least I haven't seen it anywhere in this form. It is based on two well-known daytime methods and combines them:

• The "daytime method" described by Robert Casady
• The "kitchen table method" described by Russ Slater

Both methods should also work for all Cassegrain type scopes. I only have a Mak so I can only confirm that it works very well that. Now what do you need to try this method:

1. checked A4 sheet or graph paper (if you want to be very precise)
2. a piece of cardboard, similar size than the A4
3. a compass to draw circles
4. pen
5. double sided tape
6. a mobile phone with flashlight (LED next to the rear camera)
7. photo tripod
8. your telescope and the wrenches to collimate
9. a table
10. your eyes (and optionally a camera or a Cheshire eyepiece [can also be homemade]

And now it's time to tinker

I) The collimation target:

To make one, you first draw a crosshair in the middle of the A4 sheet, whereby the lines should be at least as long as your telescope aperture.

Then you take the compass and draw concentric circles from the centre of the crosshair. In my case the telescope has an aperture of 140mm and a central obstruction of 44mm, so I make the following circles with these radii: 25mm, 35mm, 50mm, 63mm, 70mm and 90mm. One circle should be slightly larger than the telescope's obstruction (that's 25mm for me) and one slightly smaller than the aperture (that would be 63mm for me).

Now the A4 sheet is glued to the cardboard and the central hole from the compass puncture is enlarged with a needle to about 1 mm in diameter and extended through the cardboard.

At the back of the cardboard you tape the mobile phone with the LED switched on behind the hole in the target so that you have a collimation light source. The collimation target in the end looks like this:

The target is now clamped on the photo tripod and is ready to go.

II) Alignment of the telescope:

Put the telescope on the table with the dovetail bar down and place the collimation target in front of it. I start with the front edge of the telescope about 50 cm away from the collimation target.

You have to align the telescope and the collimation target so that the optical axes are the same. To do this, you first set the height of the tripod such that the innermost bright circle of light that is reflected by the telescope is exactly at the level of a drawn circle and the horizontal crosshair line. For horizontal adjustments you can move the tripod or the telescope.

I align my scope and target by centreing the smallest shadow ring and the following bright (is the brightest) ring so that it fits exactly into my R=25 mm circle (see picture).

III) Collimation

Now you can go ahead and work with the collimation screws until all projected circles (dark and light) are as concentric as possible. The picture above shows a good collimation for me.

Important! If you turn a collimation screw, the inner brightest ring moves out of the circle on the collimation target. After each movement of a collimation screw you have to (!) re-center the brightest ring on the collimation target to align the optical axis.

You can do this either on the tripod (especially if it has moved vertically) and also horizontally on the telescope itself by turning it slightly on the table. The checkered paper or graph paper helps with the exact adjustment. You will be surprised how accurate and sensitive you can adjust with this method.

IV) Final Collimation check

 After you are done with the projected circles, you should look through the eyepiece clamp or use a camera (or Cheshire eyepiece) to check the adjustment from eyepiece clamp side of the scope.

In the middle is a huge Poisson spot (rather a disk, right?) which helps you to find the optical axis here as well. If you are satisfied here too, the telescope is officially collimated.

You can of course fine-tune the collimation on a real (or artificial) star. Colleagues who do a lot of photography in particular will agree. But I have to say that for me, who only works visually, this collimation method is completely sufficient. I hardly ever see a reason to improve on a star when I check.

Have fun trying this method and post a comment with your experiences. That would be great!

CS Alex

Additional info:

This method assumes that the secondary mirror is centred in the tube and also centred with respect to the centre of the eyepiece clamp. At a commercially available Makustov you can't adjust the secondary mirror position anyway. If a de-focused star does not have its Poisson spot centred when the de-focused star is centred in the eyepiece, then it is likely that the secondary mirror needs to be re-centred. Either complain to the manufacturer (if you just bought it) or try it yourself, but that's another story. I've never had to do this.

It is also assumed that the baffle tube and the eyepiece clamp are in the middle of the tube and are not tilted in relation to the optical axis. These types of adjustments cannot be made on a commercially available Mak either, so the same applies as above for the secondary mirror.

[alex_stars]

you can also find my German version here

https://forum.astronomie.de/threads/praezise-kollimation-eines-maksutov-cassegrain-bei-tag-projektionsmethode.339785/