Mini-Dob for the kids: finally ready!

[michael.h.f.wilkinson]

Finally I have finished my 4.5" F/4.3 Mini-Dob project for the kids. The result is a nice compact scope which the kids can set up in no time at all. The scope runs smoothly in azimuth, on a surface of ebony-star and a 10cm Teflon disk as a huge washer. A little drawer at the back of the mount houses the eyepieces, some Ahex-wrenches, the barlow, and a Cheshire EP.

The main problem was the secondary mirror. The scope which donated the optics was a Konus Newtonian (4.5" F=500). This came with a secondary of a mere 28mm. A little math shows that the minimum size (which has no vignetting in the centre of the FOV) is some 36mm. Years back I had modified the Konus scope with a 40mm secondary. This might sound big for a 4.5" scope, but this size only has an unvignetted field of view of some 5mm. I cut a piece of hardwood in the right shape, and attached the secondary with an elastic type of glue (shows my lack of experience at that time). Initially (as in a couple of years) this worked fine as a little grab and go wide field scope, but I did not use it often, so it started gathering dust.

Enter the mini-Dob project. I recycled the optics and built a new tube. Much to my chagrin, I could not get a good image, there was a clear sign of astigmatism, in which the star images are sharp horizontal or vertical lines, depending on focus, with a more-or-less round but fuzzy shape in between: a cylindrical error. I suspected that the wood had warped with time, and that the glue was no longer flexible. This puts tension on the secondary so it is no longer flat. I tested this by using the old, 28mm mirror (I never throw these things away). The result was a much sharper image, with just a bit of coma after rough and ready collimation, but not a trace of astigmatism.

So I ordered a GSO secondary, complete with mounting. This time it is a 50mm, yielding a 20mm diameter un-vignetted field of view. Because the mounting of the new mirror is longer, this involved several changes to the front of the scope, but that has now been done. The result works neatly, and we have had our first light for the boys on the moon. They were completely chuffed to see the craters on the moon at 25x.

I will make a solar filter at a later date.

[saac]

Well done Michael, looks really good. It's cool getting the kids involved - I love to hear them when they see something for the first time through the telescope - it hopefully stays with them forever. Clear skies, hope you get a lot of use out of it.

Ps - I like the eyepiece drawer, good idea; I might borrow it!

Jim

[swamp thing]

Blimey! You have been a busy bee Michael. What with the bin mount and the mini Dob, amazed you get time to do so much observing.

great job.

Regards Steve

[Gina]

That's lovely

[michael.h.f.wilkinson]

Thanks for the comments. I have been working on that little dob for quite a while. I started on December 16:

http://stargazerslounge.com/diy-astronomer/167347-new-project-mini-dob-kids.html

And my last progress report was here:

http://stargazerslounge.com/diy-astronomer/170530-4-5-minidob-project-some-progress.html

In the mean time I have not been able to post much on the progress, but the project kept me occupied when the weather was dud.