TheSkySearchers

Sounds great John, and I concur with your comments about how good these scopes are, especially with low-medium wide-fields.

Just for interest, how did you determine a 0.02mm extra spacer would improve the coma? Was this trail and error, or was one of the existing spacers thinner than the others?

I am a bit confused too, as I thought refractors were not susceptible to coma. Maybe the spacers, being a bit pinched, were causing some slight mis-alignment, hence the apparent defects at 270 degrees in your image? Is that what you mean by "axial coma"?

All the best, and happy viewing.

Dean

DeanD wrote: ↑Fri Nov 06, 2020 12:54 am Sounds great John, and I concur with your comments about how good these scopes are, especially with low-medium wide-fields.

Just for interest, how did you determine a 0.02mm extra spacer would improve the coma? Was this trail and error, or was one of the existing spacers thinner than the others?

I am a bit confused too, as I thought refractors were not susceptible to coma. Maybe the spacers, being a bit pinched, were causing some slight mis-alignment, hence the apparent defects at 270 degrees in your image? Is that what you mean by "axial coma"?

All the best, and happy viewing.

Dean

The amount of spacing was trial and error indeed.
I established the location where the coma- center was located related to the coma-disc of the centered star, marked it on the outside cell and added some material to the nearest spacer in the lens. According to the findings of Wolfgang Rohr.
Misalignment ( and maybe a tad of pinching) must have been adding to the problem too, since there was even less coma after polishing the inside cell and remounting the lens with the utmost care.
You should read "axial coma" as coma occurring on the optical axis.

My adventures with this telescope and reducing the coma on the optical axis are described here:
viewtopic.php?f=61&t=13533&p=114261#p114261

As a result coma was reduced, but another problem, which I wasn't aware of that clearly because it was shielded by the coma, occurred. That was spherical aberration of at least 0,25 Wave or more. So, the original estimated Strehl of 0.8 was even lower!

I work almost exclusively with beginner scopes. I fix up 60mm refractors and give them to promising children. Blackening the edge of the lenses is the first thing I do. For entry level scopes with a simple AR coating, changing the circumference from white to black changes the whole character of the scope. I do a number of other things to the scopes, but that is always the first.

MistrBadgr wrote: ↑Wed Nov 25, 2020 4:19 am I work almost exclusively with beginner scopes. I fix up 60mm refractors and give them to promising children. Blackening the edge of the lenses is the first thing I do. For entry level scopes with a simple AR coating, changing the circumference from white to black changes the whole character of the scope. I do a number of other things to the scopes, but that is always the first.

Great! I suppose that is quite necessary.
Can you make a topic of it in the telescopes-forum? Could be of use for some...

Hi John,

I will see what I can do about that. I need to think about it a little to make sure what I say does not either confuse people or make it sound too hard. It does require some patience.

Bill Steen