Blackening lens edges

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DeanD Australia
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Re: Blackening lens edges

#21

Post by DeanD »


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
Telescopes: 12" f5 dob, Celestron CPC800, 150mmf5 Celestron achro, Tak TSA102, TV76, ETX125...
Binos: Steiner Wildlife XP 10x26, Swarovski 8x30 Habicht, Zeiss SFL 8x40, Vanguard Endeavour 10.5x45, Fuji FMTR-SX 10x50, Tak 22x60, Orion Resolux 15x70
Eyepieces: way too many (is that possible?), but I do like my TV 32mm plossl, 13mm Nagler T6, 27mm Panoptic and 3-6mm Nagler zoom, plus Fujiyama 18mm and 25mm orthos and Tak 7.5mm LE
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John Baars Netherlands
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Re: Blackening lens edges

#22

Post by John Baars »


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! :lol:
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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MistrBadgr United States of America
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Re: Blackening lens edges

#23

Post by MistrBadgr »


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.
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
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John Baars Netherlands
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Re: Blackening lens edges

#24

Post by John Baars »


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...
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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MistrBadgr United States of America
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Re: Blackening lens edges

#25

Post by MistrBadgr »


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
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
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