John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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I just made an appature mask.Here are some photos of what I made can anyone see a problem with it?
I made one because I want to narrow down the difraction problems and help with the star test results that i'm seeing.And something to do on a cold winter day 
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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pic2
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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pic3
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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pic4
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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stevew
Now I've done it
Reged: 03/03/06
Posts: 3487
Loc: British Columbia Canada
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Hi John. Nice job on the mask. The pictures don't show your center dot as being centered, I am sure it's just the way the picture was taken.
Does your mirror have a turned edge? Thats the only reason I can think of to stop down the edge of your mirror.
If it does, that should clean it up quite a bit.
How much of the edge did you mask off? 1/4 inch seems to be enough to solve a turned edge problem.
Steve
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More telescopes than I deserve....
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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Yes the dot is centered it is just the angle of the mirror.I did about 1/4" but it may not be a perfectly centerd over the mirror.Will this be a problem?It is very close.
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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Jason D
Postmaster
Reged: 10/21/06
Posts: 6172
Loc: California
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Quote:
Yes the dot is centered it is just the angle of the mirror.I did about 1/4" but it may not be a perfectly centerd over the mirror.Will this be a problem?It is very close.
John, as Steve asked, does your mirror have TDE (Turned Down Edge)? Does the outer diffraction ring of your in-focus star test noticeably "radiate"?
With respect to how much, you need to use different OTA-front aperture sizes to approximate the amount of TDE.
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Scanning4Comets
Markus
Reged: 12/26/04
Posts: 10114
Loc: Deep Space!
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I did the same to my previously owned 8" mirror. I masked off about 1/8" and it cleaned up the infocus image quite a bit. LOTS of commercially made mirrors that are done with machines have TDE problems.
Nice job, it looks very impressive. 
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Markus
10" F/4.7 Modified Skywatcher Reflector, 38mm Orion Q70, 22mm Vixen LVW, 14mm Denkmeier, 10mm Pentax XW, 7mm Pentax XW, 5mm Vixen LVW, 2" GSO 2x ED Barlow, 2" DGM O-III, Custom Laminated Star Charts, Uranometria 2000 Custom made maps with custom made rotating map holder, Astro Cards, Seen All Messier, Hundreds of NGC, Working on Herschel 400, Star hopper for over 30 yrs.
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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I noticed the fuzzy ring on the out side of focus.here is a simulation from abberrator.I added the spikes on the outside focus (right)This is very close to what I see on a decent night of seeing.The brighter stars always have an annoying spikey edge to them.
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
Edited by John K (12/29/07 08:57 PM)
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mcoren
Pooh-Bah
Reged: 01/11/06
Posts: 1403
Loc: Virginia, USA
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Very nice, John! Thanks for posting the pics. I'd love to see actual (i.e. not in aberrator) before/after star test photos.
I'm very interested in this project of yours because I have noticed that my own mirror exhibits classic characteristics of a TDE when I do a star test. How did you determine the size of the mask you needed? Why do you feel it needs to be there permenantly?
My thought for my XT10 was to experiment with different size masks, starting at 8" diameter and going up in 1/2" increments from there, and use star testing to determine the size of the mask I need. However, I was going to build something removable to fit over the front of my OTA, so that I could take advantage of my full aperture for low/medium power viewing. Perhaps I don't fully understand the effect of TDE, but my thought was to just use the mask for higher power viewing, where it causes the greatest loss of contrast and detail.
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Mike
Disclosure: I have received, and continue to benefit from, atoms that were fused by stars in the early universe, most notably carbon, oxygen, and nitrogen, and an assortment of others.
Orion SkyQuest XT10 Dob
6" f/8 Home-built Dob
Celestron C8 OTA (late 80's)
Vixen Super Polaris mount
Suburban Washington DC
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Jason D
Postmaster
Reged: 10/21/06
Posts: 6172
Loc: California
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Quote:
I noticed the fuzzy ring on the out side of focus.here is a simulation from abberrator.I added the spikes on the outside focus (right)This is very close to what I see on a decent night of seeing.The brighter stars always have an annoying spikey edge to them.
TDE affects the in-focus star test image -- not the out-focus star test image. TDE is, in a way, an over-correction error that is confined to the very outer zone of the mirror.
You might have an under-correction spherical aberration which affects the out-focus image. Unlike TDE/TUE, an aperture mask will gradually improve the defocused diffraction rings image as the aperture mask gets smaller. For TDE/TUE case, the aperture mask will improve the star test image abruptly once the TDE/TUE zone is masked.
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Jason D
Postmaster
Reged: 10/21/06
Posts: 6172
Loc: California
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Mike,
For my original XT10 mirror with TDE, here is how I built my aperture mask:
1- I cut the aperture mask off a cardboard sheet -- the kind used for cabinets’ back-panels. It is hard, sturdy, and easy to work with. More importantly, it does not sag. I masked off around 1/8" of the mirror -- diameter of the aperture opening: 9.75".
2- I drilled 6 holes to match the mirror 6 clip-screws location.
3- I laminated the mask with left-over protostar adhesive flocking paper
4- I removed the mirror 6 clip-screws, placed the mask above the clips then secured it with the 6 screws.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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I also made this to reduce the diffraction from the mirror clips.I don't want anything permanent as I don't plan on keeping this scope long term.Just till I can finance a great 12.5"scope
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
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JimPie
sage
Reged: 11/10/07
Posts: 474
Loc: S.E.Michigan
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It looks good John
Ive been thinking about doing the same thing on my mirror.
Please let us know if it works (corrects the problems).
Jim
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Jim
12.5",f=4.8 truss dob
EQ platform,24mm pan,14mm radian, vixen LVs
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dan chaffee
sage
Reged: 03/22/06
Posts: 286
Loc: Kansas City
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Quote:
TDE affects the in-focus star test image -- not the out-focus star test image.
TDE will affect BOTH in and out of focus star images; there is no reason it wouldn't. It's effects can be easily seen when racked outside of focus--where the outer diff. ring is very bright and contrasty as compared to infra focal, where the outer ring is fuzzy and/or spikey with less distinct inner rings. It can sometimes compete with overcorrection
for detection, but if you rack the focus out far enough, the pattern persists more than it does from overcorrection.
See: Star Testing Astronomical Telescopes, p 214.
Dan
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Scanning4Comets
Markus
Reged: 12/26/04
Posts: 10114
Loc: Deep Space!
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Yes, but a "fuzzy" undulating outer ring is usually caused by a TDE and can be seen when you rack inwards and Spherical Aberration is usually seen when you rack your focuser outwards no?
I just sold my Star testing book by Richard suiter, so I have no way of reading that page myself. 
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Markus
10" F/4.7 Modified Skywatcher Reflector, 38mm Orion Q70, 22mm Vixen LVW, 14mm Denkmeier, 10mm Pentax XW, 7mm Pentax XW, 5mm Vixen LVW, 2" GSO 2x ED Barlow, 2" DGM O-III, Custom Laminated Star Charts, Uranometria 2000 Custom made maps with custom made rotating map holder, Astro Cards, Seen All Messier, Hundreds of NGC, Working on Herschel 400, Star hopper for over 30 yrs.
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dan chaffee
sage
Reged: 03/22/06
Posts: 286
Loc: Kansas City
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Quote:
Yes, but a "fuzzy" undulating outer ring is usually caused by a TDE and can be seen when you rack inwards and Spherical Aberration is usually seen when you rack your focuser outwards no?
Spherical aberration, that is third order S.A., is seen on both sides of focus; contrasty, sharp rings on one side,
soft, low contrast rings (if any at all) on the other.
The sign of the correction error determines which side of focus you see these patterns.
Dan
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Jason D
Postmaster
Reged: 10/21/06
Posts: 6172
Loc: California
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Quote:
Quote:
Yes, but a "fuzzy" undulating outer ring is usually caused by a TDE and can be seen when you rack inwards and Spherical Aberration is usually seen when you rack your focuser outwards no?
Spherical aberration, that is third order S.A., is seen on both sides of focus; contrasty, sharp rings on one side,
soft, low contrast rings (if any at all) on the other.
The sign of the correction error determines which side of focus you see these patterns.
Dan
Dan, any aberration will impact both sides of the star test to different degrees. However, let us not veer too far off the heart of this thread. John uploaded star test representation of what he saw. The extra-focus image includes a fuzzy/radiating outer diffraction ring. He believes it is TDE. Markus and I are merely pointing out that with TDE, the intra-focus outer ring will have the fuzzy/radiating outer ring. John might be dealing with another kind of aberration such as a potential undercorrection which can't be fixed with an aperture mask.
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
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I think it may be also due to diffraction from the mirror clips.When it clears here I will give it a try to see if any improvment will be attained.If not then I will simply pull the mask off and move on to the next step.What ever that may be.
I made a light baffle/dew shield today I'm shure this will help a little for my local viewing,and frost
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RASC Member (Okanagan)
Home built 15"f5 Obsession clone
8"f6 Antares Dob
Celestron Skymaster 15x70
Howie Glatter laser pointer/Blug
Lumicon 2"OIII 2"UHC
Eyepieces 31mm Nagler,21mm Ethos,13mm Ethos,10mm Ethos,6mm Delos,2X Barlow, Antares 1.6X Barlow,TV Paracorr Type1
Edited by John K (12/30/07 08:42 PM)
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buckaroo
Space Cowboy
Reged: 05/26/06
Posts: 1723
Loc: Washington
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It must just be the photo but in pic #2 it looks like you have clear glass with the coating on the bottom of the mirror? Look at the shadow of the center spot.
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Steve
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