Rob S
professor emeritus
Reged: 03/16/07
Posts: 537
Loc: NZ
|
|
John,
Not trying to teach anyone how to suck eggs, but the tube and baffle can extend in front of the secondary by 1.5 times the diameter of the tube.
Regards,
Rob.
--------------------
8" f5 Dob with Feathertouch
Brandon: 16
Pentax: 7, 10 & 20XW
TMB: 30 Paragon
TV: 8 & 13 Ethos;
5 & 13 NaglerT6;
24 Panoptic
Leica Trinovid 8x50 BN
|
Scanning4Comets
Markus
Reged: 12/26/04
Posts: 10114
Loc: Deep Space!
|
|
Quote:
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.
Actually, I masked my mirror clips with a ring for the TDE on my 8" scope. The 1/8" ring covering the edge of the mirror helped but I could STILL see a very, very slight amount of fuzziness in the outer ring, so the clips won't cause that.
--------------------
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.
|
dan chaffee
sage
Reged: 03/22/06
Posts: 286
Loc: Kansas City
|
|
Quote:
[John might be dealing with another kind of aberration such as a potential undercorrection which can't be fixed with an aperture mask.
It depends on how you mean that. If I have, say an 8" f/6 mirror that is spherical and therefore undercorrcted well below .8 strehl and I stop it down to 6" f/8. it will be very close to .80 strehl. That's certainly an improvement
even though it hasn't actully corrected the aberration.
Stopping down a given aperture increases focal ratio, producing a larger diffraction disk and correspondingly less energy falling outside of it, in addition to covering
edge defects.
Dan
|
Jason D
Postmaster
Reged: 10/21/06
Posts: 6172
Loc: California
|
|
Quote:
Quote:
[John might be dealing with another kind of aberration such as a potential undercorrection which can't be fixed with an aperture mask.
It depends on how you mean that. If I have, say an 8" f/6 mirror that is spherical and therefore undercorrcted well below .8 strehl and I stop it down to 6" f/8. it will be very close to .80 strehl. That's certainly an improvement
even though it hasn't actully corrected the aberration.
Stopping down a given aperture increases focal ratio, producing a larger diffraction disk and correspondingly less energy falling outside of it, in addition to covering
edge defects.
Dan
Dan, I still do not see where the disagreement is  !!! In an earlier post in this thread I stated that if the mirror has an undercorrection issue then as the aperture mask gets smaller the extra-focus image will improve gradually for the reason you stated. That is, as you slow the focal ratio, spherical abberation becomes less of an issue. On the other hand, once the TDE zone is masked, then there will be an abrupt and quick improvement.
Jason
--------------------
XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
|
John K
Carpal Tunnel
Reged: 12/26/05
Posts: 1649
Loc: Vernon BC Canada
|
|
The clear mirror shot is the result of the center spot reflecting off of the roof,I hope. 
I made the light baffle 8" longer.I was going to paint it flat black but the paint that I tested on a scrap piece cracked after it was dry with the slightest flex.So I don't really want to be picking little paint flecks off of my mirrors 
I am still waiting for a clear night to see if any of this will help.Its clear right now but it's supposed to cloud up at dusk  
--------------------
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 (12/31/07 02:24 PM)
|
cocobolo
Carpal Tunnel
Reged: 10/27/07
Posts: 1550
Loc: british columbia, canada
|
|
Why not take your mirror out of the cell and test it with a focault tester. There is bound to be someone in your RASC group that will either have one or at least know how to use it. You can make a simple one in just a few minutes, OK 1/2 hour, and it will reveal the most embarrassing secrets about your mirror. Then, if you have a turned down edge, you can apply a paper mask to the mirror in ever increasing or decreasing amounts until the problem is eliminated. At least you will know just how good the mirror is. We have tested a few mirrors from the typical chinese dobs, and generally find that they are afflicted with minor amounts of dog biscuit. I don't believe I have seen one with a turned edge. I have seen the turned edge with hand polished mirrors, and as long as this is caught quickly in testing, that is before the turned edge becomes large, it can be reversed. It seems to take about 10 times longer to fix the problem than to cause it!! And try the word aperture, thanks.
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
