Re: 14" SCT diffraction pattern posted, comments welcome on degree of SA

From: CLT (not_at_thisaddress)
Date: 07/16/04 

Date: Thu, 15 Jul 2004 19:27:19 -0700

Try one of the SCT coolers. You can buy one or build your own. They really
work.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
And the Lunar Picture of the Day http://www.lpod.org/
************************************

"Ian Peterson" <peterson123d56rr@hotmail.com> wrote in message
news:9DhJc.9714$kK.2099@newsread3.news.atl.earthlink.net...
>
> "matt" <mariusrf@bellsouth.net> wrote in message
> news:6ngJc.1699$oG5.410@bignews5.bellsouth.net...
> >
> > Ian Peterson wrote in message
> > <7UeJc.7565$sV2.2005@newsread2.news.atl.earthlink.net>...
> > >Hi guys,
> > >
> > >In response to my other thread, where Matt suggested using a webcam to
> > >capture the diffraction patterns through the C14, I have posted such
> > >patterns on the alt.binaries.pictures.astro site and the title is
"ASTRO:
> > >C14 stellar diffraction pattern- comments invited". Responses can
either
> > be
> > >there or here. I hope these patterns reveal any potential problems my
> C14
> > >is experiencing. These were acquired with a 3x Televue barlow and
> webcam.
> > >The patterns definitely aren't equal. If this is showing spherical
> > >aberration, just how much is it? If not, and the patterns are
revealing
> > >another problem, what is it?
> > >
> > >Thanks in advance,
> > >Ian
> > >
> > >
> >
> > Some remarks, questions and suggestions . The patterns aren't equal but
> > there are several reasons for that, and they could become equal or
almost
> > equal under better conditions:
> >
> > -it appears there are still lots of tube currents , not enough cool down
> > time . You may have to use forced cooling as sometimes the ambient
> > temperature change rate is faster than the scope temperature change rate
,
> > and instead of reaching thermal equilibrium, the difference grows larger
.
> A
> > C14 can take a long time or never reach ambient temp without help. Some
> (or
> > most) of the radial stuff could be tube currents (heat plumes) .
>
> I agree. That's one of the major problems I've been having with it. My
> current solution is to place a large fan in such a way as to blow across
the
> ota. I remove the covers, place a lintless rag into the visual back, and
> let the fan go for 2-3 hours. I have already experienced severe spherical
> aberration problems because of inadequate cooling, but most of that
> disappears once the scope cools. So, you think the scope still isn't
> cooling enough? Interesting because it may not really be. Before the
test,
> I had the fan on it for 2 hours, plus just regular cooldown for another 4
> hours. Question: When dew starts forming on the corrector, even with a
> dewcap after many hours, can the temperature still NOT be at equilibrium?
> Those images were taken about 45 minutes before dew formed.
>
> > -are these stacked images ? How were they stacked ?
>
> Yes, roughly 12-15 hand picked out of 90-100 per AVI. Stacked, very light
> processing just to better bring out the rings. Didn't want to sharpen too
> much as this can throw the ring brightness off.
>
> > -the scope seems to need collimation or the image was not centered in
the
> > field of view . After recentering or recollimating, the diffraction
rings
> > should be perfectly concentric before starting any star test . The
> > diffraction rings aren't concentric in your images .
>
> I agree. Well, the problem here is mirror flop. I can collimate on one
> side of focus, then the other side shows as being off. That was the best
> compromise I could achieve. It really shows up with the 3x barlow!
>
> > -puzzling seems to be the fact that the central Airy disk, small as it
is,
> > seems to be elongated in both images (inside and outside of focus ) in
the
> > vertical direction . Being elongated like that is not normal, and it's
not
> > astigmatism, because it didn't rotate 90 deg from one side of focus to
the
> > other . If it's not a stacking artifact then it could be drive
vibrations
> .
> > At the image scale that you seem to be using, of around or less than
> > 0.1arcsec/pixel, this is just 0.9 arcsecs vertically .
>
> Very good! I've suspected at least 0.5 arcsec drive vibrations for quite
> some time. And the elongation wouldn't be from miscollimation as that
shows
> up differently. I wonder how severely the vibration can affect planetary/
> lunar images. I'll bet it's knocking out details giving solar system
> objects the "soft" appearance I keep seeing.
>
> > - after you remove the above sources of error, you may get images that
are
> > actually equal or very close inside and outside of focus . To see what
> type
> > of aberrations and how much of each, download Aberrator and modify the
> > images until you get them to resemble closely your webcam star test .
> > Aberrator is here:
> > http://aberrator.astronomy.net/
>
> Jon Isaacs advised me to do that also, so I downloaded it and started
> experimenting earlier today. Pretty versatile program. From what I can
> gather, if we remove the miscollimation, drive vibration, and improper
> cooldown factors, the SA of this scope isn't that bad at all, certainly
not
> 1/2 wave as I originally thought. I'll keep checking with the program
> though before I try and draw any conclusions.
>
> Thanks again for your input! I think I'm starting to weed out some of
these
> problems.
>
> Ian
>
> > best regards,
> > matt tudor
> >
> >
> >
> >
> >
> >
>
>

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