Re: ATM Collimation -- how much effect?

["B. Pepin" <atmjedit@conceptsnet.com>]

Wow Silent, a quarter-inch? Pop a laser or Cheshire in that puppy to check
collimation and you will see how much you are misdirecting the light. In my
f/6 system I found that tilt as much as a millimeter induced significant
off-axis aberration and lowered contrast. F/8 is somewhat less sensitive,
but I think this is the problem.

The eyepieces you mentioned are far better than the ones the classic
observers used to routinely describe fine detail in many objects.


----- Original Message -----
From: The Silent Observer <silent1@ix.netcom.com>
To: ATM List <atm@shore.net>
Sent: Sunday, October 15, 2000 7:59 AM
Subject: ATM Collimation -- how much effect?


>
> I have an 8" f/6.8 Newt/Dob that I built, with help from this list, and
> finished about a year ago.  Under dark skies last October, I was very
> pleased with its deep sky performance, getting views comparable to a 10"
> SCT on objects like M1 and M33, and finding then 9 M Comet Lee easily
> with a chart and star hopping.
>
> Since then, however, I've noticed that my cell design seems to be
> somewhat flawed; when I change the angle of the scope from near zenith
> to near the horizon, I note that the mirror's axis shifts position in
> the tube by something like a quarter inch.  I've done my collimation
> with the telescope pointed at about 30 degrees from horizontal, which is
> where I do most of my viewing (Moon and planets, close to the meridian,
> are not much above that angle this time of year), but I wonder -- just
> how much performance am I losing on this?
>
> I ask because I've noted that I can't get beyond 200x, even on the Moon,
> without the view getting fuzzy as fast as it gets bigger, and with the
> mirror I have I ought to be able to at least approach, if not actually
> exceed the Dawes limit of 50x/inch -- which would be 400x.
>
> What I'm really after here is whether my limitation is due to a poor
> quality or possibly damaged eyepiece, or if it's due to the wandering
> collimation -- when I'm looking at the Moon, the collimation appears
> spot on, but I don't have either a Cheshire or laser to check it with,
> just a film can sighting tube and my calibrated eyeball -- my out of
> focus star images seem centered, but I don't have much magnification to
> check them against, and I haven't bought shorter eyepieces because I'm
> not sure they'd do me any good.
>
> So: is my 10 mm Celestron SMA just too cheap to support 270x with a
> Barlow, or might it be damaged (I have dropped it once, though not hard
> enough to chip the black finish on the eyepiece, and that on softish
> ground rather than, say, rock or concrete)?  Or is the collimation still
> off?  Or is my mirror not quite what I expected from the test results
> just before aluminizing it?
>
> FWIW, my 24 mm Celestron SMA with Barlow, at 108x (or slightly backed
> out of the Barlow to something like 120x), seems to show as much or more
> detail as the 10 mm alone at 135x, and the longer eye relief makes it
> easier to use.  Would I be spending better money getting another Barlow,
> perhaps a 2.5x or 3x, buying a better quality 10 mm or shorter eyepiece,
> or just finishing up my extension tubes to get more magnification from
> the Barlow I have (Orion 2x Shorty)?
>
> --
> Some of their knowledge is corrupt, and inaccurate, being gleaned from
> all manner of sources.  But like us, they are wizards too.
>                                                     -- Jaldis the Blind
>
> Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
> Rocket Pages                http://silent1.home.netcom.com/launches.htm
> Telescope Pages            http://silent1.home.netcom.com/astronomy.htm
>
> Opinions expressed are my own -- take them for what they're worth
> and don't expect them to be perfect.
>