Re: [LX200GPS] Collimation
- Note that while this will suggest a possible astigmatism, it will not define the source
of the astigmatism; such a test will reveal astigmatism that might exist in the mirror,
the secondary, the eyepiece, a focal reducer, telenegative or even an optical flat
filter that is in the light path. There simply no quick, down-and-dirty way to test
accurately for optical figure or misfigure of components mounted in an optical path,
This is a good place to start however. If you see such as described by Dave - excellent
by the way - the do the same test with different auxiliary equipment: change eyepiece
and run the same test, eliminate the diagonal and any other optical item in the light
path, etc. At least you might narrow it down. But remember that such results do not
necessarily immediately condemn the primary....too many other factors in play with the
optics of a catadioptic and with ancillary optical items in the light path.
Arkansas Sky Observatories
Harvard MPC/ H43 (Conway)
Harvard MPC/ H41 (Petit Jean Mountain)
Harvard MPC/ H45 (Petit Jean Mtn. South)
----- Original Message -----
From: "Dave Herald" <drherald@...>
Sent: Tuesday, October 31, 2006 6:36 PM
Subject: Re: [LX200GPS] Collimation
> To test for astigmatism: Point the scope at a bright star. With a high-power
> eyepiece, move through the focus. If you have astigmatism, as you move close
> to focus from one direction, the star will become elongate - and when you get
> to the other side of the focus, it will be elongate in a direction that is
> 90deg different. The elongation PLUS 90-deg difference in direction is
> definitive of astigmatism.
> If you have elongation, but it is in the same direction as you move through
> the focus, it is almost certainly collimation. Note that to do the collimation
> properly, you should
> - be using a bright star,
> - in the centre of the field, with high magnification, and
> - the star near focus.
> I fear that often people that play around with collimation often have the star
> way too far out of focus - and as a result 'stuff it up'. Your description
> below is consistent with you being too far out of focus when you collimate.
> The de-focussed image should be no larger than the planetary disk of Saturn as
> seen through that eyepiece - otherwise you will have no sensitivity in the
> collimation [although if your collimation is 'way-out' you might need to start
> off with it being more de-focussed.] You need to be doing your collimation at
> the focal point where your 'flaring out' is clearly apparent!
> Finally, when you play around with collimation there is only ever a need to
> adjust two of the three adjustment screws. And for a scope that was previously
> collimated, you should _never_ have to turn any screw more than half a turn.
> Note that if you make adjustments to all screws, and get involved with turning
> them multiple turns, there is a _possibility_ that you could undo all the
> screws - with the secondary dropping off.... If you have been making a large
> number of turns on the screws, I would recommend that you start off by
> tightening them all fully, then releasing them by one turn. Then do the
> Dave Herald
> Canberra, Australia
> ----- Original Message -----
> From: "jkeshort" <joelandkarenshort@...>
> To: <LX200GPS@yahoogroups.com>
> Sent: Wednesday, November 01, 2006 8:11 AM
> Subject: [LX200GPS] Collimation
> Hello all. I hope that someone can help me with collimating my 8"
> LX200GPS. This appears to be a straightforward process that should be
> easy to accomplish, but I have not had good results despite carefully
> following the directions suggested by various sources including Theirry
> Legault's article on collimation.
> Basically what happens is that I collimate using a 6mm eyepiece and get
> good concentric circles when defocused. To avoid "mirror flop" I
> always approach focus in a counter-clockwise direction and use the
> mirror lock. However, after collimating is done, when I try to focus
> the star it blooms out horribly on one side. Also, when I look down the
> front of the scope (during the day) after collimating, the mirrors
> appear to be badly misaligned. The best collimation/focus I can achieve
> is done by simply looking down the front of the scope and adjusting the
> secondary mirror until I get the mirrors aligned as close as possible.
> This allows for good focus on stars and deep sky objects for photography
> with my DSI-Pro, but high resolution imaging of planets are not good
> enough. (I have pictures of the collimation process, showing the
> defocused star with concentric circles, but I didn't know how to put
> them in this message).
> There are two possibilities: 1. The more likely one is that I'm just
> doing something wrong or 2. Could there be something else wrong with the
> optical alignment of my scope (astigmatism etc)?
> Any help or advice would be much appreciated.
> Yahoo! Groups Links
> Yahoo! Groups Links
- I've done all the steps with my 8" LX200-ACF.
Sent from my iPhone 4
On Jun 3, 2013, at 22:54, "masa.nagaya" <masa.nagaya@...> wrote:
> Charles mentioned while ago about Thierry Legault's webpage (http://legault.perso.sfr.fr/collim.html) was a good place for Collimation info.
> I can do First Step no problem, but even I tried several different mag 2-3 stars with different EPs I can't see the optical rings he talks about. Is this for a big observatory telescope, or should I be able to perform with 10" LX200?