collimation using a planet or by iterative method??

Alvin Wibowoo posted:

Like many fellow SCT users, when it comes to collimating on a star, the
process is anything but simple.

Once a person becomes familiar with the process of collimation, it becomes
very straight forward and fairly easy. You need a star (Polaris can be a good
one) and a quality high-power eyepiece (yielding at least 25x to 30x per inch
of aperture). If you are out of collimation, star images at high power will
tend to look somewhat elongated (almost shaped like a diffuse cone). Center
the star and look at the image to carefully note which direction it appears to
be flaring towards (ie: the broader portion of the elongated star image).
Then start adjusting a screw on the secondary until you find the one which
makes the star appear to move in the field of view in the direction of the
flaring. Center the star again and adjust the screw to move that star again
in the direction that it is flaring, and after a while, you should see the
flaring become less and less as you repeat the process. Eventually, the
star's image should look fairly round or symmetric. When that happens, you
are collimated. Remember that if you wear glasses to correct astigmatism, you
should leave them on while doing this. If you can't get good collimation
after trying this process, then there is probably something wrong with the
instrument. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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When I had an SCT, I would do the collimation using a star as best I could and
then put the scope on Jupiter. I'd then give it a little tweak to see if I
could get any better performance out of the scope. Often by just moving one or
two of the screws a tiny bit I'd have a slightly better image of Jupiter.


Todd

On Sat, 28 Feb 2004 17:31:40 GMT, "Alvin Wibowoo"
wrote:

Like many fellow SCT users, when it comes to collimating on a star, the
process is anything but simple...

I can't comment on using a planet, since I always use a star. But something is
wrong if you don't find collimation fairly simple. In particular, an SCT is
probably the easiest type of telescope to collimate- there are only three
possible adjustments, and their effect on a highly magnified star is obvious. If
you can't easily reach collimation, there is probably something else wrong with
the optics, and it won't matter what collimation technique you use.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Alvin Wibowoo wrote:
It's really too bad I can't use an artificial star. With my 12" F/10, the
furthest away I could place the star would be 75 feet. From what I've been
reading, this isn't far enough.

Actually, it isn't far enough for star-testing, but I don't see why it
wouldn't be far enough for collimation. (I haven't tried it myself.)
Anyone?

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Sun, 29 Feb 2004 14:17:08 GMT, "Alvin Wibowoo"
wrote:

Ok, guys. Guess the planet idea isn't a good one. I tried it again last
night without real success. Unfortunately, folks keep telling me how simple
collimation is, but I've been at it for over a year now, with two different
SCTs, and I must say that I continue to hate the process. I particularly
don't like all of the workaround factors involved- scope cooldowns

How is this difficult? You just let the scope sit outside for an hour or two,
depending on the difference.


seeing conditions

Yes, you need reasonably good seeing. Another thing you just wait for.


constant recentering

Because of drift, or collimation? If the former, try using Polaris or another
star near it, since this will considerably reduce drift. If the latter, don't
worry too much about keeping the star central until you have the collimation
close. Even quite far off axis, it is easy to see if you have a large error.

All of this, for me at least,
turns what should be a 5(?) minute process into 45 minutes or more. I think
the biggest delay for me comes when I can't decide how close or far off the
collimation is, particularly if the scope hasn't quite cooled or seeing is
only average. Then, the large dewcap I use must be removed so I can reach
the screws, which ends up disturbing the stable cooldown I had so long
waited for.

Leave the dewcap off from the beginning.

Collimation in a healthy SCT is very stable. With my permanently mounted 12"
LX200 I needed to do it once in two years. With a 8" SCT that gets moved around
some, I've done it a couple of times. Even if it takes you 45 minutes, once you
are done you should be done for many months. If the scope is losing collimation
it either needs something else fixed or it is being handled way too roughly.



It's really too bad I can't use an artificial star. With my 12" F/10, the
furthest away I could place the star would be 75 feet. From what I've been
reading, this isn't far enough. Is there still anyway I could make this
work? I'd feel much better about the collimation process, I think, if I
could do it during the daytime or dusk using something not prone to seeing
and would therefore allow me to accurately see the diffraction patterns.

Is this length limitation because you want to do it indoors? Surely there must
be something outdoors you can point the scope at and which is a couple hundred
feet away? But be warned: artificial stars are as subject to seeing as natural
ones. You may find the effects of rising air from the ground every bit as
difficult to deal with as normal scintillation. Although the seeing is often
quite poor here, I still prefer a natural star for evaluating collimation to an
artificial one.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Sun, 29 Feb 2004 18:11:03 GMT, Chris L Peterson
wrote:

Why don't you use a diagonal? It is a complete myth that you should collimate
without one. If you use a straight through EP, you are in fact moving the mirror
spacing out of its optimal position and introducing slightly more aberration.

If your diagonal is not exactly collimated you can't collimate the
SCTcorrectly. Changing the mirror spacing should not change the
collimation, only the amount of spherical aberration.

On Sun, 29 Feb 2004 19:44:04 GMT, William Hamblen
wrote:

If your diagonal is not exactly collimated you can't collimate the
SCTcorrectly.

As long as you are collimating using a central star, you can tolerate quite a
lot of miscollimation in the diagonal when collimating your scope. The nature of
the limited adjustments on an SCT makes it quite obvious where the _scope_ is
best collimated, independently of the diagonal. This is not the case with a
Newtonian, however.


Changing the mirror spacing should not change the
collimation, only the amount of spherical aberration.

To be clear, I did not say that the collimation was dependent on mirror
position. But other aberrations are, and I think it makes good sense to
collimate the telescope with its optics in as close to optimal spacing as
possible.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Sun, 29 Feb 2004 20:40:07 GMT, Chris L Peterson
wrote:

On Sun, 29 Feb 2004 19:44:04 GMT, William Hamblen
wrote:

If your diagonal is not exactly collimated you can't collimate the
SCTcorrectly.

As long as you are collimating using a central star, you can tolerate quite a
lot of miscollimation in the diagonal when collimating your scope. The nature of
the limited adjustments on an SCT makes it quite obvious where the _scope_ is
best collimated, independently of the diagonal. This is not the case with a
Newtonian, however.

What happens when your star diagonal is cocked is that the center of
your eyepiece isn't on the center of the optical axis any more. Your
collimation therefore won't be as good as it should be.