Help! Collimation issues

[Gir92]

Good evening everyone.

I recently purchased a zhumell 10" dobsonian. I am having some troubles with collimation, and troubles I assume are caused by the troubles with collimation (oh boy.)

For the collimation, I cannot seem to get it correctly collimated when the laser is in circle in the center of the mirror. The only way I've been able to get the return laser correctly back to the eyepiece is by setting the secondary mirror so that it points ~1 inch to the right and ~1 inch up in reference to the circle (when looking down at the primary mirror through the opening of the optics tube.)

Even when collimated this way, what should be Jupiter looks like this:

The picture actually came out clearer than how I see it. With my eyes, I see a very unsteady image of a bright white object.

Now, I may be an amateur, but I know what Jupiter is supposed to look like, and I know I was looking at Jupiter.

If anyone could advise me on how to fix this, I would greatly appreciate it. Also, please keep your advice and wording amateur friendly.

Thank you again.

[Capricorn]

Try Astro-Baby Astronomy Website Mel has a guide to collimating a scope.

Next what magnification were you using?

The image looks large and after a point the clarity gets worse faster then the magnification, so you see a blurred image.

Try starting with about 80x and see what it looks like, I would guess that anything above 150x would not be worth it. Diminishing returns.

Just to backtrack a bit at 80x the image could be too bright on a 10" scope.

Problem is that doubling the magnification reduces the intensity by 4x. So too big an image will be both poor in quality and dim.

[Gir92]

I was using a 12mm plossl eyepiece, but even with my 32mm wide angle, it looked just as bad and unclear.

[Capricorn]

Going to guess then that it is as you say the thing needs collimating.

Although you have a laser collimator have you checked that the collimator itself is aligned? If the beam is not parallel to the casing then you are starting off wrong - not you but the equipment. Need V blocks to check however, and then able to adjust the tool itself.

Alternativly is there a quick check you can do without a collimator to get an idea of how good or bad it is.

After that it is a case of checking the scope out and redoing the collimation. Probably a few times to be sure and to get the process right.

[brantuk]

A star test will confirm wether it's properly collimated or not. You should see a razor sharp pinpoint of light with diffraction spikes in a perfect cross shape. Take it slightly out of focus in both directions (slowly) and you should see perfectly concentric circles both ways. If you don't see any of that then it ain't collimated right and a check of the collimator/scope is required.

See if you can borrow someone elses collimator - if you put it in and the results aren't identical to the setup with your own collimator then it probably needs looking at (unless his is out as well)

[Astro_Baby]

I am going to put me neck out and say this sounds (at least from how the OP said they are setting the secondary) like one of the classic laser problems.

I dont use lasers for various reasons but going from your description I wonder if you are useing it correctly.

This is the correct sequence for laser

1/ Insert laser and turn on.

2/ Adjust secondary mirror until laser beam hits centre spot on primary mirror.

3/Adjust primary until the return beam shows centred on the collimator.

There should be no nead to adjust the secondary to get a return beam on target unless (and I suspect this is where the problem lies) during (2) above the secondary started to rotate. THis is one of the downers with lasers that the laser can be perfectly aligned by the optics arent. A laser can return its beam perfectly even when secondary alignment is shot to piecse and this sounds like the problems.

You need firts to make sure your secondary is aligned correctly to the focuser. I would reccomend using a collicap to get at least the secondaryt to focuser sorted out and to do a rough alignment. Then go over to useing the laser.

Take a look at my page on collimation above and see if it helps you.

[mdstuart]

Also make sure your expectations of what you will see are reasonable. Jupiter on medium power is a very bright disk about half the size of a penny coin with one dark band accross it and four little moons (white dots) around it. With a lot of patience, steady skies, with a cooled down scope you can start to make out the great (slightly grey) spot and other bands and markings but they are of course very small to the eye.

I agree with the star test. Point it at Polaris and see if you can see a spot with a small dimmer secondry star next door..

Mark

[Doc]

Your image is dark so to me this means your secondary mirror is not tilted correctly.

If you use a collicap do you see all three mirror clips? As soon as your secondary mirror is tilted correctly the image will be much brighter with more contrast.

Also while looking down the collicap/sighttube does the secondary look circular. If it looks oval then that also is a sign of a wrongly placed secondary.