Collimating an Edge 8" for the First Time

When I collimate my edge8, I use a zwo 294 and put the Asi Air in "video" mode.  I adjust the gain / exp settings to get a decent image of the donut.  I make slight adjustments and can see the result in real time.  I just take care to "center" the donut manually with the arrows between each adjustment.

Yesterday I decided to install Bob's knobs.  I followed the instructions exactly as directed.  After installation, I found them cumbersome and difficult to turn in tiny amounts.  I could not achieve good collimation with the knobs and decided to  remove them. I did so very carefully, one at a time, making sure that the other knobs were tight and aiming the scope slightly downward to avoid the secondary mirror falling off.

I replaced the knobs with the original Celestron screws and then proceeded to achieve good collimation in less than 10 minutes, and this was my very first time collimating a scope.  I found that the original screws are much easier to turn in very small increments.

My question is this:  I think Bob's knobs were a little  longer than the original screws, and a tiny bit thicker as well.  One was difficult to turn to remove. 

Might I have damaged the secondary mirror or stripped the threads of my new telescope by trying to install these?  And how would I know if there were any damage?

I won't hob nob with Bob's knobs any more!!

Just my 2cents :   I avoided the Bob's knobs, just too many alternating views on how they held collimation.  I did however replace my phillips head collimation screws with allen head screws of same depth and made the process much easier.  Never felt that the phillips head screws were very accurate and always felt to be slipping or stripping when I turned.

Living in Switzerland, that would be a long stretch and a costly support ;)

You may want to have a look at these instructions to get you started:
* http://www.bobsknobs.com/resources/Literature/Instructions_English_a_Cass.pdf
* http://www.bobsknobs.com/faq/faq.html#How_do_I_perform_coarse_collima --> particularly the section "How do I perform coarse collimation after installing Bob's Knobs?" which is useful whether you have Bob's knobs or not, and the following section about star collimation "How do I collimate using a star?".

More detailed instructions, especially if you aim for high resolution photography, can also be found here: http://www.astrophoto.fr/collim.html.

Star collimation is quite easy to perform with a planetary cam.
Accessories such as Bob's knobs are really helpful in this respect - no risk of scratching the corrector plate with a screw driver.

Hope it helps.

Thank you Rodolphe and Michel!


If I am using the ASIAIR and a tablet to collimate, does the above chart still apply, or is everything inverse?

One thing to be aware of with these (Unless Celestron have changed the design) is that the mirror sits on a post and the screws pull it back against and tilt it around the post. The three screws need to be snug to ensure the mirror does not move around (Not tight, just snug) and when you tighten one you have to loosen one or both of the others. That is opposed to a typical Newtonian or similar primary mirror where the cell floats in the center and the three screws around the edges act more or less independently of each other.

If you really tighten the screws up it might even distort the mirror, I can't recall how the mirror is mounted on it's cell.