Secondary mirror alignment

OzStarGazer · #1 16-01-2014, 01:37 PM

Hello,

I tried to collimate my scope because it was not perfectly aligned anymore (I have been using it almost every day lately, in and out of the box, then on the balcony etc.). I had no problem at all aligning the primary mirror, but the secondary mirror seems to be a little bit trickier to align (with the red dot of the laser collimator perfectly at the center of the primary mirror - there is a tiny circle in the middle of the primary mirror to check and the red dot seems to always bit just outside or on the border of the circle). I have noticed that if I slightly tilt the secondary mirror upwards (towards me) the red dot does hit the centre, but it doesn't seem to last very long, maybe just for a viewing.
My question is - do you know exactly what I should do to make this most permanent? I guess maybe a screw is too loose, but I don't know which one? It is probably difficult to know without seeing or knowing the scope, but maybe those of you who own a Skywatcher Heritage 5" know.

PS: Searching for some detailed info about my scope's collimation on the net I have just found this thread and I think post #5 could also be a problem for me. It doesn't look like that angle is perfectly 90 degrees in my telescope either...
http://stargazerslounge.com/topic/19...or-mount-help/

glend (Glen) · #2 16-01-2014, 02:52 PM

Whoops, you moved the secondary. So you will need to learn to collimate from the beginning. Shining a laser in there is going to get you nowhere until the secondary is aligned. Suggest you download Astrobaby's very good instruction set. If you have a cap for the focuser that might be made into a Cheshire device by putting a small hole in it to look through at the secondary. When you look down the focuser tube the secondary mirror (don't worry about the primary at this point) should be directly under the focuser tube and you should see it as a circle (not an ellipse or other shape). Then you can use the secondary adjustment screws to position it so that you can see all the primary mirror clips through the hole in the focuser cap - that gets you close. Then if you must use the laser that's when it can be used.

Read the instructions on collimation, do it, and do it again.

BTW its not uncommon for a laser to be off target on the centre spot, this is due to two factors: the laser device may not fit the focuser tube precisely and there is slop in the fit that allows the laser to lean over alittle or be pushed to the side by the focuser lock screw, and two the laser itself maybe out of collimation (hope you bought one that could be collimated and know how to do that - if not read the article on laser collimation in the Articles section)

Allan · #3 16-01-2014, 02:58 PM

When you say you tilt the secondary up to get the laser dot centred in the primary donut, do you mean you are tightening one of the secondary collimation screws? Tension between the screw and the mirror stalk should not normally work loose. One trick people use is to place a large nylon washer between the screws and the mirror stalk. The screws torque into the washer, then push onto the back of the mirror stalk. I find this holds the collimation more permanently on the secondary. You can also make a washer by cutting a disc out of a plastic milk container.

OzStarGazer · #4 16-01-2014, 05:35 PM

Thanks for the replies.

Unfortunately I don't have a cap for the focusser. However out of curiosity I removed the focusser and looked through the focusser tube and the secondary mirror was perfectly circular (with my eye in the middle

). I guess this is at least a good sign?
Maybe I will try to make a cap with some thick paper. Would it work? something I can then fix to the focusser tube with tape for example, or an elastic band. Then I can make a hole in the middle. I have to go to Sydney tomorrow, but if you think it would work I could try on the weekend.

Due to its construction my scope is not 100% rigid (because of the flexitube) so yes maybe this might be the reason why the secondary mirror didn't appear to be exactly aligned?

Allan, yes, I meant that I "tried" to tighten the screws, but I can't even see their heads and the screwdrivers I used didn't feel right, like nothing (or not much) was happening! They are so far down, that's why it is difficult to do a good job. I have ordered a set of allen wrenches. I should get it in a couple of weeks. Maybe I can do a better job then? They mention allen wrenches in the manual for the screws of the secondary mirror, so hopefully...

Thanks!

barx1963 (Malcolm) · #5 16-01-2014, 11:13 PM

A collimation cap has one purpose only, to allow the eye to be centred in the focuser. No matter how hard you try it is nearly impossible to accuratey centre the eye over the focuser tube.
Easy way to make one is find somewhere local than sells camera film (remember that stuff!!) and take the film and throw it away (unless you can use it of course) then remove the lid from the canister, throw that away and then use a drill to put a small hole in the base of the canister. There is your collimation cap.
With regard to the angle of the support for the secondary, that is completely irrelevant to the ability to collimate the secondary. The secondary holder should rotate so you can adjust the secondary and the only important thing with the secondary stalk is that the centre of the holder is in the middle of the tube. simply measure the distance from the centre of the holder (there should be a screw head there) to the edge of the tube and if it is the same in say three different directions, that is fine. If it is out, there is not much that can be done with a simple stalk structure, personally I wouldn't fuss about it too much unless it is out inches!
I am not sure why you cannot see the secondary screws? They are clearly visible in the pic post 5 on the SGL thread you inked to. Does your scope look anything like that?

Malcolm

barx1963 (Malcolm) · #6 16-01-2014, 11:14 PM

Also when you said you "removed the focuser" what does that mean?

Malcolm

l3gendluk3 (Luke) · #7 17-01-2014, 03:39 AM

I never liked laser collimators so I stick with some cardboard cutouts, a homemade colimation cap and a Cheshire collimator. For the cap, I got my focuser dust cap and did some geometry with a ruler and a compass that found the exact center and drilled a hole. If you are going to do this, use a hobby knife to put a guide hole in the center to stop your drill from slipping (like mine did). Please note that I only did this because I have 2 dust caps, one for my barlow and one for my telescope so when I stuffed up I had a spare.

Luke

OzStarGazer · #8 17-01-2014, 09:19 AM

Thanks for the new replies.

I will measure the distance from the centre of the holder.

About the secondary screws, the big one in the middle is visible, but the small ones are further down. I cannot see exactly what their heads look like because they don't stick out, so I was not sure which screwdriver to use. (I had a torch but is seemed to have died so I could not check.) I am not much of a person who does a lot of manual work, so I didn't have many screwdrivers or tools at home either. But I got the allen wrenches this morning (they arrived much earlier than expected), so I will try them later today or tomorrow. I would rather not touch the secondary mirror too much right now to be honest as it seems OK, but maybe I should try anyway and have some guts...

The focusser is a cheap focusser. I plan to upgrade it one day. When I said I removed it I mean I screwed it out. I don't know if this can be done with all focussers, but mine is adjusted by screwing it in and out and it can be screwed out completely/removed. I don't really like it. When I have to screw it out to focus sometimes it feels so unstable...

Unfortunately I once asked and there are no shops around here that sell film canister caps but I could try Sydney (I intended to go today, but I was sick last night, so I didn't go).

barx1963 (Malcolm) · #9 17-01-2014, 08:05 PM

I see now, the focuser is a helical style one, did not realise they used that on this scope. Guess it is a cheaper option than a standard R&P or Crayford focuser!

Malcolm

OzStarGazer · #10 18-01-2014, 08:50 AM

Quote:

Originally Posted by barx1963

I see now, the focuser is a helical style one, did not realise they used that on this scope. Guess it is a cheaper option than a standard R&P or Crayford focuser!

Malcolm

Yes, that's the term! I will upgrade it one day, but at the moment my priorities are a nebula filter and a camera adapter bracket.