Do I need to collimate my scope?

[redneon]

Last weekend I took this photo of Jupiter via my 4.5" reflector and SPC880 webcam (using Registax 6 to stack it):

I was quite happy with it but I've been told that I should be able to get better results and that my scope may need collimating.

I do have a Cheshire and I've read a bit about collimating. I read that if you focus on a star and defocus the spider should look symmetrical with the shadow of the secondary mirror in the centre. So a couple of nights ago I tried doing this by defocusing on Jupiter. I took a couple of single frame shots with my webcam and these are the results:

What do you think? It looks fine to me. If it is out it's not by much and I don't want to make it any worse. It seems like a complicated process (as I don't have a mark on my primary mirror) and I've never done it before.

[LukeSkywatcher]

It does appear to be out. It doesn't look like much though. If you are happy with the views then i would leave it alone.

[ronin]

Reflectors operate as they imply by reflection.

If the error between the incident light and the mirror surface is (say) 1 degree, then the light reflects with a 2 degree error. It doubles.

Then this light heads up to the secondary with a 2 degree error where it reflects again and gets doubled. So a 4 degree error. This assumes that the secondary mirror is EXACTLY flat and at 45 degrees - will bet it isn't.

So if the whole lot is not aligned as good as you can get the errors build up very quick.

The effect of these errors is a poor image, either visual or imaged.

Now 1 degree is large on a scope but a 1 minute error (very very small) suffers the same, by the time you image or view it there is a 4 minute erros and a degraded image.

You will find that a professional observatory scope does not have a secondary, they put the imaging sensor there and so reduce errors by half.

[kniclander]

it was recently pointed out to me that in a a fast scope (not sure what your focal ration is) the shadow of the secondary won't be central because of the offset when the scope is correctly collimated.

[LukeSkywatcher]

it was recently pointed out to me that in a a fast scope (not sure what your focal ration is) the shadow of the secondary won't be central because of the offset when the scope is correctly collimated.

I remember that being said also somewhere here. Seemed strange but what do i know.

[Tinker1947]

I do believe you can adjust the Primary Mirror while its out of focus on a Star, just a quarter of a turn on one screw see if it moves the right way or not...

[redneon]

The thing that worries me is that I could end up making it worse, rather than better. My scope is a fairly slow one (f/8, I believe) but that quote sounds a bit dubious. Not that I know much about these things I just can't understand why that would be the case

Part of me thinks I should just throw myself in at the deep end, though. Just to say I've done it. I bought some binder re-inforcement rings to put a mark on my primary mirror so taking that out to put the ring on would knock it out of alignment a fair bit. Then I'd be forced to collimate.

[kniclander]

f8 would be considered slow and there would be no noticeable offset. in that case i would say that your primary is a fair bit out of collimation (although cant tell anything about the secondary from the star test). There is quite a lot on the forum if you're interested in understanding why there is an offset in fast scopes.

ps was the star central in the fov? if it is off to one side the doughnut won't be symmetrical anyway.

[Zachariah]

For the life of me I cant workout if my scope is out too. When I look down the eyepiece tube the secondary mirror doesnt lool central. My scope is A Tal 1 do I need to adjust thd secondary or primary to get it central? Help

[Peter Drew]

I think a Tal 1 deserves better collimation than that , If you are hoping for better images it will have to be addressed sooner or later. This seems daunting to most beginners, but once successfully completed they nearly always remark on how easy it turned out to be. I would suggest that tweaking the main mirror would be a good start.

[Zul]

It doesn't realy need collimation but if you want to have perfect collimated telescope then you can try to

[Zachariah]

I have put a centre spot on my primary and it appears central in secondary, but the secondary looks off centre in the tube. I am going to star test it tonight if we ever get clear skies again in the UK.

[Trull]

You need to collimate every single time you get your telescope out, particularly if its "fast" (f/ratio of 6 or less).

Size of sweet spot:

F/ratio Diameter mm

f/4 1.4

f/4.5 2.0

f/5 2.8

f/6 4.8

f/8 11

f/10 22

I have used a laser collimator and shown many people how to wield one to great effect - only takes 3 minutes!

Andy's Shot Glass how to collimate is very instructive too.

Have fun!