Collimating a C11 without stars

[Trentend]

Planning on using my C11 this evening for the fist time as clear skies are forecast and the moon should be in a good position. Only problem is it doesn’t get dark up here this time of the year so I won’t have any stars to use for collimation. Do I have any other options? I fitted Bobs knobs per instruction (one at a time) but am pretty sure the scope will now be out of alignment. Have just googled “C11 collimation” and difficult to know which of the many methods to follow. Any advice much appreciated.

[dweller25]

Collimate from the front.

Sit away from the scope at least the distance of it’s focal length.

Using one eye look at the middle of the scope - all the rings should be concentric.

If they don’t use the collimating screws to get concentric circles.

It will get you close but not perfect.

[dweller25]

Or make a star by putting a pin hole in a piece of tin foil, place it in a shoe box (to try to get some darkness against which to see the “star”), put a torch behind and put it at least 100 feet away.

Then collimate with an eyepiece as normal.

Edited June 17, 2021 by dweller25

[dweller25]

Or use it as is and wait for darker nights and a star - which is defo the best way to collimate

[Trentend]

6 hours ago, dweller25 said:

Collimate from the front.

Sit away from the scope at least the distance of it’s focal length.

Using one eye look at the middle of the scope - all the rings should be concentric.

If they don’t use the collimating screws to get concentric circles.

It will get you close but not perfect.

Trialled the scope tonight for the first time on the new mount/tripod. Doddle to set up and balance but, as expected collimation a mile out probably a result of recently fitting Bobs knobs (moon didn’t focus at all, but not a focuser travel issue). Can I check the above technique. When I look down the scope from 3m away am I trying to line my eye directly opposite/in front of the centre of the mirror? I tried this but wasn’t sure what concentric circles I was looking for. Are there any pics of this? Naff view of what I was seeing below.

 

 

[paul mc c]

if you have just replaced the screws for bobs knobs,what i would do is remove the secondary housing,it just screws off and then adjust the screws until the spacing at each screw is the same and put the secondary back,then it should be fairly close to collimated,then cover a small  bright flash light with tin foil and put a small pin hole in it and look at it from as far away maybe 40-50 feet,and focus on the pinhole.

[Trentend]

I’ll give that a try thanks. 

[Trentend]

Am sure this will be clear once I’ve removed the spacer but when you say “adjust the screws until the spacing at each screw is the same” does this mean they’re all finger tight (wondering if the distance can be the same but for very levels screwed in)?

[dweller25]

This shows how it should look

http://www.robincasady.com/Astro/collimation/

[Starwatcher2001]

When I first put "bob's knobs" on my C925, the resulting collimation was all over the place. "Eyeballing it" during daylight as described above got it pretty close. Moving towards and away from the scope helps you check if everything is concentric. Mine looked pretty good under the stars and just needed a tweak to get it right. I do a quick test most sessions, but it holds collimation pretty well and rarely needs touching.

 

Edited June 18, 2021 by Starwatcher2001
can't spell

[Trentend]

Ok so took the housing off today and noticed a few things:

- the part that the Bobs knobs screwed in to presumably would come out if you tried but is (highly) not recommended? It was loose and not flush at one point but I quickly squared up and put the housing back on

- does the above part need to go in further? There’s a small allen screw that sits at the end of a recess per the picture below when the unit (above) was squared up. Wonder if the whole unit though needs to go in further?

- It was impossible to tell how far in each bob knob was to get them roughly the same as any slack/tension was taken up by the white spacer and I couldn’t see how far in the actual screws were sitting

Next time I’ll just leave the (probably perfectly collimated) factory screws in place! 

One good thing though, when looking down the tube I could see the shadow of the central obstruction was misaligned so now just need to work out what to adjust to centre this with a bit of trial and error…

[Trentend]

On 18/06/2021 at 06:04, dweller25 said:

This shows how it should look

http://www.robincasady.com/Astro/collimation/

That helped a lot thanks. Didn’t appreciate how loosening one screw then allowed the others to be further tightened (Newbie error I was just just tightening everything more and more - have a similar set up on my dob but that holds collimation so well I’d forgotten the basics). Looking down at the secondary everything is nicely centred and concentric now. Could see it clearly moving in to place with minor adjustments. So just waiting for a clear night for a star test. Thanks again.

[dweller25]

@Trentend

Yes the Allen screw needs to go in further to ensure it’s correctly seated.

On my C6 it sits right at the bottom of the cut out notch.

You may need to re-collimate once the secondary is seated properly.

[Trentend]

First light since my ball park/daytime collimation effort back in June. Looked good to me. Moon and stars sharp pretty much straight out the box. May have another look to see if I can get the secondary mirror/Allen screw to fit further in the notch. No excuse now getting out to a darker site!