C11 collimation

[DrRobin]

Hi,

Has anyone out there got a good method to collimate a C11 (Edge HD)?

I have tried a few methods including:

Defocused star;

Metaguide;

Collimation mask;

Visual alignment.

both with real stars and a home made artificial star.

I managed to get the the scope close using the visual method, just look down the tube from the front, but then seem to bog it up when making the final fine adjustments. I probably did best with an artificial star and the collimation mask.

Finally is there anything in a C11HD that makes it harder, I can't think why?

[billhinge]

you probably answered your own question with real and artificial stars, where I live I find the atmosphere isn't always steady enough or as soon as I take the dew shield off to collimate I'm knackered as the corrector dews up

I don't have the space to do the artificial star method so I bought one of these http://www.firstlightoptics.com/collimation/hotech-advanced-ct-laser-collimator.html . Does work and you can do it in daylight

in a small area but it is expensive (I also have a C11 and I have had the front corrector removed for cleaning and flocking so it was in bad need of collimation)

[Uplooker]

When collimating my C8 I have used two differing methods:-

1. Duncan mask - http://www.astrovox.gr/forum/download.php?id=17658&sid=646453d99bbd6eef6845f3d3254b6ff9
   
2. Al's Collimation Tool - http://stargazerslounge.com/topic/136286-als-collaid-sct-collimating-assistant/
   

Certainly with Al's Collimation tool it requires the use of a camera / web cam connected tot the sciope and a laptop but I also use a webcam when using the duncan mask as it gives a view similar to that of a 6mm EP.

I think the duncan mask is more accurate but Al's Collimation Tool is quicker

Ian

[DrRobin]

Thanks Ian, it was a Duncan mask I had the most success with, I think. Thing is I got it nice in the Duncan mask and then tried another method and thought it was out, so adjusted and ended up with it miles out.

Bill, thanks for your comments, I agree what you say about the artifical star. Interested in the Hotech and I saw your other thread this morning, looks very good. Unfortunately they are currently out of stock at FLO, so might have another go with my Duncan mask and artificial star and then look to get a Hotech in the new year.

[alan potts]

I colimate mine on the stars I am looking at, I think mine is the same as the HD but just a bit bigger. I was worried at first and wanted to buy all sorts of bits and bobs to help, it really is not necessary and it is easy once you get into it..

Alan.

[DrRobin]

One more quick question, I put my Laser collimator in the eyepiece and the returned beam wasn't even visible, unless I slackened the screw and moved the laser to one side.

I wasn't expecting it to be centred when correctly collimated, but I would have thought that it would be somewhere near to visible?

Has anyone else seen this before and does this indicate that I am still not correctly collimated?

[Tim]

If you have a video camera with live view screen, and can connect it to the rear of the C11 with next to no wobble and held dead central, then you can use the camera to zoom in on the secondary and then centre the reflection in the view screen. Sounds complex but is dead easy and intuitive once you play with it a bit.

[DrRobin]

Hi Tim,

I tried that whilst trying to collimate by the visual method from the front. I expected to see the secondary, the primary and the front opening all concentric, but they weren't. I think I could get the primary and secondary concentric, but not all three and used this method when I took the secondary out of my newt. It was invaluable getting the secondary back in to the starting point.

I might give this a try as well. I downloaded a program which overlays concentric circles which works really well.

By the way I used a webcam with the lens intact and a 1.25" mounting bezel. Even if the webcam is not dead central, the primary and secondary should be centered on each other, correct?

[DrRobin]

Okay, another collimation session with my artificial star. Actually my artificial star produces three bright stars and one much dimmer.

I collimated using a defocused star and arranged the rings to be concentric. I then tweaked it using a Duncan mask and got it as close as I could. I then checked it with Metaguide, which had it just out so I adjusted it until it was spot on with Metaguide. Then a quick re-check with my Duncan mask and the defocussed star and it was miles out.

So again, I collimated it with a defocussed star, then checked it with my Duncan mask and the results are shown below. Measuring the circles, they are fairly concentric but elongated very slightly in north-east south-west direction, so slightly elliptical. The Duncan mask image is from the dim star and shows a very slight error.

Unfortunately, it was too icy and too much cloud to test it on a real star, but I now think it is very close to been collimated.

[Tim]

Hmm, not sure you could do it from the front?

The method I have used needs an optical zoom camcorder really for best precision.

All I do is manually focus the camcorder on the reflection of its own imaging chip in the secondary mirror of the SCT, and then zoom in on it, using the collimating screws to keep it central.

There needs to be just the right amount of light available inside the SCT.

I have actually broken the camcorder I used to do this by going under a waterfall with it, and haven't tried my new one yet, I think I need a couple of adapters to get it attached properly.

Just a point about artificial stars, dont forget when using them to let the tube become thermally stable first, otherwise the currents in the tube will make it more difficult.

Cheers

Tim

[DrRobin]

Hi Tim,

I saw a method (now can't find a link) that involves looking at the tube from the front, about 10 feet away for my C11 and looking at the central obs and the various rings caused by the mirrors and reflections. Then just adjust the collimation until they are all concentric. If it is way out, it is very obvious and this method appears to get you some where near, but not as accurate as a Duncan mask.

My scope is kept in a dry outdoor building so it is always at or near to ambient temperature. I did notice that over around 30m my artificial star showed some variance/disturbance due to the air between, so even this is not perfect seeing.

[Leebert]

A very interesting thread.

I have recently got hold of a C9.25 and, if I am honest, I think the views and images of Jupiter through it could be slight better (more contrasty).

I used Metaguide and that showed the little dot slightly off to one side but not much. I haven't tried any of the other methods for collimation but was concerned to read above the differences between the methods (Duncan mask verses Metaguide).

I have heard of the looking in the front of the SCT method. I read about it here: http://www.robincasady.com/Astro/myimages/collim.html

I like the idea of the camcorder method too but, as I mentioned before, I wouldn't be sure which method would get me closest without actually setting up and looking at a defocussed star. I'd be very interested to hear how you get on with your C11 DrRobin.

[Masvingo]

One thing to be aware of with the Edge HD is that, according to the Celestron Edge HD White Paper, the Edge HD 'scopes require much tighter centring and alignment tolerances than a classic SCT (see page 8).

James

[billhinge]

Hi Tim,

I saw a method (now can't find a link) that involves looking at the tube from the front, about 10 feet away for my C11 and looking at the central obs and the various rings caused by the mirrors and reflections. Then just adjust the collimation until they are all concentric. If it is way out, it is very obvious and this method appears to get you some where near, but not as accurate as a Duncan mask.

That method is only a very, very rough alignment to get you into a state that you can start to collimate properly, in reality you can be a long way out, remember the secondary is multiplying the focal length by a factor of 5

I used the Hotech laser collimator and when I deliberately screwed my collimation to test the Hotech I had to use your method to even register the reflections from the Hotech lasers on the collimating disk before I did a collimation.

There is still a lot of tweaking to get proper collimation, a tiny turn of the screws makes a big difference. Proper star collimation is supposed to be done as close to focus as possible, ie small rings, not with the giant out of focus diffraction rings you sometimes see on the internet (source https://www.willbell.com/tm/tm5.htm )

[DrRobin]

James, thanks for sending me the link to the white paper, I hadn't seen that, but took delivery of my scope before it was written. I think you are probably right with all of the corrector plates it will need to be spot on. Mind heaven help me if one of the corrector plates or the primary is out of alignment and needs adjustment, going on what it says in the white paper.

I have decided that trying to collimate on a real star and to some extent an artificial star is too subjective and for the most part too hard/cold at this time of the year, so [bill] I have taken the plunge and bought a Hotech Advanced from FLO now that they appear to be back in stock.

I collimate my newt with an laser and then check on a star and it is perfect every time so no reason why I can't collimate my C11 and get it right.

[kirkster501]

I got into a heck of a mess when installing my Bob's Knobs with my CPC1100 miles out of collimation. But got there in the end by the concentric circles from 10 feet away method to get roughly in the ballpark and then a star test. But I still don't think I have it bang on. Took me ages to get it sorted and traumatized me a little if I am honest!!! But may have another go with the tips on this thread.

[DrRobin]

Hi Kirkster,

My experience also. I made a very simple artificial star with a small aluminium torch from Poundland (guess how much), some cardboard, a piece of tin foil and a small piece of drafting film (tracing paper).

I made a cylindrical section to go over the torch and then a square piece on the end folded over which holds the aluminium foil and on the inside the drafting film. The drafting film acts a defuser to stop the LED from shining straight out.

Before the tin foil is mounted I made 3 or 4 holes in the tin foil of different sizes with a sharp pin. One is about 0.2mm in diameter, the others are around 0.5 - 0.7mm.

I put the completed assembly around 30m from the scope and use the bright stars first and then use the feint one when trying to get it really accurate. The performance is more stable than a real star, but even over 30m you see a fair bit of variation with the air in between.

If you want I will post a picture of the unit later, it didn't take more than about 20 minutes to make and was used for those pictures shown above in my post.

[kirkster501]

Hi drrobbin, would be interested to see that contraption please.....? I tried to make a artificial star for the same reasons but because the hole was not round I did not get circles when out of focus. This means collimating under our fickle skies of course, two weeks later as it turned out for me. Would be great to do this at will...

Reds, Steve

Sent from my iPhone using Tapatalk

[DrRobin]

Hi Steve,

I am still at work, so will have to take a photo when I get home in about another 2 hours. I am not sure how round the hole is or how much difference it makes. The idea is the same as a pin hole camera so I guess if it small enough, far enough away and reasonably round, it won't make any difference.

I took the plunge and bought a Hotech, so it will be good to compare the my artificial star with the scope when it is collimated with the Hotech. Certainly a home made artificial star is around 387 quid cheaper.

[Tim]

When conducting star tests, eventually you need to use a very high magnification for accurate results. IIRC up to a 3mm eyepiece is required, and a good quality one at that.

There is also a lot more to artificial light sources and the distances they are used at too, the theory of it all is quite complex. H R Suiter's "Star Testing Astronomical Telescopes" contains all the theory, equations and notes to enable accurate testing.

[billhinge]

You can buy a cheap but useable artificial star for $25 inc p&p http://www.hubbleoptics.com/artificial-stars.html I got one but my back garden was just short of space to test (ie achieve focus), alternatively most telescope sellers have more expensive versions eg astrozap etc. Yes Suiter is the bible for all this collimation malarky

[Peter Drew]

There is quite a difference in the accuracy requirements of the light source for collimating optics and testing optics for optical quality. The former can be almost any small distant spot that's bright enough, the solar image reflected from a pylon insulator for instance.

[DrRobin]

Thanks Peter and Tim for your expert advice on artificial stars, I am sure that there is considerable variance in the quality and almost any artificial star will not be as good as the real thing, but I think they can plug the gap between basic collimation and full and proper collimation on a real star, or stars as a couple of different brightness appear to be needed.

When I made my artificial star, I just spent a few minutes knocking something together to see how good/poor it might be. It probably isn't perfect but the cost was nothing as I had everything so there was nothing to loose right? I can't even claim it's an original idea, I say someone who made something similar, I just simplified it a bit.

Anyway a few pics for those who are interested....

The torch and completed A*

The front view of the A* showing the 4 pin holes in the foil, three larger ones to the top and a smaller one to the bottom

The rear view showing the drafting film to defuse the light. Note there is a small air gap between the drafting film and the foil, this is deliberate to help defuse the light better.

Finally, the A* working. Only the 3 bright stars are visible on my desk.

I hope this gives some people some ideas, it was meant to be low tech, to see if it would work and I reckon it can get me within a Gnat's whisker and make final adjustment on a real star easier.

[DrRobin]

One final think occurred to me. If the holes aren't perfectly circular and this has an impact on the quality of the light, then if 3 or 4 holes are made, rotated the foil between each one, then they should by out in different directions right? Therefore align the scope on all four and if the diffused pattern is out in the same direction in the scope on all four, it must be a function of the scope and not the holes.

And finally, I am not sure the torch came from Poundland after all, I think it came from a toolshop but didn't cost much more than £2.

[cathalferris]

It will not matter if the pinholes are out of round or not.

Firstly, as the smaller sized holes should be under the angular size of the airy disk, the shape of the actual pinhole is going to be irrelevant.

Also, once you defocus, that really makes it a lot harder to see if there is any non-roundness in the larger ones.

Any non-roundness will be generated by miscollimation, and if there is still a large out-of-roundness, please try to tilt your head while looking through the eyepiece to see if the axis of the out-of-round follows your head or stays with the scope. Any slight astigmatism will show up clearly with the out of focus test unless you use a camera of some type.

I know that with my 80mm, all of the stars on the hubble artificial star show airy disks from ~40 feet away, so if you can get your artificial star 40-50 metres away you will be golden.