Collimation with a difference

[alan potts]

I am picking up a Hotech laser from a site member when I am in England but may be I don't need one. How do you check the collimation on a refractor? Can it only be done by start test?

When I have looked either side of focus in the past you just seem to see a paler round ball of light.

Alan.

[Moonshane]

never done it but this seems reasonable http://philjay2000.tripod.com/usefulstuff/adventures.pdf

[Stargazer_00]

I put my Hotech in my ST120 just for a laugh. I put the cap on the objective end and took the small 30mm cap off which was dead central and held a piece of paper over the hole to see if the laser dot was central or not. It was. If it wasn't I wouldn't have had any way of correcting it was the cheaper 'fracs like this offer very little if any way to collimate them.

I would expect higher end 'fracs to have collimation screws of some kind but I'd probably never touch them myself. Like i'd never strip down an SCT. Newts are definitely the way forward for me because I like to 'get in there'

[Adrian Condon]

Hi Alan,

I was not aware that you could collimate a refractor at home.

I read in the book "The Backyard Astronomer's Guide" by Terence Dickinson &

Alan Dyer on collimating the optics the following sentence:

"Commercially made refractors or Maksutovs are collimated at the factory and generally offer

no user-adjustable settings. In the event that the optics do require collimating, it usually means a trip

back to the manufacturer."

There is a link that may be helpful in determining whether you need to have your refractor collimated,

http://www.ozscopes.com.au/collimating-a-refractor-telescope.html

Trust your scope is perfectly collimated.

I have a Swarovski birding scope, it has been used in various locations, even at SGL8 and it keeps perfectly

collimated.

Cheers

Adrian

[alan potts]

Adrian,

Thank you for you input. I agree most refractors are not adjustable mine however is as it is a bit better than run of the mill. Having said that I took the dew sheild away and saw the mass of alen screws to make the adjustment with. I think it's a hope it is OK situation as I don't know which to start with, there are 24 of them and that to me is 24 mistakes waiting to happen.

Alan.

[E621Keith]

I read somewhere you can check refractor lens cell collimation with a Cheshire, but alignment between various individual elements are so critical it has to be done in the factory.

There was a thread on refractors collimation a few years ago

http://stargazerslounge.com/topic/57879-refractor-collimation/

Hi Alan,

I was not aware that you could collimate a refractor at home.

I read in the book "The Backyard Astronomer's Guide" by Terence Dickinson &

Alan Dyer on collimating the optics the following sentence:

"Commercially made refractors or Maksutovs are collimated at the factory and generally offer

no user-adjustable settings. In the event that the optics do require collimating, it usually means a trip

back to the manufacturer."

There is a link that may be helpful in determining whether you need to have your refractor collimated,

http://www.ozscopes....-telescope.html

Trust your scope is perfectly collimated.

I have a Swarovski birding scope, it has been used in various locations, even at SGL8 and it keeps perfectly

collimated.

Cheers

Adrian

Alan's APM LZOS 115 had a nasty fall. It was a miracle the objective survived. I wouldn't be surprise it was knocked out of collimation by that kind of impact.

http://stargazerslounge.com/topic/183373-not-a-good-week-for-televues/

[andrew s]

I have in the past collimated a refractor using a Cheshire eyepeice. However, that was in the days before high quality anti-reflection coatings on all lens surfaces which I suspect will dim the reflected images. I uses the Cheshire in the normal way and coverd the lens with a cloth. I then adjusted the collimation screws until the various reflected images were concentric. It was much simpler than using a star test (at least it was for me).

Andrew

[John]

I use Andrews method with the cheshire. It's a good way of working out if the objective needs to be tilted a bit. Before you do that however, it's worth checking to make sure that the focuser central axis matches the optical axis of the scope. I use a well collimated laser collimator for this, putting it in the drawtube (no diagonal) and seeing where the beam exits the objective lens. It should be dead center of course. Often it's not and you need to tilt the focuser slightly and sometimes shim it to keep it that way and the laser in the centre of the objective.

[alan potts]

Nice new picture John, I guess that is your 12 inch. Thanks for the advice, I am going to have a go at my 70mm ED which is not good from a collimation poitn of view using your method. I feel confident to try this on a small scope

Good news I have checked and re-checked the 115mm APO and as far as I can see it is fine, saw the Pup so it can't be bad can it. The out of focus star both sides of focus showed many concentric rings and as far as I could see the pattern was a perfect circle. According to the infomation Moonshane sent me this is as good as it gets. I have got off lightly as far as I can see.

I used the 13mm Ethos for most of the night, what an eyepiece!

Thanks to everyone for the tips and info.

Alan.

[John]

Your 115 frac sounds great Alan. I've not been able to split Sirius with my ED120 as yet. The Dog star is getting very difficult to target here now though.

My experience of refrectors is generally of units that are not built to the quality of an APM so the mechanical tolerances would have been somewhat more "relaxed" I reckon.

Yep, I thought the 12" OO deserved a spell as my avatar

[andrew s]

I use Andrews method with the cheshire. It's a good way of working out if the objective needs to be tilted a bit. Before you do that however, it's worth checking to make sure that the focuser central axis matches the optical axis of the scope. I use a well collimated laser collimator for this, putting it in the drawtube (no diagonal) and seeing where the beam exits the objective lens. It should be dead center of course. Often it's not and you need to tilt the focuser slightly and sometimes shim it to keep it that way and the laser in the centre of the objective.

One of the advantages of the way a cheshire works is that it does not need to be exactly square on the optical axis to work as its mirror is rough and uses specular reflection. This measn it will collimate the objective accuratly even if the focuser is off.

John's point on checking teh focuser is a good one. Personally I would colimate the objective first. Also, using a laser to adjust the focuser can be difficult (in my experience) as the locking mechanism can shift the laser wrt the focuser as it is tightened various amounts. When adjusting the focuser check the beam exits at the same point as the focuser is racked in and you over the full range you use.

Regards Andrew

[alan potts]

Andrew,

I will take note of what you say because I want to sort out this 70mm ED that is not very good, it's a long story. I have two of these so I know how good it should be and isn't. It will be a month before I am can do this so I will re-read the thread a few times so I know what I am up to. If I hit any snags maybe you could help out by way of a PM.

John,

I don't think there is anything wrong with your scope it is where I am that makes the difference, I never saw this from Hull it was just too low down, I know your better placed than I once was but it will only be a small amount higher in the sky. It was so still tonight, no scintillation to speak of, it was that good i had to check it was Sirius twice.

Alan.

[Stargazer_00]

really pleased the scope survived alan. close call for sure but it sounds like its come through ok.

[andrew s]

Andrew,

I will take note of what you say because I want to sort out this 70mm ED that is not very good, it's a long story. I have two of these so I know how good it should be and isn't. It will be a month before I am can do this so I will re-read the thread a few times so I know what I am up to. If I hit any snags maybe you could help out by way of a PM.

Alan.

Happy to do what I can to help by way of PM - Andrew

[Dunkster]

John,

I don't think there is anything wrong with your scope it is where I am that makes the difference, I never saw this from Hull it was just too low down, I know your better placed than I once was but it will only be a small amount higher in the sky. It was so still tonight, no scintillation to speak of, it was that good i had to check it was Sirius twice.

Alan.

Yeah the latitude counts, Canis Major actually resembles a dog from Florida

Glad to hear the APM didn't suffer any ill effects from being overcome by gravity