35mm Film Cannister Collimation Cap

[Charic]

Just checking, and for the benefit of others, new to collimating, I  have just read two articles (there will be more)  and the implication was to discard the lid when constructing a collimation cap using an old 35mm cannister ?  

I was under the impression that the hole goes in the lid, with a white or reflective insert on the inside of that lid, to aid the visual appearance,   then the  base of the canister is cut off ( this piece  discarded)  The assembled unit is then inserted into the focuser tube.

Having the hole at the far end of the tube then inserted would be a visual hindrance, as your eye needs to be close to the hole to see through it.

Has anyone else built theirs upside down?

[Scott]

I don't think there's a right or wrong way as long as the end with the pinhole is on the outside, as you say, you need to be able to get your eye rightup to the pinhole. the main advantage of having the endcap on the canister with the base removed is that the lid stops it from falling into the secondary

[Charic]

.......cheers that was my interpretation too. 

[Stub Mandrel]

I did this canister trick  and it worked enough to give me round defocused stars, but what it didn't tell me that a laser collimator did, is that the secondary was badly lined up. The laser dot appeared about half way between the centre dot and the edge of teh mirror.

[beamer3.6m]

I did this canister trick and it worked enough to give me round defocused stars, but what it didn't tell me that a laser collimator did, is that the secondary was badly lined up. The laser dot appeared about half way between the centre dot and the edge of teh mirror.

Its not suposed to. You use it to centre the secondary.

A cheshire or laser is used to align the primary etc.

[Charic]

I chose a collimator,  had to collimate it on arrival, set to  about 18 feet to ensure rotational accuracy, and still found inaccuracies in using the laser in the Skyliner`s focuser tube, due to the fitting? Looked good one minute, take the laser out, re-insert and off again. I then discovered the Barlow-laser method, and now use the laser solely for this purpose when out in the field?

Always go for secondary mirror alignment first., then the primary second. Using just your own eyes, or a cap is the basic first steps. The tools just ensure accuracy!  You state that your defocussed Stars ( the Airy Disk appears to be concentric, that's good) but don't always trust the laser, unless you can re-fit it 100% accurately EVERY time.

The method I prefer now from scratch is to use the cheshire Tool. The other methods work once understood, I just prefer the Cheshire.

[Warped Drive]

I think the idea behind that one is that the end under the lid is perfectly round ( well almost ) from the factory and if you cut the bottom off it may be a bit ragged or elliptical thus making it a bit more difficult to judge if the secondary is lined up properly, but this is wide open to, as mentioned above, slipping straight through the focus draw tube and straight on to your secondary.

When I made mine the only black 35 mm canister I could find was too big to fit in the focuser of my Heritage 130p so I had to use an opaque white one which I cut the bottom of as carefully as I could and then I put some 100 grit sand paper on a flat surface and rubbed the cut end around until it was flat and even. Then I ran a piece of 400 grit around the inside and outside edge to remove any burr then I sprayed it Matt black inside and out and the inside of the lid gloss white and of course drilled a 2mm hole in the centre of the cap, works fine but because the Heritage has no draw tube as such I could do with getting a tube another inch or two longer than a film canister to help judge the secondary alignment a bit better or maybe I should just buy a Cheshire.

[Digger1895]

Can't remember which manufacturers they are but the black canisters with the grey lid always fit.

[Warped Drive]

I think the black and grey lid were Kodak, the all black one I have that doesn't fit is Fuji and the one I am using is from a cheap'o brand that One Hour Photo used to give out with every set of prints they developed.

[Moonshane]

I just bought one of these http://www.firstlightoptics.com/other-collimation-tools/rigel-aline-collimation-cap.html

[Moonshane]

http://stargazerslounge.com/classifieds/item/5743-film-canister-collimation-cap/

[John]

I've tried quite a few collimation methods and tools over the years including Baader and Hotech lasers, barlowed laser, cheap lasers and Tectron tools including their "Autocollimator"

The one I keep coming back to is a simple, cheap, plastic "shorty" cheshire eyepiece that came with a Skywatcher scope that I had years ago. I've carefully compared the results from other tools and methods though star testing and this plastic job consistently delivers the goods.

I'd imagine it would cost just a few quid to buy one of these - but it works

[Moonshane]

I agree John.  I always use that type for my primary collimation. It gives easily readable, repeatable results. I do use a standard cheshire for everything else though.

[Alfian]

I had to experiment quite a bit with film cannisters, they are not all the same. Some have a lip that the cap clips around and thus if you want to use it the other way round you need to snip this off. If you make a hole in the cap you need to snip the other end off which makes the tube less sturdy. I now use the unscrewed inner part of a worthless plastic barlow that came with my 70mm Travelscope and a focuser end/blanking piece, suitably holed, that neartly clips onto it. So effectively a collimating eyepiece. Simples.

[triton1]

I still use a film canister cap had to cut two notches in the end so it push fits into end of focuser,works well so does the cheapest laser I bought as long as you check its collimated then defocus on a star as John said to check.Certainly don,t have to spend a fortune to get good collimation that,s for sure.