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4lefts

please help me build a refractor

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hello,

i recently got a 60mm f16.7 carton lens from richard day at skylight telescopes, with the idea i am going to build a refractor. richard has been really helpful with questions, but frankly i have no idea what i'm doing, and kind of learning as i go.

can anyone point me to any good resources for designing this sort of thing? for example, working out how long the tube should be? how can i work out how long to cut the tube so that, when the focuser is fitted, and the diagonal, it will be the right length? i'm guessing i need to know the length of the light path through my diagonal?

my plan at the moment is to get dan at www.crawmach.com to make me a focuser and a lens cell (the lens came in a plastic one, and i hear they're not so hot). i need to source a tube (i'm pretty sure i'm going flock it with that protostar board stuff).

can anyone recommend what kind of tube i'd need? this thread is really useful (tanks!), so i'm thinking of getting a meter of 2.75" aluminium tube from http://stores.ebay.co.uk/Forward-Metals-Ltd? 16 swg? or is that too flimsy and i should get 10 swg?

as you can probably guess, i've never done anything like this before. my plan is to research thoroughly and take my time, i'm not in any rush. any help is much appreciated.

cheers.

stephen

btw - i forgot to mention, my dad can help me with any machining and stuff, he's an experienced model engineer and has a lathe and things.

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I can't help much but the focus point should be 60mm x F16.7=1002mm so 1m with focusser.

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thanks, that's what i was thinking. i just don't want to cut the tube, then find out that the diagonal adds way more to length than i thought and i can't get it to focus.

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Great project :) Something I'd like to tackle some day. Good luck and please keep us informed of your progress :) I love these DIY projects :)

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thanks, that's what i was thinking. i just don't want to cut the tube, then find out that the diagonal adds way more to length than i thought and i can't get it to focus.

No no no, that's no problem at all. That means you just take it apart and cut some more off. What you don't want to find out is that you've cut the tube and it's now too short to come to focus.

If you're buying a diagonal and focuser I'd expect the vendors to be able to tell you the length of the light path of each, so personally I'd choose a length of tube that would put the focal plane close to the full extension of the focuser travel, assemble it and make sure it focuses and then shorten the tube a little more. How much more depends on what range of movement you have in the focuser I guess.

James

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No, that's not right. I don't mean full extension. I mean full retraction.

Another tip. Don't design it after drinking a decent proportion of a bottle of wine with dinner :)

James

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thanks for the help, that makes sense. should have realised that obviously you can alway cut more off but you can't put it back. the diagonal i'm using is a 1.25" wo dialectric. i was hoping their site would say what the length of the light path through it was, but no. not to worry - i think it'll be a bit trial and error anyway...

@gina. i'll keep this thread updates with progress. it might be slow going though.

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You could always drop them an email to ask. An average figure for 1.25" diagonals is probably around 70mm.

James

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A mirror diagonal doesn't alter the light path, it just redirects it, so WO will not have anything to publish. I suspect you mean how much of the light path is inside the diagonal.

Why are you having a focuser made ? They can be bought pretty easily, TS sell them, least expensive they have is 27 Euro for a refractor focuser.

One of the best things to get would be some sheets of graph paper and produce a scale drawing.

One without the diagonal in place one with it in place - you could do it on the same sheet.

Are you familiar with producing the path of light rays through a lens to form an image at the focal plane?

You need to do this to get the baffle position and size correct - assumes that you intend to baffle it.

Material, aluminium tube would be best, just costly. Not sure if you could use a rigid plastic tube.

From playing with a few plossl's I think, and it is just a first impression, that the focal plane of the lens should coincide with the top of the diagonal eye piece holder tube. I only have this idea from trying to determine something concerning placement of a reticule so could be incorrect. You had better check it for yourself before you cut anything.

The 1.25 diagonal I have appears to have 85mm of the path in it, assuming I have measured from the correct place, the 2" diagonal has some 130mm inside it. I presume that you will use a 1.25" diagonal as the lens is 60mm.

Which to me, and this is just a first guess, means the tube and whatever the diagonal slides into would be some 915mm in total length, the final 85mm being in the diagonal making a path of 1000mm. Usually the physical mid point of the lens thickness is wihere you measure from.

Now I am puzzled: You are in Exeter, Dan of Crawford Machine is in North Carolina, the least expensive focuser he does seems to be $110, using say $25 shipping and say $10 insurance that makes $145. Close to £100, then 25% VAT and duty, so you are looking at around £120-125 for a focuser only.

Edited by ronin

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A mirror diagonal doesn't alter the light path, it just redirects it, so WO will not have anything to publish. I suspect you mean how much of the light path is inside the diagonal.

I didn't read it as suggesting that the length of the light path would be changed, but as you say, the relevant issue is how much length of light path the diagonal "consumes" in the optical train. That varies between makes of diagonal and types of diagonal (prism diagonals have a totally different light path to mirror diagonals for instance, and the light path varies depending on the seating of the diagonal in the focuser and on the length of the diagonal's eyepiece holder) so it's a useful piece of information to know and something I'd expect WO to have the information for even if it isn't on their website.

James

Edited by JamesF

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A mirror diagonal doesn't alter the light path, it just redirects it, so WO will not have anything to publish. I suspect you mean how much of the light path is inside the diagonal.

Why are you having a focuser made ? They can be bought pretty easily, TS sell them, least expensive they have is 27 Euro for a refractor focuser.

One of the best things to get would be some sheets of graph paper and produce a scale drawing.

One without the diagonal in place one with it in place - you could do it on the same sheet.

Are you familiar with producing the path of light rays through a lens to form an image at the focal plane?

You need to do this to get the baffle position and size correct - assumes that you intend to baffle it.

Material, aluminium tube would be best, just costly. Not sure if you could use a rigid plastic tube.

From playing with a few plossl's I think, and it is just a first impression, that the focal plane of the lens should coincide with the top of the diagonal eye piece holder tube. I only have this idea from trying to determine something concerning placement of a reticule so could be incorrect. You had better check it for yourself before you cut anything.

The 1.25 diagonal I have appears to have 85mm of the path in it, assuming I have measured from the correct place, the 2" diagonal has some 130mm inside it. I presume that you will use a 1.25" diagonal as the lens is 60mm.

Which to me, and this is just a first guess, means the tube and whatever the diagonal slides into would be some 915mm in total length, the final 85mm being in the diagonal making a path of 1000mm. Usually the physical mid point of the lens thickness is wihere you measure from.

Now I am puzzled: You are in Exeter, Dan of Crawford Machine is in North Carolina, the least expensive focuser he does seems to be $110, using say $25 shipping and say $10 insurance that makes $145. Close to £100, then 25% VAT and duty, so you are looking at around £120-125 for a focuser only.

hi ronin - lots of really useful info there, thanks for your help. you're right, it was a poor choice of words. i meant that the diagonal consumes a certain amount of the light path, so i was just thinking out loud about how to calculate the length tube would need to be.

the idea of using graph paper is a good one. it might even be feasible to draw it out full size and tape some sheets together. i wasn't going to make baffles, just because i can't for the life of me see how i would get them into such a narrow tube. i was hoping protostar flockboard would be enough?

i know crawmach isn't the cheap option for the focuser, but my understanding is that the ts r&p ones are just that, i.e. cheap r&ps. the reviews i've seen suggest that the crawmach crayford is a much better product, and i thought, if i'm going to get a lens cell/counter cell made there, i might as well get a focuser too. the crawmach is what skylight use in their 60mm scopes. i think if i went to all the trouble of making a scope and didn't use the best parts i could, i'd always wonder whether i should have.

you're right of course about the cost/taxes. still... cheaper than a moonlight :Envy:

like i say though, i'm really thinking out loud here. i haven't actually pulled the trigger on the materials yet, so it's all subject to change.

thanks again for your help, guys

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in case this info is useful for anyone - i had a reply from william optics. the light path of their 1.25" diagonal is 80mm (and 110mm for the 2").

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Hi,

I`ve recently built a scope with an 8" mirror and used 10" pvc vent tube but its very brittle in cold weather so you have to be careful whilst handling. I`ve been told that carpet roll tube is good and comes in various diameters. You just have to paint it to make it weatherproof.Well there`s a start I suppose.

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A length of 3" class D or E or class 7 (thicker) if pvc tube should well encompass the diameter of a 65mm lens. Just keep it out of the sun or any direct source of heat or it will warp.I think Aluminium as suggested is the best route to take on such a small but intricate scope. And with the machining facilities available the most viable option.

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16swg aluminium tubing will be perfectly adequate for the main tube. :smiley:

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Hi.

Congrats on joining the Carton 'club' ;)

Brilliant wee lenses!

Looks like Sheldon Faworski still has 1000mm FL Carton tubes left. Fully painted inside n out and also baffled(well, mine is). http://www.cloudynig...&cat=500&page=4

I got my lens n cell and tube from him, added a crawmach 1.25" focuser, shortly after.

After a long search, got lucky with a super cheap, but great little set of rings. Not easy to source rings for these wee tubes.

Got some pics on here, if you're interested.

Cheers,

Andy.

Edited by AndyH

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andy - wow, looks nice. thanks for the info - were there any issues fitting the crawmach focuser to Sheldon Faworski's tube? i guess dan has made lots for these tubes and it's a standard size?

thanks

stephen

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andy - wow, looks nice. thanks for the info - were there any issues fitting the crawmach focuser to Sheldon Faworski's tube? i guess dan has made lots for these tubes and it's a standard size?

thanks

stephen

No issues whatsoever. I think a lot of these 1.25" focusers have been going on the backs of 60mm Cartons, even before Richard @ Skylight started his 60mm Carton lensed beauties. A lovely focuser which matches the length of the tube perfectly.

You'll notice the alluminium cell that my lens came in. It is threaded to suit the thread on the outside of the tube. So it was just a case of screwing the two items together. No need for a counter cell, as the alignment is spot on.

My only head scratcher was trying to get some cheap tube rings. One day when money isn't tight I might get some nice Crawmach rings.

Sheldon and Dan at CrawMach know each other so if you were to get a tube, perhaps he could send it to Dan. Both are nice guys. Just a thought.

Andy.

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