Amateur telescope making

Re: Crayford Focuser Project

hpmachining — Tue, 28 Jul 2009 19:30:57 GMT

Well, I got busy and had to put this project aside for awhile. I believe I have a completed design now.

After getting my hands on some springs I decided to go with a little larger diameter and heavier wire spring. I have a 1/4" diameter x 3/8" long, .029 diameter wire spring designed in now. I am backing it up with a 5/16-18 x 1/4" set screw. If the design proves out to work the way I like I will probably make some knurled head thumbscrews for adjusting the tension, but the set screws will work for testing.

I also added bushings to the ends of the focusing shaft. They will have flats on the top and bottom and fit in a slot to allow movement back and forth but not up and down.

Here are links to some renderings and a drawing with section views.

http://www.hpminc.com/Focuser/Focuser_1.pdf

http://www.hpminc.com/Focuser/Focuser_2.pdf

http://www.hpminc.com/Focuser/Focuser_3.pdf

http://www.hpminc.com/Focuser/Focuser_4.pdf

http://www.hpminc.com/Focuser/Focuser_5.pdf

As some of you may notice I modeled my design and footprint based on Moonlite's focuser. I really like the way theirs looks on their web page. I have also read good things about them. If I was going to buy a focuser I would buy one of theirs.

As always, any comments are welcome. Thank you.

Re: Crayford Focuser Project

hpmachining — Sat, 04 Jul 2009 15:07:18 GMT

Thanks again, Randy.

savyone:
It looks like you've decided to use a 10-32 thread on your tensioners.

I have 3/16" dia. springs with a 1/4-28 screw figured right now.

savyone:
I would design the housing around a focusing barrel with a .250 wall. The flat on my focuser is .75 wide, plus you'll have more material to drill and tap for a thumbscrew to hold your eyepiece in place.

I'll play around a little with the wall stock. Right now I have .125 wall stock with a flanged head that has .250 wall stock for the thumb screw. The flat is .695 wide giving me .070 wall stock at the flat. How much clearance do you have between the barrel and the body? I have .062 on the diameter and was wondering what others have.

savyone:
Also try to incorporate some sort of bushing on each end of the focusing rod.

I was thinking about this but wasn't sure how I should go about this and still have adjustment. Maybe bushings with flats on top and bottom in a slightly elongated pocket to allow travel in one axis?

Thanks again for all the help. I'll post the latest revisions we discussed here when I get them done.

Re: Crayford Focuser Project

savyone — Sat, 04 Jul 2009 06:30:21 GMT

Your design is looking good. It looks like you've decided to use a 10-32 thread on your tensioners. If you find that springs of that size don't give you adequate pressure, you can eliminate the springs all together. This leads to another design consideration you might be faced with later on. I would design the housing around a focusing barrel with a .250 wall. The flat on my focuser is .75 wide, plus you'll have more material to drill and tap for a thumbscrew to hold your eyepiece in place. Also try to incorporate some sort of bushing on each end of the focusing rod.

Randy

Re: Crayford Focuser Project

hpmachining — Sat, 04 Jul 2009 01:54:27 GMT

Thanks for all the helpful input, Randy. I have plenty of 6061 so I will probably use that. I had decided on the 3/16” diameter shaft, as well. The 1/8” just seemed a little small.

I liked your thoughts on the tension adjusting screws, also. I tweaked around the locations to get them putting force on the flat as you suggested and added the thumbscrews. Here is a link to plan view of what I have so far: www.hpminc.com/Focuser/Tension_Adjuster2.pdf

Thanks again for all the great tips!

Re: Crayford Focuser Project

savyone — Fri, 03 Jul 2009 23:01:17 GMT

My suggestions on material are as follows: 6061 or 7075 T6 aluminum for the housing and the draw tube. A precision ground 3/16" dia. drill blank would work awesome for the shaft. Flat stone any machine marks from the barrel flat and polish. Keep the surface as flat as possible and as parallel to the axis of the barrel.You can apply a hard anodized coating (clear or black) to the barrel if you think wear is an issue. (I wouldn't worry about it.) You might even consider trying to find a replacement focuser knob with a microfocuser on it. Hope this helps

Randy

Re: Crayford Focuser Project

savyone — Fri, 03 Jul 2009 22:29:40 GMT

P.S. to my last post: Keep your tensioners so they apply direct force to the flat on the barrel. The sketch I posted is in my opinion a bad design because if not properly engineered, it could have the tendency to bend the focus rod...

Randy

Re: Crayford Focuser Project

savyone — Fri, 03 Jul 2009 22:25:11 GMT

Personally. I think your tensioner design if be placed symetrically, one on each side of the barrel axis, it should work just fine. One suggestion would be to use a thumb knob on your adjustment screw. Applied tension depends alot on the amount of weight at the e.p. and the fine feel you desire. Having a thumb screw will keep you from fumbling around in the dark for a allen wrench / screwdriver. Also, If you build a 2" focuser, you can have the best of both worlds. Once bitten by the Astrobug, you never know what lies down the road...

Randy

Re: Crayford Focuser Project

savyone — Fri, 03 Jul 2009 21:58:02 GMT

Here's a layout I pulled off the web for you. Good luck!...

Re: Crayford Focuser Project

hpmachining — Fri, 03 Jul 2009 20:25:45 GMT

Hello again. I have been working on designing a tension adjuster for this project. Here is a link to a PDF file of what I am thinking: http://www.hpminc.com/Focuser/Tension_Adjuster.pdf

I am planning on two of these. Do you think this will work well? Any comments or criticism is welcome. Also, as far as spring pressure needed, is there a "rule of thumb" to get me in the ballpark of what will be required?

Thanks again!

Re: Crayford Focuser Project

hpmachining — Fri, 03 Jul 2009 15:53:37 GMT

Thanks for the link, Bill.

Re: Crayford Focuser Project

Oriontaage — Fri, 03 Jul 2009 14:30:39 GMT

Hi,

Take a look at this link for some pictures of a very nice homebuilt focuser:

http://bedair.org/Focus/focus.html

Bill

Re: Crayford Focuser Project

hpmachining — Fri, 03 Jul 2009 11:52:59 GMT

Thanks for the reply, Chip. I am not a memeber of a club but there is one in the area. I think I will visit on one of the public viewing nights and check some out like you suggested. I am still in the design stage and trying to gather as much info as I can. This is going to be a side project to keep me busy when work is slow, so I am in no big hurry.

chipdatajeffB:
Where you really need finer control is for imaging, and it doesn't sound like the scope is in that league ...

I agree that the scope is not appropriate for imaging. I may look in to a dual speed design down the road just for educational purposes, but single speed is the plan for now.

Re: Crayford Focuser Project

chipdatajeffB — Fri, 03 Jul 2009 03:06:55 GMT

Are you a member of a club, or have a friend, where you could actually see a Crayford focuser before you start this? Might help answer a lot of question.

I think the wear and tear issue is not that much of a problem, given the way the Crayfords work.

I've used both single-speed and dual-speed Crayfords and my opinion is that a really well-made single speed is just fine for most purposes. Where you really need finer control is for imaging, and it doesn't sound like the scope is in that league ... so I would say that your shaft diameter should be large enough to hold up to repeated use, but not so large as to provide a coarse "feel". If you can make the action smooth, then "coarse" is a relative term here.

Crayford Focuser Project

hpmachining — Thu, 02 Jul 2009 23:10:11 GMT

Hello. I was given a used Meade 114EQ-DH recently and am interested doing some do-it-yourself improvements. The first project I want to make is a Crayford style focuser. Any links, information or tips will be greatly appreciated.

I am planning on using aluminum for the body and drawtube. Is this a good choice for these items? If not, what would you recommend?

For the focusing shaft I am planning on steel, but I was wondering if it should be something with some sort of softer coating on it. What do you think, will the steel against aluminum be able to be adjusted to give a smooth feel and not wear out the aluminum drawtube? While I am on the subject of the focusing shaft, will .125" diameter be a good size? If my calculations are correct it would give me .393" of travel per revolution of the knob. Does this sound appropriate for the travel per revolution? I am also a little concerned about the strength of the shaft being that small. Any thoughts on this will be appreciated.

Like I said before, any information you can pass along will be appreciated. Any pitfalls I should be aware of, etc. Thank you.