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Jaeger 114mm f/4.5 ATM frefractor help please

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#1 woodscavenger

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Posted 23 June 2014 - 02:13 PM

I bought one of these from SS. I scrounged the scrap metal places and found a perfect OTA from an aluminum drive line. It has a 5" ID that steps downI to 4" ID before ending at what would have been the drive line yoke which I have cut off. I had a PVC fitting laying around from another plumbing project and with a little help from my small wood lathe the lens will fit perfectly into the PVC for a lens cell and in turn the PVC fits perfectly into the aluminum tubing.

Using a cheap focuser from a small NEWT I cobbled the scope together, found the FL and walla had a neat little scope. Stars looked pretty clean but Saturn was not clean. Collimation needs work and I need to find the best mounting specs for the lens pair.

The lens has some fill on it looking like residue with an irregular discoloration in a red/orange rainbow type hue that is very thin.

1) How do I remove the discoloration?

I taped the lens pair together using the alignent mark from SS when they did rhonchi testing. They also had 4 small sections of tape that I left attached. The lens set is separated by the thickness of this tape.

2) How do you test for the best rotational alignment and separation distance for the lens pair?

3) Where do you find a reasonable cost metal focuser? The current one is plastic and has too much slop.

thanks for your collective brain power!!!

#2 bvillebob

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Posted 23 June 2014 - 04:31 PM

I got a 5" from SS and it had what looked like dried cement on the inside with an irregular edge, which sounds like what you're describing. I tried quite a few solvents and finally discovered acetone dissolved it pretty quickly.

With a scope that fast you need a large diameter focuser, at least 2". I made a 3" crayford focuser and for the 5" it's barely big enough with the steep light cone.

#3 Chuck Hards

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Posted 23 June 2014 - 04:43 PM

I got away with a 2" focuser by using a very short drawtube.
I'm using a 3" Baader Steeltrack for my Jaegers 6" f/5 build.

Here's my 4-inch Jaegers, and it's even on a Jaegers mount.

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#4 woodscavenger

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Posted 23 June 2014 - 05:34 PM

The point of a DIY refractor was to be cheap. Where do you find a cheap 2" focuser? Does that mean I need 2" EPs as well or can I step it down to 1.25"?

Any thoughts on the spacing?

#5 Chuck Hards

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Posted 23 June 2014 - 05:59 PM

I've been in this hobby since about 1968. In that time, I've accumulated... a lot of stuff. I had the focuser on-hand, it wasn't purchased just for the refractor project.
The drawtube was too long, and it did cause vignetting. What I did was swap it for the short drawtube from a low-profile Newtonian focuser from the same maker. No more vignetting.
Yes, a 2" focuser can accomodate 1.25" eyepieces with the proper adapter. But I think you'll need the 2" star diagonal anyway. A 1.25" star diagonal uses at least 2" of back focus and that may cause some vignetting.

Remember that a refractor will need to be used with a star diagonal. That's why you need a larger diameter focuser. If only used straight-through, you could use a very short 1.25" focuser, helical perhaps, and not vignette. But to put enough back-focus in for the star diagonal means a larger diameter focuser. If I didn't have short drawtube on hand to swap-out, I would have gone with at least a 2.7" ID focuser.

I mocked-up my system before I cut any tubing, using the eyepieces I thought I'd use most with the objective. You'll need to determine just what combination of star diagonal, focuser, and eyepieces you intend to use, in order to nail down the final tube length.

Cheap DIY refractors are the long f-ratio ones. The short ones will cost you more money.

#6 bvillebob

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Posted 23 June 2014 - 11:58 PM

Here's another option I've seen used and I may convert my scope over to this. Put a diagonal mirror in it, like in a reflector, and put a focuser on the side of the tube.

I need to do a CAD drawing to follow the rays but it seems like you ought to be able to put the mirror a little further down the light path so a smaller focuser would work. A newtonian secondary would be best, but I have a spare 2" diagonal which could loan its mirror for the project to see how it works.

#7 woodscavenger

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Posted 24 June 2014 - 12:13 AM

Chuck, thanks for the info. Guess I didn't do my homework first.

Bob, thanks for the offer on the mirror and the great idea with the Newt type arrangement but since I will have it on a GEM mount needing to change EP orientations would probably diminish the benefits. In an alt/az it would probably be a great option using a simple first surface mirror.

#8 bvillebob

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Posted 24 June 2014 - 12:26 AM

Doing a quick drawing and it's pretty striking. Putting a 2" diagonal at about 480mm from the lens and the light cone is only 18mm wide where it exits the side of the tube so it looks like even a 1.25" focuser might work.

#9 Chuck Hards

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Posted 24 June 2014 - 06:48 AM

Bob, I'm really liking the Newt secondary idea. Woodscavenger, it's not as impractical on a GEM as it may sound. If you can figure out a way to inexpensively rotate the tube in the saddle, you can place the eyepiece in a convenient position. Even with Newts on Gems without a rotating tube, you can position it so that the focuser is typically in an accessible position.

This could be made very inexpensively, and perform very well.

Reminds me of the old unperforated Cassegrain design. A diagonal mirror is used just in front of the primary to deflect the optical path out the side of the tube, instead of through a hole in the primary and out the back end.

#10 figurate

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Posted 24 June 2014 - 07:12 AM

Bob, I have something similar in mind for a 60mm finderscope I'll be building in two or three months. The problems are not insurmountable, but you do have to have provision for alignment of the secondary mirror, as well as probably needing to adjust or shim the focuser (I have a simple design that allows for fine-tuning of focuser axial tilt). Also the quality of the mirror becomes more of an issue, the closer it is to the objective. Old newtonian secondaries come in handy for these kinds of things. The point about eyepiece rotation is a good one.

I have a couple of the SS lens sets and apparently the light-colored stains are very common. Mine aren't too noticeable but both lenses have them, and there has been speculation on CN that it is smoke residue. Based on what I have, I wouldn't worry about an impact on performance.

Fred

#11 figurate

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Posted 24 June 2014 - 09:28 AM

Bob and Chuck, actually you could cut four slots accurately with a hacksaw at the rear of the refractor, and use the whole newtonian secondary hub and mirror assembly, provided the vanes were stiff enough at the hub junction. It would center itself and give you your adjustments that way. Just a matter of sawing off some vane length and getting a friction fit inside the slots. Alternatively, drill a hole to fit the threaded secondary shaft in a wooden or aluminum plug closing up the rear of the tube.

Fred

#12 woodscavenger

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Posted 24 June 2014 - 01:29 PM

Bob, you mentioned a quick drawing. Are you talking about pen to paper? I have been looking for a raytracing program but cant find one for refractors. Is the FL (495mm) measured from the point of first glass contact or at the moment the light exits the back of the lens?

#13 bvillebob

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Posted 24 June 2014 - 05:21 PM

I was talking about a CAD drawing. I just drew in the 127mm lens and a 630 mm focal length, creating a triangle showing the edge of the light cone. I put the mirror about 480mm from the lens, which cut off the light cone, then cut that portion of the light cone with the CAD software, rotated it 90 degrees and mirrored it so that it was pointing straight up so I could measure the width of the cone where it hit the tube. I'll try to figure out how to post the image this evening, easier than it sounds. I'm not pro at this so I think that's a valid way to model but if I'm wrong I'd like to know.

Like figurate said, my thought was just to build a spider assembly and mount it in the tube, it could be accessed from the rear of the tube for collimaton.

#14 bvillebob

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Posted 24 June 2014 - 05:25 PM

OK, here's my quick sketch just to show the idea.

It looks like with some playing around the mirror could be moved back a little further to bring the focal point closer to the tube, but it's pretty close as is. The cone is 18mm wide where it crosses the tube.

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#15 figurate

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Posted 24 June 2014 - 08:21 PM

A couple of points: (and I'm no expert either) your drawing shows a diagonal positioned closer to the lens than is required; as shown, eyepiece height would be three or four inches above tube. The light cone depicted shows full illumination for center of field only. You can get a better idea by tilting the cone a desired amount to get a given area of full illumination (move the vertex, for example, .5" or .75" up or down in your sketch). That gives a better representation of what you will see in the eyepiece.

Under the circumstances, there would be no upper limit to the secondary size other than the tube itself, but understand that with a focal length that short, a 1 1/4" 25mm EP can give you a nice RF power of 25x with a 5mm exit pupil. Also, field curvature becomes visible at very low powers with wide field EPs, meaning full illumination of that 2" eyepiece field lens could backfire because stars become fuzzballs near the edge. Plus CA gets bright.

Fred

#16 Chuck Hards

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Posted 24 June 2014 - 08:55 PM

What is the sketched size of the fully illuminated field? Difficult to tell, but it looks to be on the small side. I'd make it larger on such a fast telescope, to take advantage of the wide-field potential.

#17 woodscavenger

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Posted 24 June 2014 - 11:52 PM

OK now my puny brain is getting stretched too far talking about these technical issues. I hope one day to be able to talk in these terms with understanding and clarity about things like light cones, full field illumination, field curvature and exit pupils. You guys are great.

As we speak I am building. Right now waiting for paint to dry....literally.

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#18 woodscavenger

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Posted 24 June 2014 - 11:54 PM

Putting the finishing touches on the lens cell. It is a PVC fitting. I epoxied 4 T-nuts from the inside which should allow me to fine tune the lens collimation. The OTA will have elongated holes to allow for forward/backward adjustments.

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#19 figurate

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Posted 25 June 2014 - 12:32 AM

Works for me. Your tube looks great, by the way. If the cell is easy to get to, I would test for best rotational position using trial-and-error and a star test at 100x. Wait for a night with good seeing. I think you'll enjoy it!

Fred

#20 bvillebob

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Posted 25 June 2014 - 12:35 AM

That looks great. I wish I had tried the PVC fitting idea, I machined mine out of aluminum, turning and threading the .040 or so retaining ring wasn't fun.

Regarding the sketch, the fully illuminated area is 18mm, it's just a preliminary proof of concept sketch, it still needs to be optimized.

#21 Chuck Hards

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Posted 25 June 2014 - 07:10 AM


Regarding the sketch, the fully illuminated area is 18mm, it's just a preliminary proof of concept sketch, it still needs to be optimized.


Thanks for clarifying that.

#22 dan_h

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Posted 25 June 2014 - 08:07 AM

Here's a layout I have used for an RFT refractor to maximize illuminated FOV using a small mirror and an inexpensive low profile focuser.

dan

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#23 Chuck Hards

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Posted 25 June 2014 - 08:14 AM

I like that, dan. A lot. Effective yet inexpensive.

#24 woodscavenger

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Posted 25 June 2014 - 08:56 AM

So last night I stuffed the lens cell in place, bolted on my cheap focuser and jumped out the the partly cloudy skies. Pinpointed Acturus and took a peek. Nice orange color. As I rolled in and out of focus the star would flatten then going the other direction would rotate 90 degrees and flatten again. Am I dealing with colimation issues or pinched optics? When I put the lens in this time I used a few more layers of tape and it was a bit tighter fit.

#25 Chuck Hards

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Posted 25 June 2014 - 09:55 AM

Pinched optics sounds very likely.






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