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J.T.'s 8" f/10.6 Ed Jones Chiefspiegler build

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#26 jtsenghas

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Posted 07 September 2019 - 11:24 AM

Thanks, Mladen! 

 

The main reason for my question is that I am in fact considering masking a wee bit of the edge to improve the image and slightly reduce the primary tilt or scope length.

 

The secondary could  be a little lower for the same tilt angle, or a little closer at the same height for a lesser tilt angle.

 

If I were to mask it at 6.9", that would be a 26% reduction in light..... ( (6.9/2)^2)/4^2 = 0.74

 

Decisions, decisions.... 



#27 MKV

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Posted 07 September 2019 - 11:50 AM

Before masking anything off, I would do a star test first to determine the quality of the image. Likewise, I would test for contrast (planets, the moon, US Air Force resolution chart, etc.) to see if there's any perceptible loss of detail. If not leave it as it is. Even if the primary were left completely spherical -- it's still about λ/7 wave optic!  My guess is the mirror may be the leats of your worries! ;o)

 

Mladen



#28 jtsenghas

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Posted 07 September 2019 - 01:17 PM

That may be. It's just that, unlike a Newt , the prescription for lens tilt and prescription doesn't lend itself easily to later masking and changing dimensions to take advantage of the smaller aperture. Unless, of course I make a  7 foot Newt star tester first.

 

I'll probably use it as is and dimension for all 8".



#29 jtsenghas

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Posted 14 September 2019 - 01:31 PM

Okay, today the last of the purchased optical components for this project arrived, and it has presented a problem on this project, which may in fact be an opportunity in disguise.

 

Ed Jones told me last month by PM that approximate distances for my 8' f/10.6 optical design are: primary to lenses 73.25", distance between lenses 0.45", and distance from lenses to focal plane, 11.57". Remember, the secondary will go between the primary and lenses. The larger the secondary, the shorter I can fold it. 

 

Except for the lenses, which I got from Newport Lenses, all other optical components were obtained through CN classifieds. I opted for a Meade focuser salvaged from a 90 mm refractor since it was sturdy, cheap, and appeared to have an adequately long draw tube to allow me to set up either with or without the diagonal. That length expectation was more than met:

 

20190914_140931_compress53.jpg

 

Now this focuser is a whopping 9.25"long! The dielectric diagonal, according to its literature, is 9 cm, or just over 3.5" of optical length. Together they add up to 12.75" approximately.

 

With Ed's rough figures, that puts both lenses slightly INSIDE the draw tube. That's the problem. 

 

So here's the opportunity. I could trim an inch off of the draw tube easily, and another 3/4" if I drill and tap a couple of 4-40 or 2 mm holes to move the rack (I haven't checked screw sizes yet). 

 

I could in fact make a cell for the tilted lenses that is attached to the draw tube, much like a Jones Bird has the negative lens in the drawtube. That would make alignment, distance, AND tilt easy to set with respect to the focal plane. If the first iteration is off a little, a replacement cell could be easily built. This would ride with drawtube inside a box that has the focuser at one end and the secondary at the other. 

 

I could build that cell or 3D print it to attach to the 1.75" approximately draw tube, and that would keep it accurately distanced from the focal plane, which has a pretty tight tolerance anyway. This would mean I would have to always use the diagonal, but I'm okay with that. It is worth it to me for the improved ability to center and set the tilt and distance of the lenses.

 

Thinking ahead, if someone wanted to make Chief kits for standard optics, say, with Discovery Optics 8" f/8 primaries, a standardized cell to attach to a standard refractor focuser could be made for that prescription!

 

That would make an 8" chief only slightly more difficult to build than a dob! 

 

What would we call such a correcting cell?

 

An "ASTIGMACORR", perhaps? 


Edited by jtsenghas, 14 September 2019 - 02:13 PM.


#30 Ed Jones

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Posted 14 September 2019 - 02:53 PM

 

I could in fact make a cell for the tilted lenses that is attached to the draw tube, much like a Jones Bird has the negative lens in the drawtube.

Well I wouldn't do that, the BFL from the lenses should be fixed not moving with the eyepiece unless you leave it fixed and maybe add helical to focus the eyepiece?.  You are too long however; cut it off and use extension tubes if needed.   Did you get the refractor tube with it?  That's a very easy way to hold the lenses, focuser and secondary and also allows rotation for alignment.

 

A guy at work has promised to print me a lens holder cell but we'll see if he delivers.


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#31 jtsenghas

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Posted 14 September 2019 - 03:09 PM

Okay, if the lenses should be fixed with respect to the primary, not the focal plane, I'll just trim the draw tube as needed for clearance. 

 

I don't have the corresponding tube, so I plan to build a lightweight box to hold the secondary and lens cell. A square cross section may make it easier to make lens holder and adjust tilt.


Edited by jtsenghas, 14 September 2019 - 03:12 PM.


#32 jtsenghas

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Posted 14 September 2019 - 03:14 PM

I know the tilt of the lenses will depend in part on the tilt of the primary. That will depend in part on how large a secondary I use. I'll try to get my prospective secondaries boxed up this weekend and off to you for evaluation. Thanks! 


Edited by jtsenghas, 14 September 2019 - 03:16 PM.


#33 jtsenghas

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Posted 14 September 2019 - 03:20 PM

I understand how the refractor tube in your Chief video gave you a couple more degrees of freedom, including rotation. There are a number of ways to accomplish alignment though. I might get an appropriate sized tube, or I might construct something that better lends itself to the trusses I'll be using. 


Edited by jtsenghas, 14 September 2019 - 03:21 PM.


#34 jtsenghas

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Posted 14 September 2019 - 03:27 PM

Since I'll almost be certainly using an elliptical secondary turned sideways, a built tube may have additional advantages in my case over a cylindrical tube.... 

 

That is, unless anyone following along offers for sale at a decent price a round flat secondary in the 3" to 3.5" diameter.... 

 

Anyone? 



#35 Ed Jones

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Posted 16 September 2019 - 02:33 PM

Well a rotatable tube gives only one more degree of freedom for alignment but an important one.  You might find a used scope on Craigslist for a tube or Public Missles has a 3.9 inch phenolic tube for ~$20+.  Of course a 3.5 elliptical doesn't work with it.  Ring clamps make a good mounting method too.



#36 jtsenghas

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Posted 16 September 2019 - 04:29 PM

It appears I now have 3" round ZERODUR flat on its way from a CN member who responded to my request. If this is indeed at least the 1/10 wave it claims to be it is the most likely contender for this project.  I was offered a 1/20 wave one from another that no doubt would have been excellent, but was a bit out of my budget.

 

Here's a question for Ed. If a round flat has a little astigmatism, would it be best to orient it so that curvature is in line with the primary tilt? I imagine that when  lens cell tilt is tweaked it could not only adjust it some astigmatism from slight power (if regular) of that tilted secondary, but also some inherent astigmatism in the flat itself. 



#37 jtsenghas

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Posted 16 September 2019 - 05:20 PM

Well a rotatable tube gives only one more degree of freedom for alignment but an important one...

Understood, and a tube clamp also allows for a little longitudinal adjustment for focal length. My focuser appears to fit a tube with an inside diameter of 86 mm. That would allow the 3" round mirror I now hope to use as a secondary to fit into it, even at the required angle.

 

That seems an odd size in either inches or millimeters. A 3 7/16" OD tube would have to have a wall thickness of only 0.026" or 0.66 mm to match that. A 3.5" tube would have to have a thick wall of about 1/16" to match. 



#38 Ed Jones

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Posted 16 September 2019 - 09:34 PM

 

If a round flat has a little astigmatism, would it be best to orient it so that curvature is in line with the primary tilt?

Hopefully it is just regular astigmatism then you align one axis to the tilt axis.  The same holds for the primary too if it has astigmatism.


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#39 jtsenghas

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Posted 10 December 2019 - 03:14 PM

Okay....

 

I'm returning to this project as a wintertime diversion to hopefully get done before spring along with my 12.5" f/4.3 dob.  I don't want to have another contender for my "Who among us is the record holder for slowest build" thread, now, do I?

 

Ed Jones and I have been exchanging a few emails and CN PMs regarding the lens cells.  As a reminder, the lenses must be carefully positioned for angle, distance, and centration.  Ed's Chiefs so far have had adjustments for some or all of these dimensions as shown in his videos and website, and he's fine-tuned them in the scopes based on aberrations seen at the eyepiece.  Kevin Frederick, on the other hand, had an aluminum box made with water jet components that fixed these relationships and has done well without adjustment except to the secondary, lenses, and focuser as a UNIT.

 

I've worked out a way that I believe will allow for nailing these dimensions on a wood lathe that will require a few more dimensions to be taken off the Zemax design files, and I may still go that route if Ed can help me on the optical design dimensions.  If I do, I think I can show a way to do this with just a couple of easily confirmed dimensions using manual woodworking tools on a lathe.

 

Ed wants to pursue his idea to 3d print this tricky component, which may make this design a bit more within the realm of most DIYers. He sent me a  couple of Solidworks files for an 8" f/8 and a 8" f/12 lens cell.  While these won't be quite right for my 8" f/10.6, I'm going to print these in PLA at work to check out the feasibility of using these designs and hopefully to offer Ed some useable cells for himself in exchange for his time on my project.  I've learned that online 3d printing services are getting increasingly common and cheap, so Ed may have a real winner for bringing such a project almost down to a dob level for complexity.

 

His files, though in an older version of Solidworks updated to 2019 and generated .stl files for the printer.  I'll try it in PLA material to try to minimize shrinkage.  We could always scale it slightly if shrinkage is an issue for the lens seats. I'm going to try first to print the cell with one face flat on the printer bed like this:

 

lens cell flat.jpg

 

I may also try it with supports in the direction the data shows, which appears to be with the optical axis normal to the printer bed like this, which would need some supports:

 

8f12 lens cell print.png

 

This method may be more problematic but will be more symmetrical with printer artifacts.

 

I'll try to print tomorrow and let you know how it works out!


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#40 Ed Jones

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Posted 10 December 2019 - 07:19 PM

JT here is an 8 inch F/10.6 Chief design using Newport lenses and tweaking the curve on the concave lens.  Across the spectrum the on axis spot is better than any APO and there is almost to coma in the field.  

Attached Thumbnails

  • 8 inch Chief f10_6.jpg
  • JTs 8 inch F10_6 Chief.jpg

Edited by Ed Jones, 10 December 2019 - 07:19 PM.

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#41 Jeff B

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Posted 10 December 2019 - 08:27 PM

I like it!



#42 jtsenghas

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Posted 10 December 2019 - 09:01 PM

JT here is an 8 inch F/10.6 Chief design using Newport lenses and tweaking the curve on the concave lens.  Across the spectrum the on axis spot is better than any APO and there is almost to coma in the field.  

Nice! I see that's with a full 3 degree tilt of the primary and a sufficient secondary mirror angle to allow for decent baffling.

 

I notice that with a 1.447" radius of the secondary, that my round 3" secondary will have a margin of only about a millimeter for the rays shown. It is a tiny bit under 3" for the surface. That doesn't give me much of a margin of error for placement, but considering that I don't want the scope any taller and I won't be using it much off-axis, that appears to be VERY close to what I envisioned for an optical design.

 

Yes! With the star diagonal this 8" f/10.6 should have a working eyepiece height not much higher than that of an 8" f/6 dob! choo-choo.gif



#43 Ed Jones

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Posted 11 December 2019 - 09:01 AM

Since the secondary is flat you can move it forward if you'd like more clearance or change the angle with no effect.

The image is only 37 inches (in Z) from the primary.
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#44 jtsenghas

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Posted 11 December 2019 - 09:18 AM

It's happening!

 

We're trying nylon since it is black and was already in the printer from the previous job. As a first pass we are using default settings, which I believe are for a 0.8 mm shell and a triangular interior grid system. Shown is the 8" f/12 cell about one third printed. We set the layer thickness to 0.2 mm, which I hope will be fine enough. Less than 0.1 mm is available but REALLY slows the printer down. 

 

20191211_090339_compress40.jpg


Edited by jtsenghas, 11 December 2019 - 09:53 AM.

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#45 jtsenghas

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Posted 11 December 2019 - 10:37 AM

Not bad at all! 

 

20191211_102710_compress60.jpg

 

I haven't measured my Newport lens diameters, but if they are right on 2", we may have to scale this up 1 or 2% and print again. The lens seats are 0.002"-0.003" undersized, but quite round. I remember from your tolerance analysis in one of your videos, Ed, that the distance between lenses isn't as critical but I can adjust the scale in X, Y, and Z individually.

 

What were you planning on for screws? Were you thinking of letting some #4 screws self tap, for example? I could run some tests before trying again.


Edited by jtsenghas, 11 December 2019 - 10:38 AM.

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#46 Ed Jones

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Posted 11 December 2019 - 10:56 AM

JT,
Looks good, how long did it take? I'm expecting to need to use a reamer on the lens seats. I don't remember what size screw I intended to use,likely #4s, whatever works. I'd like to see this one and I'll design a cell for your design before you run another one.

#47 macdonjh

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Posted 11 December 2019 - 11:39 AM

Not bad at all! 

 

attachicon.gif 20191211_102710_compress60.jpg

 

I haven't measured my Newport lens diameters, but if they are right on 2", we may have to scale this up 1 or 2% and print again. The lens seats are 0.002"-0.003" undersized, but quite round. I remember from your tolerance analysis in one of your videos, Ed, that the distance between lenses isn't as critical but I can adjust the scale in X, Y, and Z individually.

 

What were you planning on for screws? Were you thinking of letting some #4 screws self tap, for example? I could run some tests before trying again.

Cool, now all you have to do is sand it with 120, 240 and then wet sand from 400 through 800 grit and you'll be ready for paint. smile.gif


Edited by macdonjh, 11 December 2019 - 11:40 AM.


#48 jtsenghas

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Posted 11 December 2019 - 11:50 AM

Cool, now all you have to do is sand it with 120, 240 and then wet sand from 400 through 800 grit and you'll be ready for paint. smile.gif

Actually, I plan on roughening the ID with coarse sandpaper to avoid being too shiny in the optical train. This is nylon and won't paint well, but it is good and black. 


Edited by jtsenghas, 11 December 2019 - 11:51 AM.


#49 jtsenghas

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Posted 11 December 2019 - 01:03 PM

JT,
Looks good, how long did it take? I'm expecting to need to use a reamer on the lens seats. I don't remember what size screw I intended to use,likely #4s, whatever works. I'd like to see this one and I'll design a cell for your design before you run another one.

It was just under 2 hours with printer warmup and 0.2 mm layers. This is not a high end printer but is surprisingly adequate.

 

The cell is quite lightweight, perhaps only half an ounce or 15 grams or so due to the hollow grid construction. 


Edited by jtsenghas, 11 December 2019 - 01:54 PM.

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#50 jtsenghas

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Posted 11 December 2019 - 01:39 PM

Okay, Ed and I have exchanged a few PMs on this. I'm going to send him the first cell without any rework for evaluation after I check the fit of Newport lenses. I suspect we'll want to increase the shell thickness to allow for a little more robustness, especially at the holes. My suspicion is that partially tapping with a taper tap would give really good "Nyloc" behavior with machine screws, perhaps #4 diameter. 




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