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8 inch F/7 CHIEF group build

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#26 steveastrouk

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Posted 15 August 2020 - 12:02 PM

Does the  corrector cell need adjustability ?



#27 jtsenghas

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Posted 15 August 2020 - 01:33 PM

Does the  corrector cell need adjustability ?

Ed made his first Chiefs with each lens adjustable for centering and the angle between the lenses adjustable and the overall tilt adjustable. 

 

The beauty of these printed cells is that lens centration (within the few thousandths of an inch clearance to avoid pinching) and the angle between lenses are fixed.  You should only need to tip the lens cell as a unit. He's done this with long threaded rods and springs. 

 

If you look carefully at the printed cell in my photograph on the previous page you'll see one of two holes intended as a pivot. 

 

Once the focuser is aimed at the center of the cell and that beam hits the secondary reasonably on center, the whole "top box" or top tube assembly of those components can be treated as a unit. At most, a one-time tune in of lens cell tilt should be needed the way he and plan to build the next Chiefs. Ed got to this incrementally with sanded tapered doughnut and eccentric Delrin lens clips on a more recent build.

 

Kevin, on the other hand, had the entire top box water jet cut to targeted dimensions. I can vouch for the fact that he nailed it from my brief views through his scope at Cherry Springs. He just aims the top box and secondary with a laser through the focuser. 

 

EDIT: I see that photo is too dark to see the pivot. Here's a view of his Solidworks model rotated slightly in .stl format to lay flat on the printer bed.  You can just see the pivot holes on the left side of the image and the inside of the right side:

 

20191210_145923_compress93.jpg

 

 


Edited by jtsenghas, 15 August 2020 - 05:13 PM.

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#28 PeteDCard81

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Posted 15 August 2020 - 02:31 PM

I am in for the cocave lens.

 

 

Mark Petersen



#29 ksdowd

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Posted 15 August 2020 - 02:38 PM

I've been emailing Ed this week daily asking questions on feasibility of an 8" f/7 Chief and he has kindly sent back daily emails.  (Thanks, Ed!)

 

My 8" f/7 has been a good scope for me, but I'd like to see it even better and this looks like the best way to do it. 

 

I appreciated the question on focal length tolerances.  My f/7 is actually an f/6.96, so I think it will be okay, but it is always wise to check.

 

I loved the comment about elliptical cross section tubes.  I'd been contemplating an oversize cylinder, but elliptical will allow weight savings that will help keep the balance point closer to the mirror as well as lower the weight for this hopefully portable scope.

 

I'm trying to keep the balance point close to the mirror as the plan is for this to be a split-ring project.  The ability of the Chief to intercept a larger light path closer to the primary seems to be a good match for this goal.  And the corrections of the chief also seem a good match for the moderate astro-photography I plan on.

 

And if I can minimize the distance from the balance point to the mirror, that allows me to reduce the RA ring size/weight significantly, which will enable me to also use the scope more easily visually.

 

Respects,

 

Kevin


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#30 hamishbarker

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Posted 15 August 2020 - 05:09 PM

Yes please! Now that I saw the spot diagram, looks incredible. I am up for it!

 

Will you order an test example first so you can check their quality before the group order?

Thanks again Ed!

 

 



#31 jtsenghas

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Posted 15 August 2020 - 05:35 PM

Yes, on axis all frequencies in the visual spectrum should be well within the Airy disk which, when you think about it, means that a slightly lesser optical quality of components can still result in a scope diffraction limited on axis. 

 

There will be some losses from the secondary and each lens surface.  If the concave lenses are coated those will be somewhat reduced. 

 

Note that there should be NO ghost reflections from uncoated lenses due to lens tilt. 

 

Properly constructed I imagine that views through such 8" f/7 Chiefs should be comparable to those of 7" apochromatic refractors, and slightly better for resolution due to the increased aperture. 

 

This thread is motivating me to finish my current build so that I can properly start my slightly longer 8" f/10.8 Chief build.  I have the optics for that and if I get going on it soon enough, I may be useful for those of you considering these builds. 

 

No one scope does it all, but it was in anticipation of a Chief planetary scope that I finally took the plunge this year to invest in some eyepieces with green lettering:

 

20200815_183402.jpg



#32 Mark Harry

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Posted 15 August 2020 - 06:13 PM

"If you use a 6inch f/9.3 mirror with identical radii and spacings in that design, you can afford to have the mirror be spherical and it is still diffraction limited on axis."
*******

I don't see virtually all the blur in OSLO be confined to the diffraction disc on a 6" spherical mirror until you get to F/12. (across most of the whole field of view)
I like this added security, for it can  counteract errors from other random variables in the whole OTA. Just my 2¢.



#33 RajG

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Posted 15 August 2020 - 07:04 PM

"If you use a 6inch f/9.3 mirror with identical radii and spacings in that design, you can afford to have the mirror be spherical and it is still diffraction limited on axis."
*******

I don't see virtually all the blur in OSLO be confined to the diffraction disc on a 6" spherical mirror until you get to F/12. (across most of the whole field of view)
I like this added security, for it can  counteract errors from other random variables in the whole OTA. Just my 2¢.

Mark,

You don't need the full blur inside the Airy disc to be diffraction limited, though I hear you about the added security. I posted OSLO files for both the 8in f/7 and the 6in f/9.3 version on the design thread. The PV wavefront error of the 6in spherical version is less than 1/4 wave, RMS is about 1/20 wave, and Strehl is about 0.87. A parabolic mirror would be even better, but this is pretty good already. Also, I think Vlad Sacek pointed out on his website that the difference between spherical vs parabola for a 6in f/9 is less than 1/4 wave PV.

 

Rajesh



#34 Ed Jones

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Posted 17 August 2020 - 01:29 PM

Update.  Wow! I have 13 or maybe 14 people who want this lens!  I had the supplier tighten the surface irregularity spec to 1/4 wave which involved making it thicker to meet this spec. and $5 more per piece.  However I had to increase the order since the original order was for 10 pcs. to cover 14 pieces plus one for me and a few extra for any late comers and that with shipping and Paypal fee worked out to $52.75 per lens.  thewave.gif

 

The Chief has more options in the layout than a Newt. I'm recommending a 90 degree fold shown here.  It will stick out farther than a Newt but you get all the BFL for a binoviewer or whatever.  If you aren't using a tube then you need to put a baffle by the secondary to keep out stray light.  If you don't want it sticking out so far and don't want all that BFL you can fold it after the lenses provided the lenses an holder don't obstruct.

You can also fold it back at some angle but again you need a baffle near the secondary to keep out stray light.  The cell and lenses can't obstruct and at F/7 things get a little tight.

Forgot to mention they are ordered and due the 4th week in Sept.

Attached Thumbnails

  • 90degree.jpg
  • Lenses first.jpg
  • Baffel.jpg

Edited by Ed Jones, 17 August 2020 - 01:54 PM.

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#35 jtsenghas

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Posted 17 August 2020 - 05:38 PM

Good going, Ed! I think your first option would work best for most and make aligning the focuser, lens cell, and secondary easiest. That configuration probably would be the most practical for many for overall scope length, too.

 

As drawn in that first image, what is the distance between mirrors? 

 

My Chief at 8" f/10.8 wil be folded back with a round 3" Zerodur flat. Thank you for testing that with me in your Zygo. I know I have good optics. As much as I'm tempted to join this group build, I have to build with several sets of optics I have already....



#36 jtsenghas

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Posted 17 August 2020 - 05:41 PM

By the way, I know it's been only a couple of business days since I inquired, but I haven't heard back from Discovery regarding my inquiry about 8" f/7 primaries. One of you may want to follow up with multiple attempts by phone.  You may be able to get a quantity price break on that size if you work together. 


Edited by jtsenghas, 17 August 2020 - 05:42 PM.


#37 steveastrouk

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Posted 17 August 2020 - 08:36 PM

What's the angle between the two lenses ? I eyeballed it at 23 deg



#38 jtsenghas

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Posted 17 August 2020 - 08:59 PM

What's the angle between the two lenses ? I eyeballed it at 23 deg

It appears to me from the Zemax design of this 8" f/7 that Ed posted on the the optical design thread,  that the difference in rotation of the two lenses is 24.262 degrees. He tilted the primary 3.4 degrees and the secondary an arbitrary 20 degrees there.  A different secondary angle would yield different individual lens angles, but their difference should remain the same. 



#39 Ed Jones

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Posted 17 August 2020 - 09:05 PM

I need to edit the posted design, for one thing the radius was one I had measured and not -112 in. and a few other small changes.  


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#40 jtsenghas

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Posted 17 August 2020 - 09:18 PM

Ed, I'm so grateful you are continuing to make the Chief design more and more accessible to ATMers, and think that at 8" f/7 you are making this a practical size, and reasonably priced. 

 

I would encourage you to promote the first configuration shown a few posts back. With the focuser at 90 degrees and the focuser, lens cell, and secondary in line, I expect it will be easier for most to configure. 

 

Would you be so kind as to post that design on the optical design thread? I'd like to see the resulting overall dimensions. It looks reasonable to me as one that would have an OTA that needn't be broken down for transport. 

 

Thanks again for the great work! 


Edited by jtsenghas, 17 August 2020 - 09:20 PM.


#41 steveastrouk

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Posted 17 August 2020 - 09:32 PM

Is this a workable form for  the holder of the tilted components ?

A cheese shaped wedge like this ? I'd make mine out of aluminium, with traditional screwed clamp rings.

 

 

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  • Corrector 2.jpg

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#42 jtsenghas

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Posted 17 August 2020 - 09:51 PM

Is this a workable form for  the holder of the tilted components ?

A cheese shaped wedge like this ? I'd make mine out of aluminium, with traditional screwed clamp rings.

If you research Ed's Chief building videos you'll see he once did something similar with a round object that he sanded to a precise angle that he measured with a digital angle gauge with resolution of 0.05 degrees. 

 

The thing with that kind of a design is that lens centration has to be carefully adjusted, and I doubt many of us have his eye for adjusting both overall tilt and individual lens positions.  He made that particular version adjustable with eccentric Delrin clips he turned on his mini lathe. 

 

You can see from my previous posts on this thread that the 3d model has a recess for each of the 2" diameter lenses.  He designed these cells working from the optical axis of the cell. 

 

Incidentally, I found that rotating the model so that one face was parallel to a 3d printer bed and printing in PLA at 101.5% scale worked very well. I would expect that either machining or printing to get lens spacing, relative angles, and centration (alignment) would greatly simplify construction. It would take just one among you to print cells for all of you. Since this comes out better with a fairly fine resolution (thin layer) setup, each may take more than an hour, however. You may owe that individual more than just filament and mailing costs. 


Edited by jtsenghas, 18 August 2020 - 07:05 AM.


#43 jtsenghas

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Posted 17 August 2020 - 10:07 PM

Oh, and a couple of more things on construction:  

 

A round lens cell has the advantage that it is more easily fit with clearance within a round tube, making it easier to adapt refractors, or at least to use refractor focusers and tubes for the top end. 

 

One reason I favor the first design shown a few posts back is that a standard secondary would be used at close to its optimal angle and could also be fit like a periscope mirror to an angled cut or roughly elliptical hole in the tube.

 

Such a cell printed, even with a thicker than default shell setting, would weigh much less than an ounce, helping to reduce top end weight on a typically dob style mount.

 

I'd love to see the variety of mechanical designs some of you come up with.  Equatorial mounts aren't out of the question. 

 

I'm leaning towards a hexapod dob type mount on the longer version I need to get back to.



#44 steveastrouk

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Posted 17 August 2020 - 10:13 PM

Given the CTE of plastics, I wonder how badly the tilt of the components is affected by typical temperature changes. The Al part I showed weighs 1.2 Oz


Edited by steveastrouk, 17 August 2020 - 10:14 PM.

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#45 tim53

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Posted 17 August 2020 - 11:15 PM

Update.  Wow! I have 13 or maybe 14 people who want this lens!  I had the supplier tighten the surface irregularity spec to 1/4 wave which involved making it thicker to meet this spec. and $5 more per piece.  However I had to increase the order since the original order was for 10 pcs. to cover 14 pieces plus one for me and a few extra for any late comers and that with shipping and Paypal fee worked out to $52.75 per lens.  thewave.gif

 

The Chief has more options in the layout than a Newt. I'm recommending a 90 degree fold shown here.  It will stick out farther than a Newt but you get all the BFL for a binoviewer or whatever.  If you aren't using a tube then you need to put a baffle by the secondary to keep out stray light.  If you don't want it sticking out so far and don't want all that BFL you can fold it after the lenses provided the lenses an holder don't obstruct.

You can also fold it back at some angle but again you need a baffle near the secondary to keep out stray light.  The cell and lenses can't obstruct and at F/7 things get a little tight.

Forgot to mention they are ordered and due the 4th week in Sept.

I don't suppose it would be possible to put the secondary on the far side of the tube from the lenses?  That way, the back focus wouldn't need to be so large.  But I suspect there's not enough distance between the lenses and the mirror?



#46 kfrederick

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Posted 18 August 2020 - 05:44 AM

Given the CTE of plastics, I wonder how badly the tilt of the components is affected by typical temperature changes. The Al part I showed weighs 1.2 Oz

The 20 inch chief the lens holders are plywood ,.and works fine .For this group build it might be best making it like Ed did on the 20 inch . Having it hinged at the bottom .I show you with the telescope why .



#47 jtsenghas

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Posted 18 August 2020 - 05:46 AM

I don't suppose it would be possible to put the secondary on the far side of the tube from the lenses?  That way, the back focus wouldn't need to be so large.  But I suspect there's not enough distance between the lenses and the mirror?

The slower the focal ratio and the less the primary tilt, the more one has options for various configurations. At f/7 it appears that Ed had to tilt the primary 3.4 degrees to get what he he shows as options that don't vignette and are barely adequate for baffling.  That's what Ed meant by things being a bit tight at f/7.

 

A much slower Chief could have a larger, perhaps round or rotated sideways elliptical secondary that returns the beam backwards quite a bit. That comes with a more awkward focuser angle, though, unless a tertiary is used. (My f/10.8 will have 3 degrees of primary tilt, a round 3" secondary,  and an optional dielectric mirror diagonal as a tertiary, but I'm using a long enough focuser I can dispense with using it if desired).

 

The slower the ratio and the less the tilt, the more forgiving the design is. I think it's great, though, that Ed has worked out a reasonably economical f/7 version.  For such tight spot diagrams it does require one custom lens, but he appears to have found an inexpensive enough source for it. Larger apertures would require larger lenses, which get rapidly expensive. More primary tilt requires greater lens tilt angles. Everything is a compromise.  

 

If the primary were tilted more and a larger secondary were used, then the beam could cross back before the lenses. That complicates the lens cell design quite a bit and deteriorates the view more off axis even a little. 

 

Moving lenses closer to the focuser may free up room between secondary and lenses for crossing the OTA and for larger apertures make 2" lenses large enough, but that creates problems too.  The lenses would have to have more power and a greater difference of radii. Tolerances get tighter and lateral color increases due to the difference in radii.

 

If Ed hadn't found this source for concave lenses slightly weaker than his preferred COTS convex lens I would have expected this design to excel only at slightly slower f ratios.  It looks like he has squeaked it in with a commercially available primary, though. It doesn't look, however, like we can quite cross the OTA with these optics without some vignetting. 


Edited by jtsenghas, 18 August 2020 - 06:53 AM.

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#48 Oberon

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Posted 18 August 2020 - 07:31 AM

Also there is no need to have a tube to cross, no need to encircle the incoming beam.



#49 jtsenghas

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Posted 18 August 2020 - 07:36 AM

Given the CTE of plastics, I wonder how badly the tilt of the components is affected by typical temperature changes. The Al part I showed weighs 1.2 Oz

FUGGEDABOUTIT!

 

The cell will be mounted by its geometric center and any change of angle between the opposite faces should be essentially zero with temperature changes due to the isotropic (homogeneous with direction) nature of the PLA. We do have to be careful to size the cell so that there is no pinching at low temperature or excessive looseness at higher temperatures. With slight shrinkage of this material while printing I found 101.5% scaling to  work well. 

 

To put numbers on it, I just looked up the CTE of PLA and found a value of 68 microns per meter per degree C. That's only 2/3 that of ABS plastic and less than 3 times that of Aluminum 6061. 

For a lens spacing of, say, 20 mm, that's a mere 40 microns for a 30 degree C swing. Each lens would move about 20 microns from the cell central pivot  and maintain the same relative angle. Lens spacing has loose tolerances in these designs anyway. 

 

Incidentally, I inspected with a Mitutoya CMM one of the PLA cells I printed for Ed and it was within 0.013 degrees for angle,  within the couple of hundredths degree uncertainty in the measurement itself. 


Edited by jtsenghas, 18 August 2020 - 07:59 AM.


#50 Ed Jones

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Posted 18 August 2020 - 09:45 AM

Steve, you could make it out of aluminum, I use ABS tube for my 6 inch F/12 Chief cell shown here. I had to machine the ID first then cut the angles on a table saw then used a sanding disk to true the angles with a digital gauge.  It's a lot easier than cutting metal.  The lenses were retained with black delrin clips that had the center hole drilled off center to allow some centering adjustment.  The centering in X was easily measured from the side with calipers but the Y centering was trickier.  It's a lot more work than a 3D printed cell but it is adjustable.  Also a layout of the correctors from Zemax.

Attached Thumbnails

  • 1-2.jpg
  • correctors.jpg

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