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8 inch F/7 CHIEF group build

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#276 jtsenghas

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Posted 20 September 2020 - 06:15 AM

That's terrific, Ed! Clearly you've found a good competitive source for such custom lenses. This is a huge step forward for faster Chiefs that require slightly different radii on the two lenses.

It suggests to me that, in the future, slightly larger diameter lenses may be possible at moderately increased cost and, perhaps more importantly, lens focal lengths between your current constraints of about either 300 mm or 500 mm (positive and negative sets) may be possible at about the same price if ordered in quantity!


Edited by jtsenghas, 20 September 2020 - 06:23 AM.

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#277 Ed Jones

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Posted 21 September 2020 - 08:25 PM

Well larger lenses are more expensive and the good price I got on these was due to the quantity I ordered but you not going to find so many folks interested at a higher price.  Newport Optics is out for PCX lenses, Optosigma has 80 and 100 mm PCX lenses ($185 and $261) but their CTs are a bit on the thin side and it's probably better to have both made but I wont start another group buy, it'll be way too pricey for me.  I'm hoping to just break even on this group buy.  It might be more an item for some place like Surplus Shed.  



#278 Ed Jones

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Posted 22 September 2020 - 02:07 PM

When it comes to larger lenses I think it would make more sense to just make them yourself and save costs.  It wouldn't be any harder than making a small achromat, actually easier.  Both lenses have flats which you can check on 4 inch flat and they don't have to be dead flat .  Then I would grind both curves together and polish the concave just enough to use as a test glass for the convex testing it with a KE.  Once the convex is done then tweak the radius on the concave to flatten it to a little slightly longer radius.  Of course you need to use a wedge gauge to control wedge but center thickness isn't critical.  Overall it isn't a very hard project.  Other than the blank costs which can't be that much, the only other cost is having them coated and that's optional.


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#279 jtsenghas

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Posted 22 September 2020 - 05:44 PM

Yes, and as you've pointed out to me, Ed, there wouldn't be any ghost reflections either if the lenses are uncoated.  The angles keep reflections off the lenses entirely away from the focal plane. 



#280 kfrederick

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Posted 23 September 2020 - 10:13 AM

 Galvoptics has 100 mm off the shelf listed in PCX   the concave lens should be much easier  to make than the convex . They did say they do custom lens up to 400mm



#281 Mike I. Jones

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Posted 25 September 2020 - 06:02 PM

Sure is looking like you're going to have to make your 8" f/7 yourself.

 

Maybe the way for non-mirror makers to approach this is to see what IS available:

 

JMI has 8" f/6 mirrors at http://www.jimsmobil...buy_mirrors.htm

 

GSO has 8" f/6 mirrors at https://agenaastro.c...-f-8-ad011.html

 

Orion UK has 8" f/6 mirrors at https://www.orionopt...opemirrors.html

 

These f/6-ers are super common but are not going to perform well as Chiefs. 

 

However:  HERE YOU GO!

 

Royce has either conical or standard 8" f/8 mirrors at http://www.rfroyce.c...and_mirrors.htm

 

8" f/8 would be a fine candidate for a Chief, and only a little bit longer.


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#282 PeteDCard81

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Posted 25 September 2020 - 06:29 PM

R. F. Royce has a note on his web home page that he is retired.

http://www.rfroyce.com/

 

If you want a premium 8-inch F/7 mirror I suggest Zambuto.

 

 

Mark



#283 Mike I. Jones

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Posted 25 September 2020 - 06:52 PM

R. F. Royce has a note on his web home page that he is retired.

http://www.rfroyce.com/

 

If you want a premium 8-inch F/7 mirror I suggest Zambuto.

 

 

Mark

I didn't know he'd retired.  Regarding Carl Zambuto, several posts back in this thread I related what he told me about this project, that he wasn't interested in a batch run of 8" f/7s but would be glad to quote individuals for making them.



#284 jtsenghas

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Posted 25 September 2020 - 07:53 PM

Considering that an 8" f/7 Chief with a right angle arrangement of the mirror before the lenses is only as tall as an 8" f/6 dob and has considerably better spot diagrams than an 8" f/6 Chief, I'm hoping you guys find someone willing to turn out a batch of 8" f/7 or longer. 

 

I'm in the midst of a major water jet programming project at work for a product launch that has brought all hobbies to a screeching halt for a bit. I worked seven days last week and will work at least six days this week. 

 

I'm hoping to show soon my idea with proof of concept prototypes for making lens cells in a manner that should easily make it possible to adjust to different prescriptions for tilt, spacing, and Y offsets. 

 

I know it sounds like more than a garage workshop could do precisely, but I'm confident a combination of two tricks will make it quite possible to stay well within the tolerances Mike and Ed are designing to for the cells. 

 

I think the printed cells Ed designed are still the best idea for a standardized design for those without the tooling experience and equipment to go the route I'll try, but I'm confident that at least two of you on this group build could duplicate my efforts. 

 

This should help the one-offs for the unique mirrors a few of you already have or are making, as well as to help builds of homemade mirrors whose focal lengths are a bit off target.  I do recognize, though,  from the tolerance studies done so far that these designs can suffer a bit of range of primary focal length. 


Edited by jtsenghas, 25 September 2020 - 08:16 PM.


#285 kb58

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Posted 25 September 2020 - 08:29 PM

I'm still looking as well. While a few with "close" numbers have turned up, I really want to stick to the f/7 design to avoid problems. I know some of you are very familiar with optical design and it's not a problem, but for me it's an unknown I don't really want to get into. Zambuto will make a primary for around $1200, https://zambutomirro...om/mirrors.html but I haven't made any decisions yet.


Edited by kb58, 25 September 2020 - 08:57 PM.


#286 Ed Jones

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Posted 25 September 2020 - 10:46 PM

Well it's not a big deal to use other higher F numbers mirrors if you can find them as well. F/7 and longer mirrors used to be common years ago.  



#287 Steve Dodds

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Posted 26 September 2020 - 12:54 PM

There is a 8" f/7 for sale over on the classifieds.  $225

https://www.cloudyni...primary-mirror/


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#288 kb58

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Posted 27 September 2020 - 10:08 AM

What's BVC?



#289 coinboy1

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Posted 27 September 2020 - 10:12 AM

BVC = black vitrified ceramic. A low expansion material for mirror blanks.



#290 jgraham

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Posted 27 September 2020 - 11:25 AM

Interesting. I know that I already have a couple of 6” f/8 mirrors on-hand, one was ground by my father in the 1950’s, the other is mine from 1974. I also have a 6” f/11 that could prove interesting.



#291 Ed Jones

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Posted 27 September 2020 - 07:31 PM

Looking through my "forgotten" mirrors that could work with these lenses I found 2.   One a Pyrex 8 inch F/11.5 that would be pretty close to my 6 inch F/12.  I'll need to check it because it might be a sphere from another project.  The other one is a BVC 12 inch F/7 and surprisingly it can work with these lenses.  I already have a 12 inch F/7 Chief so maybe someone would be interested in it, I'm not likely to do anything with it.



#292 jgraham

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Posted 27 September 2020 - 07:37 PM

Ed; what do you think of my 6” f/11? Lengthwise it shouldn’t be too bad in a somewhat normal Newtonian configuration and being unobstructed it could make a decent high resolution scope. I might also be able to pack it into a reasonably sized tube similar to the old Optron Herschellian.

 

Thoughts?

 

(FYI... I’m working on sending you a prepaid mailing label.)



#293 Ed Jones

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Posted 27 September 2020 - 08:31 PM

John,

In the newtonian configuration the eyepiece would be roughly 5 feet from the primary.  One of my favorite scopes is my 6 inch F/12 shown here, the eyepiece is in a convenient location, it breaks down easily and the refractor tube gives it another very useful alignment option.

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#294 jgraham

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Posted 27 September 2020 - 09:08 PM

Perfect! Refurbing this 6" f/11 was on my to-do list. Turning it into a CHIEF sounds like a winner. I also have a 6" f/15 achro on my to-do list and I recently refurbed a 6" f/20 classical Cassegrain. Last, but not least, I bought a 4.25" mirror grinding kit to make a 4.25" Kutter Schiefspiegler.  This will make an interest group to compare. :)

 

Fantastic!


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#295 steveastrouk

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Posted 28 September 2020 - 04:51 PM

Has production started again, or is this a new old stock?

New production. The original maker has restarted.


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#296 Steve Dodds

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Posted 28 September 2020 - 05:16 PM

New production. The original maker has restarted.

If it's the same guy doing the same BVC stuff, I will be steering well clear.



#297 ksdowd

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Posted 01 October 2020 - 12:50 PM

MY CONCAVE LENS ARRIVED FROM ED YESTERDAY!!!!!!

 

Now I just need to figure out the best way to mount it.

 

Some of you mentioned flexing tubes into elliptical tubes.  That sounded intriguing.  I'm interested in any details you can provide on how to do that for sonutube or aluminum cylindrical tubes.

 

Respects,

 

Kevin.


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#298 rjaszcz

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Posted 01 October 2020 - 01:17 PM

MY CONCAVE LENS ARRIVED FROM ED YESTERDAY!!!!!!

 

Now I just need to figure out the best way to mount it.

 

Some of you mentioned flexing tubes into elliptical tubes.  That sounded intriguing.  I'm interested in any details you can provide on how to do that for sonutube or aluminum cylindrical tubes.

 

Respects,

 

Kevin.

Kevin, I believe Mark Cowan mentioned flexing the tubes.
 



#299 mark cowan

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Posted 01 October 2020 - 03:14 PM

Yeah, you can flex Sonotubes - I did this to decrease the size of what I could get (18") into what I needed (16").  I just cut a slot out and then pulled the tube down into a smaller size glued into a joint piece I'd routed out to accept the cut sides.  

 

Here's what it looks like finished (to the point of putting an end ring on anyway).

 

IMG_84401.jpg

 

What I figure you could do is apply the same sort of technique but end up with a tube that's not round (i.e., tapered or elliptical).


Edited by mark cowan, 01 October 2020 - 03:17 PM.


#300 kb58

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Posted 01 October 2020 - 07:32 PM

Borrowing from moldless construction used in experimental aircraft, you can also cobble together whatever shape you want it to be, drape fiberglass ply over it and let it cure, then remove the interior construction, leaving just the shell. If done right it can end up being exactly the shape you want and very light.


Edited by kb58, 01 October 2020 - 07:33 PM.



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