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8 inch F/7 CHIEF group build

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#51 jtsenghas

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Posted 18 August 2020 - 09:56 AM

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

Ed, since this quantity brought the cost of the coated custom concave lens down to under that of the uncoated lens of slightly shorter radius you usually use, might it be worth asking if the same supplier could make a coated convex lens as well that is otherwise a match for the KPX one? Such an option may also help to ensure the refractive index matches too, right? 


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#52 jtsenghas

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Posted 18 August 2020 - 10:41 AM

.... you know you're laying the groundwork for coated custom 3" lenses for a standardized 12" Chief, don't you? whistling.gif



#53 Ed Jones

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Posted 18 August 2020 - 11:37 AM

 

might it be worth asking if the same supplier could make a coated convex lens as well

 I've thought about that but my reasoning is   1 I'd like to see how good a job these guys do    2 With Newport if the lens has a problem (you can always have me Zygo them) you can always return them  and 3 these took a good chunk of change to buy.

 If they are a good supplier then doing a bigger lens is possible but I don't think there would be as much interest and volume drives the cost.  It might be more appropriate for someone like Discovery or Surplus Shed to supply these.


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#54 jtsenghas

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Posted 18 August 2020 - 12:00 PM

Thanks, Ed, that was a bit of a tease about the 3" lenses, by the way. 

 

I also realize that differences of refractive indices of same glass types should be trivial among the suppliers and their sources are likely identical anyway. 

 

I'm really stoked that so many readers of this thread are interested enough to request a lens, though. 



#55 kfrederick

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Posted 18 August 2020 - 12:00 PM

 I be interested in some 4 inch lens sets . Like used on the 20 chief . Thanks 



#56 ksdowd

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Posted 18 August 2020 - 12:41 PM

Actually J.T., I was nodding YES to your comment on upscaling this 8" group project to a 12" ++) project.  I'm not sure if I'm ready for a 20" like Kevin's yet,(need land and an observatory on it) but something in the 12"+ range is of interest.



#57 Ed Jones

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Posted 18 August 2020 - 01:28 PM

Well don't get too far ahead.  I have a 12 inch F/10 design but the cost will really go up for a 3.7 lens set plus a small volume penalty.  Both would likely need to be outsourced.


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#58 jtsenghas

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Posted 18 August 2020 - 02:07 PM

Small steps.  Let's not turn my teasing comment into a full scale hijacking. I did investigate 3" lenses in the past and things get expensive fast. 

 

This group build is a great idea in my mind at its scale, and 8" f/7 is small enough for most for it to be portable and relatively uncomplicated.  Considering that these lenses cost about the difference between a good 10" primary and an 8" primary, these 8" Chiefs should cost little more than 10" dobs to build. 

 

Those of you who are in for this group build should be thinking about the other purchased components.  I've decided my longer 8" Chief will only use 1.25" eyepieces, which also made an optional dielectric tertiary cheaper and lighter. 

 

I imagine most of you would be satisfied with a 1.25" focuser. If you make your upper end a box structure you'll probably want a newt focuser that attaches easily to a flat surface. If you build as Ed did his last couple you may want to affix a refractor focuser to a tube.  Some refractor focusers are nice and long, which allows for flexibility for binoviewers or cameras. Just be careful you don't create a crash condition between your draw tube and mirror cell. 

 

A tube in a tube clamp allows for a little more adjustment in the build,  too, both for length and rotation.  Ed discovered last year, however, that the tube clamp sizes in the practical diameters for these builds are scarce. Salvaging such hardware from larger refractors or building your own may be required. 

 

I haven't worked out all the details on my upcoming Chief build, but it will probably be a bit more intensive on the woodworking side than most would want to delve into, and I'm still most likely to have hexapod adjustment on my setup for collimation control of both ends. 

 

I urge you to watch Ed's YouTube videos and his "Optical Ed's" (opticaleds.com) writeups on the evolution of these scopes.

 

I believe the corner frame mount he currently favors was in fact first used on Kevin Frederick's 17" Chief. 

 

I'm anxious to see what mechanical designs some of you come up with.  A fork design sounds excellent to me. 

 

These 8" diameter primaries are small enough that only particularly thin mirrors wouldn't be adequately supported with a three point mirror cell with no moving parts and the back supports anywhere near 40% of the mirror radius. 

 

Don't forget, though,  to build your cell at the tilt angle of the optical design, which is 3.4 degrees here.  This could be as simple as angling a dob tailgate at that angle. 


Edited by jtsenghas, 18 August 2020 - 02:13 PM.


#59 kfrederick

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Posted 18 August 2020 - 05:16 PM

Well don't get too far ahead.  I have a 12 inch F/10 design but the cost will really go up for a 3.7 lens set plus a small volume penalty.  Both would likely need to be outsourced.   

 Ed If I could get  a set  exactly like on the 20 I buy you a set . If the price is not too high .



#60 Ed Jones

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Posted 18 August 2020 - 05:58 PM

I no longer have access to a generator for that size lens.



#61 steveastrouk

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Posted 18 August 2020 - 07:14 PM

What would United Lens charge for blanks ?



#62 Ed Jones

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Posted 18 August 2020 - 07:34 PM

I haven't got a quote from them in years, likely over $100 each?



#63 Pierre Lemay

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Posted 18 August 2020 - 08:06 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

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

Ed,

Just saw your thread. I would like to build one if you have any lenses left over in your order. I will pay whatever the extra cost of shipping to Canada.

 

Thanks.



#64 Ed Jones

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Posted 18 August 2020 - 08:31 PM

Well that the last of the extra lenses.


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#65 jtsenghas

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Posted 19 August 2020 - 05:46 AM

Well that the last of the extra lenses.

Oh, good.  You didn't overinvest and I've been relieved of the temptation to buy for too many future builds...



#66 kfrederick

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Posted 19 August 2020 - 06:07 AM

Well don't get too far ahead.  I have a 12 inch F/10 design but the cost will really go up for a 3.7 lens set plus a small volume penalty.  Both would likely need to be outsourced.

 Do you have a design  posted for the 12 inch . And why F10 ? I thinking that 12 inch F6 . Descovery sells  Thanks



#67 Ed Jones

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Posted 19 August 2020 - 06:52 AM

I'm not considering starting a larger aperture thread at this time. 



#68 Oberon

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Posted 19 August 2020 - 07:20 AM

Ed, I’m curious why you’ve put the secondary before, not after, the lens set. 
 

post-3008-0-82689300-1597427574.jpg



#69 Oberon

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Posted 19 August 2020 - 07:34 AM

Also, where do I find the “corrector length” (as distinct from focal length)? I’m guessing that the two most critical dimensions once the optics are determined are distance from Primary to Corrector, and the tilt angle of the beam. Or is it the corrector to eyepiece? Or a ratio? Sorry, I can’t work it from from the tables.


Edited by Oberon, 19 August 2020 - 07:34 AM.


#70 Ed Jones

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Posted 19 August 2020 - 07:36 AM

The secondary can be placed after the lenses, see post #36.  Some may not want all that BFL sticking out the side so this is an option.  You just need to be sure the lens cell doesn't obstruct incoming light.

 

 

Line 14 is the lens to image distance, about 7 inches and is the most critical.  Line 2 and 6 together are the primary to lens distance.


Edited by Ed Jones, 19 August 2020 - 07:41 AM.

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#71 jtsenghas

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Posted 19 August 2020 - 07:43 AM

... I’m guessing that the two most critical dimensions once the optics are determined are distance from Primary to Corrector, and the tilt angle of the beam. Or is it the corrector to eyepiece? Or a ratio? Sorry, I can’t work it from from the tables.

Yes, the former two. On my build I suggested putting my lenses at the end of the draw tube as an "Astigmacorr". Ed informed me that I'd be better setting them fixed with respect to the primary. 

 

The lens cell position and lens relative tilt are indeed matched to the primary tilt angle and focal ratio. 


Edited by jtsenghas, 19 August 2020 - 07:44 AM.

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#72 Oberon

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Posted 19 August 2020 - 08:38 AM

The secondary can be placed after the lenses, see post #36.  Some may not want all that BFL sticking out the side so this is an option.  You just need to be sure the lens cell doesn't obstruct incoming light.

 

 

Line 14 is the lens to image distance, about 7 inches and is the most critical.  Line 2 and 6 together are the primary to lens distance.

Aha! Thx. OK, so, most critical dimension is Corrector Lens to Focal Plane, in this case virtually 7” (or 177.25mm) and measured from...the closest lens surface or the center of the lens set?
 

And will this distance change if my focal length is slightly short (I’m about f/6.9)?
 

Sorry for all the questions!



#73 Ed Jones

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Posted 19 August 2020 - 08:42 AM

You can even lose the secondary and use a star diagonal instead.


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#74 Oberon

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Posted 19 August 2020 - 08:43 AM

Yes, the former two. On my build I suggested putting my lenses at the end of the draw tube as an "Astigmacorr". Ed informed me that I'd be better setting them fixed with respect to the primary. 

 

The lens cell position and lens relative tilt are indeed matched to the primary tilt angle and focal ratio. 

Did you look at option of no secondary mirror? 

 

So long as I can keep the eyepiece within 1500mm of the ground then why not dispense with the secondary?


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#75 Oberon

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Posted 19 August 2020 - 08:46 AM

You can even lose the secondary and use a star diagonal instead.

Of course! And I have all these bits, including a 3” focuser...I don’t suppose it would be possible to fit the corrector lens cell within the tube of a 3” focuser?




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