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8" f/15 refractor project

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#51 rwiederrich

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Posted 12 January 2011 - 06:04 PM

My 5 baffle ring/stringer was 13ft long and slit nicely into the tube of my 12" OTA. It doesn't need to be secured because it rests up against the back plate...and since it is firmly placed and the scope aims high...it isn't going anywhere. However...I cn easily grab it and slide it out if need be..after I remove the cell adapter from the tube.

Rob

#52 Sean Cunneen

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Posted 12 January 2011 - 07:14 PM

Stringers are the way. You can't see them from the tube and you have precise control making them square. Baffles that you press fit or screw in to attach can look slightly off unless your lucky...


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#53 Jeff B

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Posted 13 January 2011 - 08:20 AM

With a larger instrument, tube currents are an issue. The warm air can pile up and spill over the baffle and into the light path. so I've adopted a best practice of "ventilating" my baffles. This is basically leaving a small gap between the tube and baffle OD, which allows the warmer air to cling to the tube wall and not pond or pool behind the baffles. There are many ways to do this. One of the simplest with the stinger arrangeement is to simply allow the stinger to stick out from the baffle OD a bit, thus creating the desired gap between the tube ID and baffle OD. Also, this makes assembly into the tube very easy as the baffles tend to self-center and you're only sliding them in along stingers instead of the baffle ODs. With a long & large baffle array, sliding them in on the baffle ODs can be a challenge as each baffle adds its own friction against the tube wall. This couple with the fact that the tube will not be perfectly round at some points along its length can make baffle installation with that design a very fustrating and tense experience.

And don't forget the PICTURES (!!)

Jeff

#54 GShaffer

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Posted 13 January 2011 - 09:03 AM

Thanks guys.....I had considered using wood but want the scope to remain as light as possible....the carbon graphite shafts and thin aluminum sheet for the baffles seemed the best solution.....

Jeff B....I had not considered this regarding the tube currents so I will put some thought into that....thanks!

#55 Mirzam

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Posted 13 January 2011 - 10:19 AM

I think you would want a pretty close fit of the baffle rings to the tube, but not so tight as to make it difficult to insert the baffle assembly. The reason for this is to avoid leakage of low angle reflected light around the outer edges of the baffles. If you build an assembly that is too loose you will see what I mean (been there already).

JimC

#56 Darkenergy426

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Posted 13 January 2011 - 01:02 PM

The optic world lost out, but I'll bet that the lawyers didn't.

#57 rwiederrich

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Posted 13 January 2011 - 01:19 PM

And as one can recall..that is the very design I utalized for my 12" tube. It is(IMV) a must to allow heat access..and as you accurately described, it permits 3 points of contact instead of the entire OD of the individual rings.

It worked for me that's for sure. :grin:

Rob

#58 rwiederrich

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Posted 13 January 2011 - 01:22 PM

Visit my yahoo site..*Homemade refrator telescopes* and check out the images of what I did for my 10"f/15.
Jeff B has some images there as well of what he did.

Rob

#59 Jeff B

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Posted 13 January 2011 - 02:01 PM

Thanks Rob!

And even more helpful hints:

I usually use a fully illuminated field spot at focus of ~35-40MM and I don't recommend using the IDs of the focuser drawtube or the diagonal nose to set the spot size. Those restrictions can create a bit of scatter as they typically are not "knife edge" and they are behind the baffling, who's mission is to absorb stray light.

Rather, I use one of the stops about 30-50% back from the lens to set the spot and , even though the other stop ID's have been calculated for a certain axial position, I actually position them a couple of inches behind where they should be. But why he asks? Well even knife edge baffles have scatter and if you position them exactly where the calculations say they should go, that means their IDs are touching the light cone and the small scatter from their IDs can intrude. Backing them up a couple of inches from their "ideal' location removes them and their scatter from the light path.

TAH DAH.

Jeff

#60 rwiederrich

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Posted 13 January 2011 - 03:50 PM

Thanks Rob!

And even more helpful hints:

I usually use a fully illuminated field spot at focus of ~35-40MM and I don't recommend using the IDs of the focuser drawtube or the diagonal nose to set the spot size. Those restrictions can create a bit of scatter as they typically are not "knife edge" and they are behind the baffling, who's mission is to absorb stray light.

Rather, I use one of the stops about 30-50% back from the lens to set the spot and , even though the other stop ID's have been calculated for a certain axial position, I actually position them a couple of inches behind where they should be. But why he asks? Well even knife edge baffles have scatter and if you position them exactly where the calculations say they should go, that means their IDs are touching the light cone and the small scatter from their IDs can intrude. Backing them up a couple of inches from their "ideal' location removes them and their scatter from the light path.

TAH DAH.

Jeff


You're so smart. :grin:

Rob(Did I teach you that?) :lol: :roflmao:

#61 GShaffer

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Posted 14 January 2011 - 08:18 AM

Very nice to have the input of those who came before me :) Thanks guys!!

#62 Robert Bupp

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Posted 12 April 2011 - 07:46 AM

Mike,
I can continue the Muffoletto Optical Company story a bit. Your description so far is quite accurate, and leaves the lingering question about what happened to its people and equipment. Subsequent to the falling apart as you described, a group of (N)ine (U)nemployed (Tek)nicians regrouped formally as Nu-Tek Precision Optical Corporation, first in a basement, then in a warehouse facility, and finally in a manufacturing facility in Aberdeen MD where it (we) have been operating since 1998. Nu-Tek was born in a hostile environment, with a daunting (baseless, and ultimately dismissed) lawsuit hanging over it in its early years. There was no cooperation with at all with equipment, so the opticians were starting from scratch, with only their abilities. Not exactly a seamless transition.

The group of nine worked for several years as Nu-Tek, and over half are still working here now. With about 16 employees in manufacturing, the talent, techniques, and experiences are being passed down already through two generations from the original group.

As for the equipment, we understand that it was unceremoniously set out in the parking lot in the end, and the Nu-Tek group had no knowledge and no opportunity to purchase or acquire any of it.

We regularly work with university and government labs, as our emphasis continues to be very high quality, prototyping and low rates on optical components and subassemblies, including diamond turning and massive (up to Ø1.5m) optics.

If someone is looking to connect with one of the old Muffoletto guys that they used to know, there is enough information above to find Nu-Tek on the web, and somebody here would be happy to fill in what we know.
Kind Regards,
Robert Bupp

#63 Mirzam

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Posted 12 April 2011 - 08:31 AM

Hello Robert,

Welcome to Cloudy Nights!

I was wondering if your company does small diamond generation projects, such a generating a rough curve on 16" pyrex mirror blank? (Which I happen to have sitting in my basement not too far away from Aberdeen).

JimC

#64 Jim Curry

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Posted 13 April 2011 - 07:01 AM

So Herb, where's the pics? :poke: :4

Jim

#65 mikey cee

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Posted 13 April 2011 - 09:28 AM

Come to think of it I haven't seen any pics yet either!! :shocked:

#66 GShaffer

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Posted 13 April 2011 - 09:30 AM

Sadly due to some health issues the project has been on hold for a while now. Things are getting better though and I am back to about 90%.....Will likely start back to work on it in a month or two....

So Herb, where's the pics? :poke: :4

Jim



#67 Jim Curry

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Posted 13 April 2011 - 01:59 PM

I'm sorry to have harassed you.
Get well soon!!

Regards,
Jim

#68 JohnBuilt

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Posted 10 January 2013 - 11:02 AM

I too wonder about the old Muffoletto Optical shop. I worked there from 1978 - about 1982 I believe, on Everall Ave. There were a great group of guys, lets see if I can recall ~ Big Mike & Little Mike, Chuck the engineer, Bob, Frank the crazy grinder operator, Bill the mad machinist, Pat, Emil, John, Pete, Ron Athey, And the Coating Room Guy. I personally trained in grinding and polishing, and was working with Verne, and Chuck, training in the coating operation. I was sent to Denton for a class, after that it seemed a bit strained between Carl Jr = Vern's son, and myself, I think he wanted the coating room position, but never pursued it until I got involved. So I moved on. But yes, a tremendous group of talent, concentrated in such a small area.

#69 GShaffer

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Posted 26 December 2014 - 01:24 PM

Really appreciate those of you who stepped in with information on Muffoletto Optical.....The project is back underway and should be completed in a month or so.....cant wait :)

#70 photiost

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Posted 29 December 2014 - 09:18 PM

 

Should they be evenly spaced along the tube or is there a specfic pattern that works best?


Greg,
If you want to efficiently baffle a refractor without installing additional and unecessary baffles, the baffles will not be evenly spaced. I have built several refractors including 8", 10" and 11" and have used the following method:
http://www.berfield.com/baffles.html
Of course it doesn't hurt to add more baffles if you want to, but it won't increase the control of stray light.

Regards, Stan

 

 

All makes sense except the "desired field size".

 

What exactly do you mean ?

 

For the few refractors I helped build, we always pull two strings from the objective to the focal point and measured at different points down the tube as the light cone converges.


Edited by photiost, 30 December 2014 - 07:15 PM.


#71 Jim Curry

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Posted 04 January 2015 - 08:50 AM

I'm glad to hear this project is back on the workbench.

 

Well actually, Photoist, you might be cutting off the edge of the field with the method you describe.  You don't bring the lines to a point, you bring them to the diameter focal plane you want.  If looking for the widest possible in 2" eyepiece format for deepsky you use the diameter of your field stop of the longest focal length eyepiece you will use.  If planning on planetary use with minimum deep sky you would use a focal plane for your largest 1-1/4" eyepiece field stop. Or, you compromise and design for something between the two extremes I just described. I believe the first page of this thread shows a nice diagram from Mike Jones of what I'm trying to describe.  You can do an internet search for baffle design as well, there are a few similar sketches floating around.

 

Regards,

Jim


Edited by Jim Curry, 04 January 2015 - 08:53 AM.

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#72 GShaffer

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Posted 04 January 2015 - 10:30 AM

I'm glad to hear this project is back on the workbench.
 
Well actually, Photoist, you might be cutting off the edge of the field with the method you describe.  You don't bring the lines to a point, you bring them to the diameter focal plane you want.  If looking for the widest possible in 2" eyepiece format for deepsky you use the diameter of your field stop of the longest focal length eyepiece you will use.  If planning on planetary use with minimum deep sky you would use a focal plane for your largest 1-1/4" eyepiece field stop. Or, you compromise and design for something between the two extremes I just described. I believe the first page of this thread shows a nice diagram from Mike Jones of what I'm trying to describe.  You can do an internet search for baffle design as well, there are a few similar sketches floating around.
 
Regards,
Jim


You have brought something up I have been sweating over ever since I started this deal.....My layman's understanding of this is somewhat sketchy. What I think I do understand is that if I use a field stop size that is larger than my largest EP's stop then I am losing illumination and as a result contrast and detail......if smaller then the result is viginetting?

#73 Sean Cunneen

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Posted 04 January 2015 - 11:42 AM

Ideally you set the illuminated field to suit your observing preferences. You might think an 8" F15 scope is a long-focus planetary monster however at 8" that f15 scales to something " in between" planetary and wide-field!  So if your preferences are for planetary observations, take a look at your high-powered eyepieces and consider what your seeing will allow. Shoot for an illuminated field that matches the diameter of the field lens (or field stop) of your most used eyepiece. Speaking for my own location and my own builds, I like to use 1-1/4" Brandons and my seeing supports 200x with occasional nights of 300x and extremely rare nights of 400x. If I was building an 8" f15, I would shoot for an illuminated field of 15-20mm. I built my 6"f15  with a 25mm illuminated field figuring I would use it more generally and I haven't noticed any detriment to the performance at all. If you use 2" eyepieces, (Televue specifically) You'll want no less than 25 mm for planetary and 32-35 for a more general purpose scope. If you are going to do photography, then your illuminated field should be large enough to contain the image plane (even the corners).  If you set your illuminated field too large the risk you take is loss of contrast from letting in of more stray light. If your illuminated field is too small, some of the wider field eyepieces will vignette. Let me be clear, vignetting is WAY WORSE so in all figuring, go a tad larger in your illuminated field/Baffles and you can have confidence you'll have a great scope!

 

One reason I love televue is they provide all specs for their eyepieces, check out this chart and the "field stop" column to see where my approximations came from:

 

http://www.televue.c...page.asp?id=214


Edited by Sean Cunneen, 04 January 2015 - 11:46 AM.

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#74 GShaffer

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Posted 04 January 2015 - 12:44 PM

I should probably stop calling it an f15 as it actually calculates out to f/15.919 with a clear aperture of 7.75".....a bit closer to that planetary monster you referred to :) But I get the gist of what you mean..... 8" f/20 would be in the planetary monster category lol

My machinist buddy who is helping with this build who is also into astro and I just got off the phone and pretty much came to the same conclusion that I should go with around 32 to 34mm for the field spot size.....Thanks for the input and confirmation of our own conclusions......

It does cause me some concern though....now that I have looked at the link you posted, my 35mm panoptic has long been a favorite EP although I have not really used it in anything over f/9.....TV says its field stop is 38.7mm....if I configure optimal for 34mm how much viginetting should I be expecting to find? I do know that it is 89x at this focal length which is a bit much for deepsky viewing. It did however impress me looking at the moon in the mockup we made with no tube or baffling and poor collimation.

I do have the full set of XW's and I suppose I should find out what the 40mm field stop is and how severe the penalties for undersizing are......

#75 Sean Cunneen

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Posted 04 January 2015 - 01:06 PM

Others can provide a more detailed answer, but fall-off is not linear, you do not need a 100% illuminated field to get an image and most texts state a 50% illuminated field works just fine for the human eye. Photographically, you'll want every bit of field illumination you can get. Something like 70% of the image is formed from the central 50% of the image and the image illumination falls off  algebraically from there, so the edge of the field only accounts for a few percent of the image illumination.

 

Your experience with vignetting is also dependant on your choice of eyepiece. Some designs kidney-bean more readily than others, and I've found kidney-beaning to be exacerbated by undersizing the illuminated field. Most eyepieces show a dimming at the edge of the field when vignetted and some others kidney bean.

 For example, I use a 48mm Brandon as my low power eyepiece and with a 25mm Illuminated field the images don't show any vignetting or edge dimming(though some may be present. The field stop is about 45 mm.




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