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Jeff Morgan
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Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Folded Refractor new [Re: moe134]
      #1416769 - 02/10/07 08:55 PM Attachment (86 downloads)

I just acquired two objectives for the express purpose of folding them. My thoughts? Since mirrors scatter light and will degrade the image, I am folding them for a different purpose - ergonomics. I want an eyepiece location that is convenient, and fixed if possible. Just placing the focuser in the normal refractor position like the "N" design does is a total waste, IMHO, because then you add two scattering mirrors and *still* need a star diagonal.

In my designs the tubes do get shorter (obviously), but this is not the intended purpose. Afterall, short APOs are plentiful. Adding scattering mirrors *just* to get short makes little sense to me. I want to keep the total reflective surfaces to just two, with one of the mirrors replacing a star diagonal. The reason is that a two reflection system gives an image that easily matches a star chart.

My first design is more like a figure "4". It uses a 4" f/14 achromat. The reason? Overall size is what I was looking for, and by placing the focuser at the altitude bearing of a dob mount I can arrive at eyepiece position that is fixed in altitude. The attached MS Word file shows the layout.

My second design is a Coude arrangment. It has the advantage of fixed eypiece postion *and* equatorial motion. This is in the sketch phase, a detailed layout is still some time away. The objective I chose for this is an 80mm FPL 53 APO, which I acquired last month for a good price.

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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spaceydee
Postmaster


Reged: 04/16/04
Posts: 17311
Loc: Where the Kittens Are
Re: Folded Refractor new [Re: Jeff Morgan]
      #1416789 - 02/10/07 09:09 PM

There are some pretty good flat mirrors out there. Are you sure that the scattering is that bad?

--------------------
Dee
space-scientist
student violinist
Nexstar8i,SV80S,80/9D,FC100,94 Brandon,TMB92SS,GM8
8" f/7 Discovery,12.5" Portaball, PST



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KaStern
professor emeritus


Reged: 04/18/06
Posts: 701
Loc: TwilightZone
Re: Folded Refractor new [Re: spaceydee]
      #1417930 - 02/11/07 01:32 PM

Hello Dee,

Jeff Morgan wrote:
Quote:

Since mirrors scatter light and will degrade the image, I am folding them for a different purpose - ergonomics.




And Dee answered:
Quote:

There are some pretty good flat mirrors out there. Are you sure that the scattering is that bad?




I would like to say something to this.
Every surface has some degree of roughness. The less, the better.
Normal roughness will not degrade the views, severe roughness will do.

A mirror with equal surface roughness as a less surface will impact
more the views, because it leads to a 4x bigger deviation of light.

In addition flat mirrors are not easy to be made. The edge will allways
turned down, but one can overcome this by simply making the mirror
1" bigger, so that the needed size is really flat.

The folded refractors wich I have linked before are different sized.
Some of them are double-folded and need an additional diagonal, that
adds up to three mirrors.
In Germany there are some "Fagottrefraktoren", these are V-shape tubes
wich can be used without a diagonal for some targets, or with diagonal
for other targets with different elevation.

For not too big Apertures and focal length I would follow Jeff Morgan
and use 2 Flats so that the eyeoice will be situated in the altitude
axis so that it does not change it hight over ground at all!
With two mirrors it ha sonly one more than in a normal refractor with
additional diagonal.

Folding makes sens when a refrator becomes too long for a portable
normal sized mount. Because when the aperture of an achromatic refractor
is increased he has to be made with longer focal length to preserve
the same colour correction level. This is mostly fue to the decreasing
size of the airy disc when aperture gets bigger.

Fraunhofer stated that the size of the red and blue blurr should not
be bigger than 3 times the airy disc size of the focussedgreen light.
This is acchieved in an achromat made to the specs when the focal length
is 0.122 per 1mm Aperture. A 102mm achro should be at least f/12,44
and a 152mm should be f/18,54 wich is 2820mm focal length(!)

And more picky users feel that the f/ratio should be longer than this.
A 80/1200mm achro pleases most of them. This is 0,1875 per mm Aperture.
A 6 inch achro will be f/28,5 and 4332mm focal length. See Jon Isaacs`s Post.

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/1417762/

There it becomes very desirable to fold the tube. Disadvantages are:
- high quality flats are expensive
- a folded achro has to be collimated
- thermal situation is less good than in a streched achro

But the other posibility, making the achro faster so that it fits onto
a normal mount, has other disadvantages as we can see when we look through
a 6"f/8 achro at Jupiter, Saturn or the moon:
Severe colour aberration.

Best regards, Karsten

--------------------
Thats how they got my Brother

Edited by KaStern (02/11/07 01:59 PM)


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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Folded Refractor new [Re: spaceydee]
      #1418673 - 02/11/07 07:14 PM

Quote:

There are some pretty good flat mirrors out there. Are you sure that the scattering is that bad?




I would have thought that on the refractor forum, there would be howls of purist outrage about using mirrors at all due to the evil Scatter

There is a quantitative answer to this courtesy of the ProtoStar web site IIRC, wavefront error is equal to 1/(cos * angle of incidence). Errors could be additive ... or maybe not.

Additionally, mirror reflectivity is probably in the very low 90% range - per surface (hopefully). Yes, manufacturers claim higher, but these are for *new* coatings, very soon after leaving the coating chamber those numbers come down. So you can see very easily that a two mirror system is already down to 80% on the throughput. A three mirror system down around 65%. And remember, the star diagonal counts!

So when you think about it, you want to have a compelling reason for folding - because you are reducing the quality and aperture of that objective compared to a straight thru system. For me, the driving reason is ergonomic. I want to observing in a naturally seated position without adjusting chairs, tripods, or anything else. Such an arrangement is also conducive to heavy eyepiece loads, or great disparities in eyepiece weight. The OTA will get shorter, but this is just a happy by-product.

Now as far as your original question - it's probably in the eye of the beholder, just like chromatic abberation or coma for Newtonian owners.

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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Jeff Morgan
Post Laureate


Reged: 09/28/03
Posts: 4018
Loc: Prescott, AZ
Re: Folded Refractor new [Re: KaStern]
      #1418684 - 02/11/07 07:20 PM

Quote:

...
In addition flat mirrors are not easy to be made. The edge will allways
turned down, but one can overcome this by simply making the mirror
1" bigger, so that the needed size is really flat.

...

The folded refractors wich I have linked before are different sized.
Some of them are double-folded and need an additional diagonal, that
adds up to three mirrors.
In Germany there are some "Fagottrefraktoren", these are V-shape tubes
wich can be used without a diagonal for some targets, or with diagonal
for other targets with different elevation.

For not too big Apertures and focal length I would follow Jeff Morgan
and use 2 Flats so that the eyeoice will be situated in the altitude
axis so that it does not change it hight over ground at all!
With two mirrors it ha sonly one more than in a normal refractor with
additional diagonal.

...

Best regards, Karsten




A couple of excellent points there. The first is that like any other optic, the worst errors tend to be the edge. By oversizing, you use the best part.

Secondly, that one miror V idea is interesting. An L arrangement would also use one mirror but preserve the fixed eyepiece. The mount could present some deign issues ...

--------------------
Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making


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astroN00B
sage


Reged: 10/21/06
Posts: 393
Loc: Bakersfield, CA
Re: Folded Refractor new [Re: Mr. Bill]
      #1419413 - 02/12/07 06:01 AM

Better late than never... Mr. Bill, that is an awesome scope! Just thought I'd let you know I was so impressed

--------------------
SV 80/9D 10:1 focuser
WO 66mm SD APO
100mm f/5 ATM achro refractor
WO TR-188 tripod
Modified DS-2000 goto mount
Orion AZ-3
Vixen Polaris
Meade 5000 1.25" EPs, Telextender
GTO 7mm-22mm zoom


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Ziggy943
Post Laureate


Reged: 08/11/06
Posts: 3060
Loc: Utah
Re: Folded Refractor new [Re: Jeff Morgan]
      #1419534 - 02/12/07 08:47 AM

Quote:

I would have thought that on the refractor forum, there would be howls of purist outrage about using mirrors at all due to the evil Scatter
Jeff Morgan




Howl…howl. There is no optical advantage to folding a refractor. I wouldn't do it.


I think all of the reason given by Karsten not favoring folding are valid. I would add that a 100mm is a little small to fold anyway unless there is a compelling reason to shorten the tube.

I’ve only had one experience with a large, folded refractor. It was with Richard Brandt’s 8” in Prescott, AZ. We were not able to do a side by side but IMO, the telescope did not perform to expectations.



Quote:

There it becomes very desirable to fold the tube. Disadvantages are:
- high quality flats are expensive
- a folded achro has to be collimated
- thermal situation is less good than in a stretched achro
Karsten




I would add:

Lack of contrast reducing baffling

--------------------
May your skies always be clear,

Ziggy


War doesn't determine who is right, only who is left..

www.slas.us

4" Mak
#1, 160mm F8 TEC (born 1-18-2007, 27 lbs, 45.5" long), on AP900
6" F17.5 (D&G lens) (first light 6-7-2008)
9" F/14.9 Alvan Clark (1915), on Byers 812
14.25 F/5.5 Newt in a roll off observatory
Others, that have come and gone


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dietmar
Carpal Tunnel


Reged: 01/21/05
Posts: 2144
Loc: linz, austria europe
Re: Folded Refractor new [Re: Ziggy943]
      #1420120 - 02/12/07 02:32 PM

hi folks,

I have just found this interesting thread and since I own a 9" Tmb Apo designed as folded classic SCHAER-refractor I feel free to introduce it to You.

my scopes

sketch and details

--------------------
clear skies,

Dietmar

www.stargazer-observatory.com


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Scott BeithAdministrator
SRF


Reged: 11/26/03
Posts: 42898
Loc: Frederick, MD
Re: Folded Refractor new [Re: dietmar]
      #1420130 - 02/12/07 02:36 PM

Dietmar,
That is a gorgeous setup!
Great looking scopes!!!

--------------------
SLAP Observer --- TMB130SS, SV102V(LOMO Lens), SV80ED Deluxe
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy


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KaStern
professor emeritus


Reged: 04/18/06
Posts: 701
Loc: TwilightZone
Re: Folded Refractor new [Re: Ziggy943]
      #1420240 - 02/12/07 03:30 PM

Hello Siegfried,

"Lack of contrast reducing baffling"

you can baffle a folded refractor too. It is a little more difficult to do.
I think the situation is a little bit comparable to the situation in a Yolo,
or other types of TCT (tilted component telescopes), for example
a tri-Schiefspiegler.

Regards, Karsten


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Mr. Bill
Postmaster


Reged: 02/09/05
Posts: 5219
Loc: Northeastern Cal
Re: Folded Refractor new [Re: dietmar]
      #1420242 - 02/12/07 03:30 PM

Quote:

hi folks,

I have just found this interesting thread and since I own a 9" Tmb Apo designed as folded classic SCHAER-refractor I feel free to introduce it to You.

my scopes

sketch and details






--------------------
The night sky is the palette....
My optics are the brushes....

The Milky Way is the masterpiece

Member IDA



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dietmar
Carpal Tunnel


Reged: 01/21/05
Posts: 2144
Loc: linz, austria europe
Re: Folded Refractor new [Re: KaStern]
      #1420423 - 02/12/07 05:02 PM

hi Siegfried, hi Karsten,

Quote:

It is a little more difficult to do




...but only a littel...
honestly it took me quite a bunch of weeks to figure out how to design a perfect baffle.

the simplest way is to make an exact sketch of the optical and mechanical system and measure the best baffle.

here is what I mean (517 kb):
sketch

the optimal design was a cone shaped baffle.
but a straight tube will do - I had a severe gradient problem and light falling directly into mya 3,5" focuser passing by the mirrors.
with a paper-test-baffle ALL those problems were solved at once.
I`m just about to create a perfect baffle with some blendrings inside.

good luck!

--------------------
clear skies,

Dietmar

www.stargazer-observatory.com


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Art Bianconi
Post Laureate


Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
Re: Folded Refractor new [Re: Jeff Morgan]
      #1420450 - 02/12/07 05:15 PM

If one builds a defraction limited, folding refractor and mounts it on a Dobsonian.. . and it doesn't work, no matter what you try. . .

Does that make it a "Draconian, Dobsonian, folding, defraction limited refractor? "

Hey! If I can build a "Defractor", the first of its kind. . . . . . Why not!?

Art

--------------------
“Everything is on its way to somewhere. . . . . everything!"

____________________ George Malley (John Travolta)
________________________ "Phenomenon"


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horizon
member


Reged: 12/18/05
Posts: 92
Re: Folded Refractor new [Re: Art Bianconi]
      #1420534 - 02/12/07 05:58 PM

Hey Art,

I remember seeing pictures of a folded 10" refractor with a TMB lens mounted on a simple donbsonian mount which was build be Mathias Wirth.

Cheers
Michael

--------------------
Takahashi FS-102 NSV
H-EQ5 Sky Scan
Hyperion 13mm, Pentax XF 8.5mm, TS-HR 5mm
TMB ED Barlow x1.8
10x70 Orion Little Giants

--< I hate all light younger than 8.3 minutes >--

-Clear Skies
-Michael


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Ziggy943
Post Laureate


Reged: 08/11/06
Posts: 3060
Loc: Utah
Re: Folded Refractor new [Re: dietmar]
      #1420617 - 02/12/07 06:52 PM

Dietmar,

That is indeed a beautiful scope. What I meant by my earlier comment is exactly what you wrote, it is much more difficult to baffle a folded refractor than a straight through refractor and it is probably not as effective no matter what. At least that is my guess.

I also wanted to ask what mount do you have that on. It looks like a single arm fork or torque tube type mount.

--------------------
May your skies always be clear,

Ziggy


War doesn't determine who is right, only who is left..

www.slas.us

4" Mak
#1, 160mm F8 TEC (born 1-18-2007, 27 lbs, 45.5" long), on AP900
6" F17.5 (D&G lens) (first light 6-7-2008)
9" F/14.9 Alvan Clark (1915), on Byers 812
14.25 F/5.5 Newt in a roll off observatory
Others, that have come and gone


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trainsktg
Post Laureate


Reged: 08/23/05
Posts: 4952
Loc: Pacific Northwest
Re: Folded Refractor new [Re: Ziggy943]
      #1420795 - 02/12/07 08:06 PM

Quote:

Disadvantages are:
- high quality flats are expensive
- a folded achro has to be collimated
- thermal situation is less good than in a stretched achro




Heck, it still has to be better than a Celestron SCT, and look how many people buy them.

Unitron experimented towards the end of their amateur telescope career with a folded refractor in an effort to compete with some of the shorter designs coming out. It looked alot like the folded TMB that Dietmar owns. They selected mirrors that apparently had bad figuring/polish because most reviews were not so hot and they stopped production pretty quick. I would expect that in reality a folded refractor with good glass and mirrors like Dietmar's would be virtually indistinguishable from an unfolded design, especially an uncoated doublet using a prism diagonal like alot of the older classics... Brandt objectives also tended to be inconsistent in quality, so that possibility coupled with no coatings might have resulted in your experiences Siegfried. But...thats only an opinion and a guess...

Keith

--------------------
He was a good little monkey and always very curious.


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Ziggy943
Post Laureate


Reged: 08/11/06
Posts: 3060
Loc: Utah
Re: Folded Refractor new [Re: trainsktg]
      #1421191 - 02/12/07 11:45 PM

I remember the Unitron 3" folded refractors. I dont recall them offering any other models.

That time with Brandt's personal 8" folded refractor was a one time shot. It's hard to assess the quality in one short observing session. And one night doesn't make me an expert by any means. I think you're right about the inconsistant quality of Brandt's objectives. My 6" happened to be great, the 8" not as good inch for inch.

No doubt with the quality parts in Dietmer's and the care taken in it's construction he has a very fine scope.

--------------------
May your skies always be clear,

Ziggy


War doesn't determine who is right, only who is left..

www.slas.us

4" Mak
#1, 160mm F8 TEC (born 1-18-2007, 27 lbs, 45.5" long), on AP900
6" F17.5 (D&G lens) (first light 6-7-2008)
9" F/14.9 Alvan Clark (1915), on Byers 812
14.25 F/5.5 Newt in a roll off observatory
Others, that have come and gone


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trainsktg
Post Laureate


Reged: 08/23/05
Posts: 4952
Loc: Pacific Northwest
Re: Folded Refractor new [Re: Ziggy943]
      #1421332 - 02/13/07 02:35 AM

Quote:

I remember the Unitron 3" folded refractors. I dont recall them offering any other models.




Yup, that's the one. More often than not owners reported not-so-good results right out of the box, but someone on the Classics Forum (I think) said that the mirrors were horribly mounted and once aligned gave substantially improved performance. I oohed and aahed over the first one I saw one up for auction...they do look cool.

Keith

--------------------
He was a good little monkey and always very curious.


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dietmar
Carpal Tunnel


Reged: 01/21/05
Posts: 2144
Loc: linz, austria europe
Re: Folded Refractor [Re: Ziggy943]
      #1421641 - 02/13/07 09:29 AM

hi Ziggy (sound somehow cooler - o.k.?)

thanks for the comments on my post and the refractor.
if the baffle is well measured is a perfect optical system.

here is my mount:
MOUNTEGRA
You`re right - a sigle arm fork mount.

--------------------
clear skies,

Dietmar

www.stargazer-observatory.com


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dietmar
Carpal Tunnel


Reged: 01/21/05
Posts: 2144
Loc: linz, austria europe
Re: Folded Refractor new [Re: trainsktg]
      #1421650 - 02/13/07 09:32 AM

...*Gosh, dang dibbity dag nabbit* right, Keith,

Quote:

I would expect that in reality a folded refractor with good glass and mirrors like Dietmar's would be virtually indistinguishable from an unfolded design




there is no difference. but the collimation has to be very good and the baffle in place.
the shorter the construction - the more difficult - cos the chance for straylight is higher...

--------------------
clear skies,

Dietmar

www.stargazer-observatory.com


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