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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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Quote:
A bunch of APO fanciers claiming a 6-inch APO will outperform any reasonably sized reflector... In fact, touting their resolution advantage over 20-inch reflectors. I realize rational arguments make no difference under such circumstances.
Hey, the person you really want to wade into the middle of that kind of "debate" is John Pons.  Or of course Ed Grissom, he knows a thing or two about making a good planetary instrument... 
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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Quote:
As I understand it (I did not read the book from A to Z and did not build one myself yet) Schupmann scopes are by far the best concerning color correction outperforming any Apo.
Once I tame my robots I'm going to start building some exotic stuff, and like I said that's on the short-list. Its been on that list for quite a while, and the idea of an all-quartz Schupmann is so appealing. 
Best,
Mark
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
The off-axis spot diagrams are not very nice and by my criteria would need correction.
At F24 and not very large aperture they are perhaps not general purpose enough.
I do have a partly ground 5" F25 mirror for an off-axis scope as featured in S&T. It relies on the focal surface being within the coma free area of that mirror. The design used a spectacle lens as a corrector. The limit is 8".
For serious planetary observing, AKA EddieG, I think my Best Telescope needs to be at least a 6" Apo, 7" MN or 8" Newt. Can you show me an image or a spot diagram 20mm off-axis for an 8" Scheif? If the detectable image is fairly star shaped I will add it to the list.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
When considering making telescopes I did investigate off-axis mirrors. With a diagonal its still a 2 surface Newt.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I am afraid I passed on a quote from Eddgie and your calculations I assume are correct.
I have a sled-focusser in my Celestron/Vixen 6" F5 with a single vane which allows a pretty big illuminated field.
I was not aware the dielectric coating adds a stress to the substrate. When AP bought out their deielectric diagonals they pointed out an aluminised coating consisted of a rough surface of tall crystals. Perhaps it needs an additional burnish like silvering.
Thanks. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
Edited by HandyAndy (07/03/08 07:44 PM)
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I did come across the Schupmann a few weeks ago delving around.
As it can work as a 13" at F10 we can add it to the list.
And the related dialyte, as made by John Wall at 30" who was carrying a 'top secret' Shupmann at a recent BAA I&I meeting.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I count a single lens or corrector plate as one surface compared to a mirror.
A reflection doubles the surface error but a mirror only has one surface to figure.
A refraction has about half the surface error but there are two surfaces so nets out at about the surface error but there are two surfaces to figure.
Yes we are not worried about cosmetics.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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Made a correction to the earlier post about illuminated fields.
Andrew, are you looking for 20mm off-axis fields, ie, 40mm field total? 46mm would be as large as a 2" EP can accept. And I guess, one other question there, is the criteria at 20mm off-axis to be within the diffraction spot? Tough one...
Best,
Mark
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Starkler
professor emeritus
Reged: 11/04/05
Posts: 692
Loc: Australia, Melbourne
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Quote:
Out of morbid curiosity I hunted down the thread you're referring to...where it starts to say No one makes a "Premium" 6" Newt. Ah, yes. No one makes one, so you can't buy one, but since no one can buy them, it appears there's no market for one...which means no one will make one.
Takahashi MT160.
I pointed one out for a friend who bought it, and a very nice small newtonian it is too.
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Geoff
15" SDM truss dob | Vixen r130sf | GSO 10" dob
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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Quote:
Hey, the person you really want to wade into the middle of that kind of "debate" is John Pons. Or of course Ed Grissom, he knows a thing or two about making a good planetary instrument...
Best,
Mark
Mark/Chris:
Thanks for your responses. I like APO refractors, don't get me wrong, but I don't think there can be any doubt the Reflectors forum is my natural home... 
I actually own an 8-inch f8 planetary mirror that I traded a club member for. It was tested by an optician in our club and has an awesome figure. Unfortunately, the mirror (which was made years ago by a legendary mirror-maker who was also a member of our club and is long deceased) had not been aluminized. When I got it aluminized, I discovered it had some pitting on the surface. It might still be a good performer, but I put the project on hold and then subsequently built that monster 22-inch Dob. I should probably post some pictures of it on the ATM forum to get advice whether to have it refigured or use it as is.
I contacted RF Royce about refiguring the mirror and got my head bitten off.  We exchanged a few more emails, and he apologized for his initial response (which I hadn't been upset by -- master opticians have the artistic temperament, after all). He told me a few things about the glass that was used for my mirror. I think it was ground in the late 50s or early 60s.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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I also got a ProtoStar secondary for it with a small mirror (maybe 1.86 inches?) and a JMI Crayford focuser, which I ended up putting on my XT-8. Still haven't used the ProtoStar secondary, which will probably still go into that scope.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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Quote:
Takahashi MT160.
I pointed one out for a friend who bought it, and a very nice small newtonian it is too.
But that's an f/6. What I think the OP has in mind are slower instruments - my 8" planetary telescope is an f/6 and it works extremely well, but one wonders how it would perform around f/8 on occasion. A 6" f/10 would be the equivalent for planetary specs.
Best,
Mark
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
I was thinking of slow enough to give a Hi-Fi image.
Looking in the 'Green' book at spot diagrams and taking off-axis distances where the spot is in the Airy disk and a F no needed to get somewhere near:
Spherical Mirror F10 - 0mm
Parabolic Mirror F10 - 10mm
Flourite Refractor F8 -
Red 0mm
Green 15mm
Blue 0mm
Optimised? Schmidt-Cassegrain F 10 -
Red 0mm
Green 20mm
Blue 10mm
M C Rumak F15 - 30mm
Scheifspiegler F25 - 0mm
Can any one produce a spot diagram for a 7" F8 Makustov-Newtonian?
Obviously one can argue about the smaller colour free field of a Newtonian or a very expensive larger field of a Apochromat.
Looking at spot diagrams for eyepieces at F10 then Plossls, Abbes/Orthoscopics and Konigs give a 2 arc minute image at 10mm off axis. A Nagler type gives a 1 arc minute image at 10mm off axis as well.
Where I live my background is probably never less than 15 mag so I can resolve 2 arc minutes at most.
I can use up to 60x with the trio and 120x with the Nagler. Fortunately a Barlow lens improves things so we can get to 240x with the Nagler.
I think.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
Before adding a Dialyte type or a Schupmann to the list I would like to see spot diagrams for a 6" aperture if anyone can supply them.
Googling:
"He describes the Dialyte telescope of Plossl (1850)"
Dialyte
Dialyte designs
Do you think I can count a cemented lens as one element?
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
Tidying the list and adding a corrected Newtonian. I think four elements is our upper limit.
I might add a Chromacorr corrected scope if someone can supply a spot diagram.
Two Elements:
-Newtonian @ F8
Three Elements:
-Maksutov Newtonian @ F8
-Doublet Refractor and Star Diagonal @ F15
Four Elements:
-Triplet Refractor and Star Diagonal @ F9
-Maksutov and Star Diagonal @ F15
-Newtonian @ F8 and Paracorr
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1447
Loc: Hockessin, De
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Here is a spot diagram of 9 1/2" f/12 Schpumann I'm building now. Note that it is diffraction limited out to 0.4 degrees and has a polychromatic Strehl of 0.997 which means all the colors are inside the airy disk across the 0.4 dgree field of view. Schupmanns also have the ablity to correct for color dispersion introduced by the atmosphere so one can achieve a color free image of even when the objects are low in the sky.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
Its very interesting that you can correct for atmospheric refraction. Does that involve tipping a component?
How many elements are there in your design including a star diagonal if needed?
I am learning new things in this thread.
Thanks for the spot diagram.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1447
Loc: Hockessin, De
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There are four elements in a Schupmann including the star diagonal. To correct for atmospheric dispersion you very slightly tilt the field mirror while looking through the eyepiece which completely tunes out the color from the atmosphere. My newtonians show more color then my Schupmann.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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wallie x
member
Reged: 07/05/08
Posts: 36
Loc: Ka'Lee'for'Knee'a
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The Chinese computer generate a lot of their optics these days. No surprise optical quality wise for a reflector as they are doing a stand-up job by mass produced standards for both types. (I own a Celestron C-6R [Synta optics] 6" achromatic refractor. Computer generated lenses with near text book diffraction patterns 4mm eyepiece with Sirius as my diffraction subject
An excellent mirror with enhanced coatings and a dielectric coated secondary should beat even Apochromats (relatively speaking) because of shear workmanship necessary to produce a quality Apochromatic lens capable of the same light grasp as say a 12.5" f-7
But then again, when I first saw Saturn with a 7" f-8 APO at the Miami planetarium I did the "Is this for real?" double take at first. The image was almost flawless with NO CA at all..at all.
Edited by wallie x (07/05/08 04:38 PM)
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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OK, but how does that fit the thread?  Nobody here's contending that a properly prepared 12.5" f/7 newt won't outperform smaller APOs, no "relatively" about it.
Best,
Mark
Edited by mark cowan (07/05/08 11:32 PM)
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