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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1456
Loc: Illinois
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These scopes supposedly have triplet lenses with an ED element but are not considered apos. What is their shortcomming? Color or shperical aberration? Thanks
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Astronomy educator/Sidewalk astronomer
Owner of Astronomy Delight franchise
18 inch f4.42 Dob on eq platform w ST120 f/5 finder
12 inch Zhumell Dob modified by Brian Reed on eq platform
8 inch f/8 eq planetkiller
William Optics red 10th Anniversary 80mm FD
PST
24lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
Edited by Darren Drake (01/10/08 01:49 PM)
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dougspeterson
Pooh-Bah
Reged: 09/01/04
Posts: 1070
Loc: Murrieta, CA
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They have some color, but not bad, perhaps somewhere in the ED doublet-ED triplet class but not a true APO.
I picked one up at an RTMC swapmeet from Gary Hand long ago. At first I thought it had spherical abberation, then realized it was designed to work with a prism diagonal. The star test still looked odd thru focus, but after comparision with the abberator program, I concluded there was a small residual of what is called 5th order or high order spherical abberation (as opposed to 3rd which is what most people are talking about), now something I see on all fast lenses. This has the effect of unbalancing the contrast between the intra-focal and extra-focal rings, i.e. inside snappy contrast, outside fuzzy.
Nevertheless the amount is quantitatively small and results in a Strehl of about 95. As a matter of fact, the lens gives snappy images and stars are perfectly round. I like it!
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18" Dob
12" SCT
8" TMB F6 + Chromacor
6" F6 APM/LZOS Fluorite triplet, 32", 32lbs
2ea. 6mm singlets, one blind
"--Granted, that's a worse case scenario. The destruction might in fact be ... limited to our own galaxy."
Edited by dougspeterson (01/11/08 12:09 AM)
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Jae
professor emeritus
Reged: 04/13/06
Posts: 609
Loc: New England
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Hi Darren,
I assume you saw this ?
http://www.cloudynights.com/item.php?item_id=907
There's actually another review by Ed Ting which I disagreed with.....Ed must have used one of the very few samples that was good for higher power. I think it was originally intended as good for high power but didn't cut it.
Jae
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Jae
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dougspeterson
Pooh-Bah
Reged: 09/01/04
Posts: 1070
Loc: Murrieta, CA
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Well Ed Ting is the best.
I see that in the cloudynights review the user was using a 2" mirror, not a long-path roof prism for which the scope was designed. I have ray-traced the effects of prisms vs. mirrors at various F#s and I can assure you at F5.6 the effect is huge, on the order of a quarter wave spherical.
In fact, my MasterBirder functioned similarly to the cloudynights reviewers with a mirror diagonal. The sharpest image, best focus was actually not even at the same point as the best color correction. With a prism diagonal, those two points coincided.
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18" Dob
12" SCT
8" TMB F6 + Chromacor
6" F6 APM/LZOS Fluorite triplet, 32", 32lbs
2ea. 6mm singlets, one blind
"--Granted, that's a worse case scenario. The destruction might in fact be ... limited to our own galaxy."
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BHunt
Pooh-Bah
Reged: 12/31/06
Posts: 1385
Loc: NJ and PA
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I did read somewhere they made two models of this scope. can anyone add to this?
Bill
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Jae
professor emeritus
Reged: 04/13/06
Posts: 609
Loc: New England
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Quote:
I did read somewhere they made two models of this scope. can anyone add to this?
Bill
Hi Bill,
I only know of the variants in the '80's to the '90's starting with the 1.25 version of the seeing is believing scope. I had one of these that had lots of color but was sharp, then shortly there after was the 2 inch version for birders. This latter one seems very much like the Master Birder optics. Later there was the APO F6 version that was a very good scope but was a bit longer using two connecting tubes. Then there is the Masterbirder - a low power birding scope......
I think Tony is pretty knowledgeable about these scopes if he can chime in.
Doug,
Who makes a roof prism for this ? I have Brandon prism diagonal and a Brandon Amici but never heard of them offering a roof prism.
Jae
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Jae
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dougspeterson
Pooh-Bah
Reged: 09/01/04
Posts: 1070
Loc: Murrieta, CA
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Jae, by roof I meant amichi, which are related:
http://www.tecplusplus.de/ManualLu/prisms.htm
As you can see, roof prisms, amichi or schmidt, etc. have a longer optical path because of the fold. In a converging beam this produces chromatic and spherical abberation, not a problem at F10, starts to generate APO damaging effects at F8, noticeable at F6, large at F5, unuseable at F4.
I believe a lot of controversy about the Masterbirder's performance is because of this. Also, the original F6.25 had a single element lens that screwed onto the front of the 1.25" prism to correct for exactly this issue. The MB does not: since it was sold with a deep amichi, it may have been designed precorrected for the abberations, the way high end binos and spotting scopes are.
The early "1800x" was a triplet consisting of a cemented doublet and a single airspace. There were a couple of spacer rings. I played around with the one I had back then and switching rings I could get better performance but never that good. The 2" version was identical but for the focuser change, I took advantage of the upgrade.
Doug
Edited by dougspeterson (01/12/08 12:49 AM)
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twhite
Pooh-Bah
Reged: 01/28/06
Posts: 1409
Loc: Broken Arrow, Oklahoma, USA
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Quote:
I did read somewhere they made two models of this scope. can anyone add to this?
Bill
Not of the Master Birder. The early 80mm f/6.25 smooth-body scope (possibly the scope referred to earlier as the 'seeing is believing' scope?) is still sought after by the birding community and is a very good birding scope, but usually is a poor performer as an astro scope, especially compared to some of the ED scopes available now. The MB was a later design (after the 80mm f/6).
There is also an f/5 ED triplet in the rubber armored tube with what I think is a Unitron focuser mated to it. I've heard mixed reviews about this scope, too, though I'd like to try one out some day.
The 80mm f/6 ED triplet rubber armored scope is the one you want if you can find it... though I think Darren knows this based on the conversations we've had in the past.  My Master Birder was a very good scope, but I couldn't justify keeping it after I found and acquired the 80 f/6. However, based on my experience and conversations with other MB owners as well as Gary Hand, I'm fairly convinced that the MBs were inconsistent performers across the run.
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Tony White
Astronomy Club of Tulsa
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18" f/4.5 Pegasus/homebuilt dob
TMB 152 - TMB 105 - AP 105EDF - Antares Elite 105/1500 - Unitron 102/1500 - Celestron C102F -Celestron C80F - Unitron 131-C - Unitron 140 - Brandon 80 f/6 - Coronado MaxScope 60 0.5A - and more...
Celestron Ultima 11, Orion Optics OMC-140, Celestron CometCatcher
AP1200GTO - AP400GTO - Vixen GP - DiscMount DM-6 -Lapides Tak Teegul - and more...
Garrett GT100-45, Celestron 11x80, Eagle Optics 10x50ED, Celestron 6.5x44ED
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dougspeterson
Pooh-Bah
Reged: 09/01/04
Posts: 1070
Loc: Murrieta, CA
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Tony,
yes indeed the focusers on the original 80 F6.25 were Unitron (Nihon Seiko sourced), at first the 1.25", same as on the #142, then later the 2", similar or a subset of the big Unitron focuser used on the 3" F6.25 and also the University and Orion "semi-apo" (really just an achromat) 3"F6.25.
I picked up my MB from Gary's swap table at RTMC a decade ago. He had about a dozen blems. Some of them looked perfect. I tested a theory and picked one with a minor but visible cosmetic blem thinking that these rejects were likely from either cosmetic or performance problems but odds against both. Sure enough my lens was a winner.
Edited by dougspeterson (01/13/08 07:58 AM)
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Ralph Marantino
Postmaster
Reged: 05/01/05
Posts: 6350
Loc: Old Bridge, New Jersey
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I have a 80mm seeing is believing asnd I love it,I also have a 94mm and a 130mm.Also 15 brandon/Vernonscope eyepieces and all of the filters. 
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Lunt H-alpha LS100THa/B3400
Rockland Astronomy Club
Member BAA Solar Section
Central Appalachian Astronomy Club
Astronomical League
DSPST,PST & 70mm Coronado Cal K telescope.2.2A
Zeiss 2 inch prism diagonal&Zeiss Zoom EP
2inch APM Herschel Wedge in a 94mm F7 Brandon
14.5 inch f4.2 Starmaster Dob(Zambuto mirror)
TeleView Genisis SDF with 1000 Oaks Glass filter
Zeiss 63mm f13.5 refractor 1000 Oaks Glass filter
Itty Bitty Solar Radio Telescope.
LHRIES Lite Solar Spectrometer
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Ralph Marantino
Postmaster
Reged: 05/01/05
Posts: 6350
Loc: Old Bridge, New Jersey
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94mm f7
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Lunt H-alpha LS100THa/B3400
Rockland Astronomy Club
Member BAA Solar Section
Central Appalachian Astronomy Club
Astronomical League
DSPST,PST & 70mm Coronado Cal K telescope.2.2A
Zeiss 2 inch prism diagonal&Zeiss Zoom EP
2inch APM Herschel Wedge in a 94mm F7 Brandon
14.5 inch f4.2 Starmaster Dob(Zambuto mirror)
TeleView Genisis SDF with 1000 Oaks Glass filter
Zeiss 63mm f13.5 refractor 1000 Oaks Glass filter
Itty Bitty Solar Radio Telescope.
LHRIES Lite Solar Spectrometer
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