drexelpbp
sage
Reged: 01/21/08
Posts: 269
Loc: Phoenixville,PA
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Has anybody seen this objective at Surplus Shed?
http://www.surplusshed.com/pages/item/l13001.html
Wondering if this would make a good RFT, How much magnification would it be able to handle?
Thanks
Gary
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Orion 8" intelliscope
Celestron CF 8' SCT
Celestron 8SE
Celestron C80ED
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Orion ST80
Orion 120 f/8.3
Orion Skyview Pro Deluxe 90 f/11
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Various Televue, Baader, Orion and other EPS
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Sky Captain
Metal Whisperer
Reged: 11/07/04
Posts: 7195
Loc: Issaquah, WA.
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I saw it too. The focal length is about 7.8 inches and the image circle is fairly large so EP's would not be able to take advantage of all the light gathering capability. Also there wouldn't be any room for a focuser with the short FL. You would have to make a lens based focuser in this case. This lens is not designed for magnification, but for wide angle, bright images for photography.
They do have a focal lengthener lens that came with it originaly, but they don't state how much longer it will make the FL. This extra lens will flatten the field for photography purposes as well.
To give you an idea, here is my lens that is similar but has a 4" front lens as opposed to the 6" at SS. Mine is F1.8, as opposed to their F1.25, and has the lens based focusing. It is not possible to get an EP to focus with it. Mine is only 7.5" long. It would make a great wide field video lens though, and that would be great for capturing metor showers.
$300 for both lenses and shipping would be a very expensive start to a RFT project. I could see it hitting $800 or so to build a workable model.
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Equipment Overload!
Kerry
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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1234
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What about barlowing?
Jess Tauber
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Sky Captain
Metal Whisperer
Reged: 11/07/04
Posts: 7195
Loc: Issaquah, WA.
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Quote:
What about barlowing?
Jess Tauber
If you think about how a barlow works in lets say a refractor, if you put one in your diag. and then insert the EP, only minor repositioning of the focuser is needed. You would need to have an optical lens that would be matched to the main lens and have it physicaly make the focal point longer. At this point you might as well just get a 6" F6.5 refractor which probably would be cheaper in the long run.
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Equipment Overload!
Kerry
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drexelpbp
sage
Reged: 01/21/08
Posts: 269
Loc: Phoenixville,PA
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Thanks for the great - it saved me from making a potentially expensive mistake.
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Orion 8" intelliscope
Celestron CF 8' SCT
Celestron 8SE
Celestron C80ED
Orion ST120
Orion ST80
Orion 120 f/8.3
Orion Skyview Pro Deluxe 90 f/11
Celesrton SP C102
Celestron SP 150N
Various Televue, Baader, Orion and other EPS
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RoverWanderer
member
Reged: 06/10/05
Posts: 35
Loc: roaming
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In general I think both lens are candidate for a good classical telephoto combination of positive+negative couple.
Having in mind the formula f=(f1*f2)/(f1+f2-d), where d is the distance between the lens, and for our purposes I assume d= f1/2 (Galilean telescope), then the final focal length would be 780mm. This would mean about f5, which is quite fast, and probably would be interesting for astrophoto.
What is unknown is how well chromatism is corrected...
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Celestron 8" SC XLT
H-EQ5 + Rajiva mod
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Meade DSI Pro II b/w
Canon 350D + 500/5.6 Rubinar
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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My surmise would be that in order to work properly the negative extender *must* be used in conjunction with the positive group. And the correct spacing between the two lens groups would have to be observed, too. And there *may* even have to be a correctly sized and positioned aperture stop somewhere in between, to boot!
Be careful of what you read on the SS site. For example, their statement that this triplet by itself seems to be well corrected should be accompanied by some description of the test method. If someone simply held an eyepiece behind the lens, then its virtually certain that only a small portion of the lens was contributing to the view due to the *huge* exit pupil that an f/1.25 lens would necessarily deliver. Such a simple test reveals very little.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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ed_turco
member
Reged: 08/29/09
Posts: 30
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Fascinating lens, but I cannot think of a use for it.
Well maybe -- How about getting two of them and making a symmetrical eyepiece? In this era of extreme everything. why not make a series of 7" OD eyepieces?
A new optical standard! I'm sure the big buys can make an 8 element version that will weight about 100 pounds or so 
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MtnGoat
Pooh-Bah
Reged: 02/18/07
Posts: 1213
Loc: Columbia Gorge, WA
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I was looking at these online and pondering. Tonight I had nothing better to do than grab a sheet of paper in the shop and sketch up an astrograph. Then I took a pic of it with an ancient *BLEEP* digital. Then I hit that with Nebulosity. It's mostly readable.
Looks to me like there might be room for a stubby crayford. you might need to cut the drawtube down.
the scope is ridiculously cartoonish, and the sheer incongruity of this glass hit me when I realized i'd just sketched out a 6" refractor on a single sheet of paper...at full scale!
so, other than tossing assumptions down on paper, are there any glaring errors here?
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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I got one of these SS 6.1 lens
it has lots of coma
however there is a small usable center
here is shot of what I built 50$ for box and focuser
and shot of ic 1338 with mallincam and big lens
http://www.cloudynights.com/photopost/showphoto.php?photo=19841
http://www.cloudynights.com/photopost/showphoto.php?photo=19842
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MtnGoat
Pooh-Bah
Reged: 02/18/07
Posts: 1213
Loc: Columbia Gorge, WA
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how about a couple more shots of your implementation?
Collimation is hyper crucial at this FL and the focus should be super sensitive..what's your focusser and how did you collimate?
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
Edited by MtnGoat (11/14/09 10:36 AM)
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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ok I'll get to it later
yes collimation is a real pain in the butt -- I call this lens the "mule". I used a laser (howie glatter's type with hologram)
pattern that helps. I used a regular orion focuser 2 inch
but like I said its ok with small ccd chips any big forget it - lke like old metrogon lens in the way i mount it
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MtnGoat
Pooh-Bah
Reged: 02/18/07
Posts: 1213
Loc: Columbia Gorge, WA
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I found a tube in the garage, a large thick walled sonotube like thing used for shipping eletronics components. ID is 153mm, close enough to mess with. It's straight, did not get whacked in shipping.
A friend bought one of these lenses, I want to go do a test fit just for giggles.
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
Edited by MtnGoat (11/20/09 01:50 AM)
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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I plan to use more as an eyepiece for my 42 foot refractor
this lens has issues - lots of coma - here is a another shot
http://www.cloudynights.com/photopost/showphoto.php?photo=19877
this lis also works better at single wave - like ha
will try to get more pictures- been busy with MMT whch I find more interesting
Edited by highfnum (11/20/09 04:02 PM)
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GlennLeDrew
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Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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highfnum,
Not only coma (which will evidence itself off-axis), but huge amounts of spherical aberration, and no small amount of lateral color!
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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thats why single wave (ha) is better - see shot from above
it works pretty well as a portrait camera
but for astronomy its more limited
I will test as eyepiece on 42 footer when I have chance
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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igot some improvement by using corrector lens (kind of barlow+reducer )
here is a test shot
http://www.cloudynights.com/photopost/showphoto.php?photo=19904
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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Much better! Any details on the 'corrector' you used?
(BTW, I added a note confirming the object you imaged as being the 'Heart', and of which only a portion of this large nebula appears in your image.)
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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highfnum
super member
Reged: 09/06/06
Posts: 236
Loc: NY
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its a combo negative lens and a positive achro where
they almost cancel out to within (5-10 %) old trick used by NVD devices
tried lens as eyepiece with forty footer -- nice image of moon -- thats the real reason why I bought it -- prime focus image of moon is 5 inches wide this is 6.1 -- it fits
but i'll do more testing as fast lens
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MtnGoat
Pooh-Bah
Reged: 02/18/07
Posts: 1213
Loc: Columbia Gorge, WA
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have you tried the one which was offered with that lens?
and post a pic of the forty footer!
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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain
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