ICEINSPACE
Moon Phase
CURRENT MOON
Waxing Gibbous 82.5%
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05-12-2018, 09:25 AM
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Registered User
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Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
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No. Note the seller has not disclosed the focal length. The strong curved internal reflections suggest that is a condenser lens ripped out of a projector, meaning the focal length will be probably 100mm, hopelessly short and the image quality terrible.
If you want a lens for a DIY effort there are:
- surplus shed, which sells suitable cheap doublets of known aperture and focal length, note these are unmounted so you’ll have to figure that out for yourself;
- APM, which sells high quality Russian objectives from LZOS in a temperature compensated lens cell.
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05-12-2018, 10:39 AM
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Plyscope
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Join Date: Oct 2006
Location: Perth
Posts: 519
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For lens in cell 5" and over there is Istar.
I am a big fan of D&G but they seem to have stopped production for a while. Even when they were making lenses there could be a wait time of 1 -2 years.
For smaller lenses try MASILMW on Cloudy Nights Classified section (ATM parts). He has Carton lenses from Japan and recently has been offering Explore Scientific lenses.
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05-12-2018, 10:42 AM
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Registered User
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Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,048
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Jason, have a look at the iStar objectives, they are very well made, come already mounted in the adjustable cell, and are cost effective.
http://www.istar-optical.com/achromatic-doublets.html
They offer regular Achromat lenses, and an Anastigmatic version with reduced CA. I used their 127mm f12 R30 in a refractor I built and it is a wonderful setup.
D&G, imho, tend to be more expensive than iStar and have limited production.
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05-12-2018, 01:12 PM
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Registered User
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Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,048
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Quote:
Originally Posted by Merlin66
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Those are out of stock Ken.
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05-12-2018, 02:01 PM
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Registered User
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Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
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05-12-2018, 03:09 PM
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Registered User
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Join Date: Dec 2015
Location: Melbourne, Australia
Posts: 349
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I-Star 250 mm F/20!!!
Thanks guys,
The i-star lenses look very promising
http://www.istar-optical.com/r30%2c-...-doublets.html
I particularly like the look of the 180 mm F/8 - most of the others would require incredibly long tubes - such as the 250 mm F/20 5000 mm FL!!!
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05-12-2018, 09:17 PM
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Registered User
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Join Date: Mar 2014
Posts: 5
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Quote:
Originally Posted by AstroApprentice
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If I'm correct 180mm F8 achromatic objective has CA ratio(F / Dia in inch) about 1.13, according to this widely referenced chart, is classed as unacceptable. I'm not sure if the claimed Strehl ratio 0.91 is for single wave length or weighted full spectrum.
https://www.cloudynights.com/uploads...3834301145.jpg
The first one in your link 5" F12 wich CA ratio 2.4 should work decently though.
My 2 cents
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05-12-2018, 11:21 PM
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Registered User
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Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
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That strehl figure is monochromatic - it’s usually measured in an interferometer.
Lateral and longitudinal colour correction will be woeful at f/8 for an achromatic that big. Even to be considered “tolerable” it would have to be f/12 and f/15 would be better.
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06-12-2018, 04:43 AM
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Dark sky rules !
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Join Date: Dec 2011
Location: 52N 6E (EU)
Posts: 1,152
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06-12-2018, 02:38 PM
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Registered User
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Join Date: Mar 2012
Location: geelong
Posts: 60
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My DIY scope using an ISTAR ''PERSEUS'' 127MM F8 lens is an excellent performer,purchased the lens here on IIS. Absolutely minimal ca,even on the moon and planets,shows pinpoint stars and performs brilliantly on dso's.
This is an older lens and its said that the current offering are something of an improvement. I'm very happy with what i have and don't feel the need to upgrade even at ISTAR'S very reasonable prices. My TSA 102 gets used a lot less now as the ISTAR is just more versatile and very enjoyable to use.
Frank.
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06-01-2019, 01:59 PM
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Registered User
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Join Date: Mar 2014
Posts: 5
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Just to correct my own statement below, the Istar 180F8 R35 is calimed to be made of special glass that has 35% less chromatic aberrations than classic flint-crown achromat, thus roughly has similar CA of classic achromat that is 35% slower @ F10.8 making the equivalent CA ratio 1.52.
Similarly, the 5' F12 R30 has equivalent CA ratio 3.12 instead of 2.4, making it an excellent achromat doublet, in theory regarding CA.
Quote:
Originally Posted by Totoro
If I'm correct 180mm F8 achromatic objective has CA ratio(F / Dia in inch) about 1.13, according to this widely referenced chart, is classed as unacceptable. I'm not sure if the claimed Strehl ratio 0.91 is for single wave length or weighted full spectrum.
https://www.cloudynights.com/uploads...3834301145.jpg
The first one in your link 5" F12 wich CA ratio 2.4 should work decently though.
My 2 cents
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09-01-2019, 11:29 AM
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Plyscope
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Join Date: Oct 2006
Location: Perth
Posts: 519
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The Istar 5" f12 is an excellent lens in my experience. Please refer to the link below for more details on my experience.
In comparison to a regular 5" f12 achromat lens in cell, the Istar is comparatively heavy due to the thicker glass elements. The reduction in CA is a benefit however if I was doing it again I would choose the standard Istar 6" f10 lens in cell as it weighs the same and has greater aperture and costs almost the same price as the 5" f12 R30 lens.
The only advantage to the 5" f12 R30 is the reduced CA on planets and bright stars and it is a personal choice whether that is preferable to greater light grasp with some CA. A standard 6" f10 or f12 can always be stopped down to 5" or less to reduce the CA for planets but will still be far superior on DSO's where the CA is not obvious.
https://www.cloudynights.com/topic/3.../#entry4171500
Andy
Last edited by anj026; 09-01-2019 at 11:49 PM.
Reason: updated
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