Lens for DIY refractor?

[AstroApprentice]

Would it be possible to use this lens for a DIY refractor?

https://www.ebay.com.au/itm/Telescope-Lens-two-element-plano-convex-170mm-diameter/273589871955?_trkparms=aid%3D111001 %26algo%3DREC.SEED%26ao%3D1%26asc%3 D20160908105057%26meid%3D9e2f55581a 9a412f85a297f038a5ef1e%26pid%3D1006 75%26rk%3D1%26rkt%3D15%26sd%3D27358 9871955%26itm%3D273589871955&_trksid=p2481888.c100675.m4236&_trkparms=pageci%3Ab001f259-f811-11e8-b2bb-74dbd180c050%7Cparentrq%3A7b4796c81 670ab6bd720bc57fff87be6%7Ciid%3A1

[Wavytone]

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.

[anj026]

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.

[glend]

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.

[Merlin66]

Jason,
If you're looking for cost effective Achromats, have a look at the Surplus Shed offerings....
https://www.surplusshed.com/pages/item/L3967.html

[glend]

Those are out of stock Ken.

[Merlin66]

Ahhh, yes...
What about the 102mm??
https://www.surplusshed.com/pages/item/L14819.html

[AstroApprentice]

Thanks guys,
The i-star lenses look very promising
http://www.istar-optical.com/r30%2c-r35---r50-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!!!:eyepop:

[anj026]

Here is a link to a home made refractor using the Istar 180 lens.

https://www.cloudynights.com/topic/562689-thoughts-on-istar-1801440-refractor/

[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/monthly_12_2011/post-2597-14073834301145.jpg

The first one in your link 5" F12 wich CA ratio 2.4 should work decently though.

My 2 cents :)

[Wavytone]

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.

[skysurfer]

TS (Germany) supplies a 80 and 90mm APO triplets for EUR 540 (A$810) and 90mm for EUR 840 (A$1260).

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4710_TS-Optics-APO-Objektiv-80-480mm---Triplet---FPL-53-in-justierbarer-Fassung.html

https://www.teleskop-express.de/shop/product_info.php/info/p6359_TS-Optics-90mm-F6-6-FPL-53-Triplet-APO-Objective-in-Cell---air-spaced---adjustable.html

[fsphotography]

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.

[Totoro]

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.

[anj026]

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/325327-5-f12-istar-refractor-project/#entry4171500


Andy