cheap(er) DIY Hyperstar ?

[bojan]

I was playing with Optic raytracing app yesterday..

It looks like it may be possible to significantly correct the SA of the prime focus of C11 (when secondary is removed) with two lenses (RASA is achieving this with 4 lenses, last image attached).

The simulation was done at one wavelength only, so it is only to prove the concept.. obviously, single lenses should be replaced with CA corrected doublets (at least).
Also, as Stefan pointed out when I asked him a for opinion, the simulation is valid for optical axis only...

The whole thing was inspired by thi (https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwid6fjB_7_aAhVNtJQKHVWCCB YQFggnMAA&url=http%3A%2F%2Fwww.iap.uni-jena.de%2Fiapmedia%2FLecture%2FDesi gn%2Band%2Bcorrection%2Bof%2Boptica l%2Bsystems1351638000%2FDaCOS12_Lec ture_Part_12_Correction_of_aberrati ons_1_112.pdf&usg=AOvVaw1YLS5dBsAbh2RtgYGHgcoy)s article and this (https://www.cloudynights.com/topic/573637-inexpensive-alternative-to-fastar/page-3) CN thread, discussing the issue.
The third image shows rays as they are without abovediscussed correction.

[JA]

For no work there's always ..........

(Not that far off)

Best
JA

[bojan]

Quite :)

[billdan]

I was under the impression the corrector plate removed the SA, and the lens for hyperstar was a coma corrector for the short f2 focal length.

[bojan]

I think you are only partly correct - corrector plate removes the SA, but for cassegrain focus only....

Lens system for hyperstar is (I think) SA, coma and field corrector for prime focus. Those things are never strictly separate from each other...
Image from CN (https://www.cloudynights.com/topic/573637-inexpensive-alternative-to-fastar/?p=7833932) illustrates this very well - it was taken in prime focus (secondary removed) and SA is obviosly dominant.

[Hemi]

Hi JA, that’s nice glass....do you use it on a film canon, or have an ef adapter? If an adapter, which? And how good is it?

Nice!

Hemi

[JA]

Hi H,

I have the Nikon equivalent, used without adapters on various Nikon F mounts. It's very sharp, as is the Canon (I mentioned the Canon here because Bojan is a Canon Man)

Best
JA

[bojan]

Yes I am.. not by decision and/or because of some particular reasons, it just happened that way..
The lens in question (http://www.iceinspace.com.au/forum/showthread.php?t=138076&highlight=canon+400mm+f2.8) is quite good...
It is mechanically modified to EOS, the work was quite substantial..

Have a look... one of the shots (eta_Car) and in action these days...

[JA]

Nice 1 :thumbsup:- Well worth your work in modifying the lens.

On the subject of your DIY Hyperstar,
it wouldn't surprise me if a lens teleconverter of the type mounted to the front of a lens filter thread (NOT the type used between the back of the lens and the camera body) had some success as the front mounted optic system for your DIY Hyperstar, as the optical structure is somewhat similar, albeit afocal, to the double concave + plano convex lens you've shown in pic 1 . If you have one of these filter mounted teleconverters, it should be easy to see if it works. In principle just remove the secondary and place the teleconverter near the corrector plate to see if you can get an image on a small piece of paper held behind the teleconverter, when looking at a distant object during the daytime to make life easier. Adjust the paper and teleconverter position and focus to try to get a crisp image at what should be a vastly reduced focal length (you may need 10 hands!). You should compare the quality of this image with that with NO front optic group to see if there is a real improvement in terms of abberation.

OR ...... you could just go for a focal reducer and be happy to be somewhat faster

Best
JA

[bojan]

Hi JA,
Interesting idea..
I think now I have a teleconverter (the type you are mentioning) somewhere in my lens graveyard... so I will try with it - but not before I sort out the OTA mounting issues, which may take some time.
I also have couple of telescope objective air spaced doublets ~ø50mm (they were available from ebay for couple of bucks some time ago)..
As for 10 hands... 3D printer will help :D

[bojan]

Just found Nikon teleconverter (https://www.ebay.com.au/itm/52mm-58mm-2-0X-Magnification-Telephoto-Tele-Lens-for-Nikon-D90-D80-D70-18-55mm-N/132486681251?hash=item1ed8d252a3:m: mLGEFqpY8bxuKnSar3H_Vsw) on ebay...
Size and price is about right for experiment to prove the concept.

BTW...
There was a lot of discussion and ideas thrown in on CN (https://www.cloudynights.com/topic/599182-anyone-use-rev-imager-fun-star/)... and experiments, here (https://www.cloudynights.com/topic/599182-anyone-use-rev-imager-fun-star/?p=8272541).
They even did much more serious simulations, using OSLO.

Hyperstar patent (https://patents.google.com/patent/US7595942/en) was also mentioned.

It is interesting that Hyperstar group looks like Canon 400mm back half...

[JA]

If no luck in the lens graveyard then let me know I have a couple you can try. One is a very large Canon Teleconverter with 100mm diam front lens and 67mm rear screw on and the other is a VERY sharp and CLEAR Ricoh converter. Happy experimenting.

Best
JA

[bojan]

Thank you for the offer, we will certainly talk about it when I solve the mounting issue :).

I found 2 converters, but they are both wide angle... one of them is also (too) large, another one is smaller.

[Hemi]

That is a great image of eta! I really have fancied this lens for a while now... but no skill or experience with modding it to my ef body. And the adapters all degrade massively I’m told.

[fragis]

Bojan, what is your solution for hyperstar alternative? tks.
fragis:)

[bojan]

No solution yet.
Also, I am wandering if f/2 is really feasible at all, considering light pollution....
My f/4 Canon picks background glow after 10 sec of exposure.... in Ballarat I can go up to 1 minute (with LPF). That means F/2 will be there after 15sec. So, there is benefit in saving time.
Anyway, I will come back to this sometime in the future...


BTW, I just bought Nikon teleconverter.

[bojan]

Adapter arrived..

M42-EOS adapter ($2 from ebay) will mechanically fit nicely (after some machining). It will be possible to adjust the distances easily with additional filter rings, if necessary.

[bojan]

Now this (https://www.cloudynights.com/topic/774785-my-homemade-f24-hyperstar-for-c925-telescope/) is what I was waiting for :-)

[Sunfish]

Looks interesting and a fine project. It would be great if you could squeeze better performance from your lense.