One step beyond Stage 1 & 2
- Merlin66
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Re: One step beyond Stage 1 & 2
Well done!
Great report and successful outcome.
Your efforts are greatly appreciated.
Great report and successful outcome.
Your efforts are greatly appreciated.
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- swisswalter
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Re: One step beyond Stage 1 & 2
Hi Theo
wonderful mod, thank so much for explaining
wonderful mod, thank so much for explaining
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE
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Re: One step beyond Stage 1 & 2
Great stuff! I have a replacement collimating lens so that I can use a PST mod on my 70mm f6 scope, it works a treat too!
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Re: One step beyond Stage 1 & 2
Hi Neo,
Very nicely done.
BTW which side of the new lens are directed to the telescope objective? Flat or concave?
Thanks,
Valery.
Very nicely done.
BTW which side of the new lens are directed to the telescope objective? Flat or concave?
Thanks,
Valery.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.
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- swisswalter
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Re: One step beyond Stage 1 & 2
Ahhhhh that site has to be given a tripple A+ rating
thank you very much modders, so much interesting stuff and fun
thank you very much modders, so much interesting stuff and fun
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE
from 7 am - 7 pm http://www.nanosys.ch
from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa
from 7 am - 7 pm http://www.nanosys.ch
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- michael.h.f.wilkinson
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Re: One step beyond Stage 1 & 2
Very interesting indeed!
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- Valery
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Re: One step beyond Stage 1 & 2
Sorry to disappoint you, Michael and Neo and, possibly, many other modders.michael.h.f.wilkinson wrote:Very interesting indeed!
I spent a fair amount of my time to see what is what with this modification. I used Zemax optical design software.
I took the design of the SW 150mm F=750mm refractor and simulate the following configurations:
1. SW 150mm F=750mm + typical 2x Barlow + PST etalon as is. Distance between the 2x Barlow and the front PST etalon lens 10mm.
Effective focal length changed to 2374mm if the back focal length (behind the PST etalon) remains intact.
2. SW 150mm F=750mm + BK7 plano-concave negative lens F= - 100mm, inserted in the PST etalon as the replacement of the PST collimator lens.
3. SW 150mm F=750mm + specifically designed ARIES negative achromatic lens to produce F/10 beam + PST etalon as is.
The results:
1. Spherical aberration at 656nm at the center of the FOV 0.54 wave. Strehl ratio 0.45
Resulting berrations at 656nm at the edge of the sweet spot (approximate) 1.98 wave Strehl ratio 0.
2. Spherical aberration at 656nm at the center of the FOV 0.46 wave. Strehl ratio 0.52
Resulting aberrations at 656nm at the edge of the sweet spot (approximate) 3.2 wave Strehl ratio 0.
3. Spherical aberration at 656nm at the center of the FOV 0.07 wave. Strehl ratio 0.99
Resulting aberrations at 656nm at the edge of the sweet spot (approximate) 0.1 wave Strehl ratio 0.99
As can be seen, home made mods with F/5 refractors are not that good at all. Images will never be really sharp
and this always will require a lot of sharpening, but this will increase the noise and still not reveal the smallest details
and their contrast will be from low to very low.
I can add that such a special Aries 2x Barlow lens, designed exactly for this case, was already made and tested.
The results are were expected from the Zemax design board (screen).
Hope this helps.
Valery.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.
Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
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- Valery
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Re: One step beyond Stage 1 & 2
Hi Neo!Neo wrote:
1. Instead a typical Barlow a 2 inch 4-element telecentric Powermate was used. I have tried a regular Barlow with horrible results. The distance between PST frontlens and last Powermate element is not 10 but closer to 40 mm. (Precise value can be measured)
2. That is close to my new situation. Strehl of 0.52 brings tears to my eyes and any ideas for improvements are very welcome!
Have you taken the thick D-ERF into account that is located somewhere down the lightpath behind the objective lens? (That may worsen things even more)
3. I've read good things about the ARIES but I assume it is not available for us modders?
In the same order:
1. No matter what kind of Barlow will be used, the results will be the same poor. And placing the PST etalon farther from the Barlow will make
things worser. The Barlow (no matter of type and manufacturer) is not designed to fix a spherical aberration at 656nm in the 150mm F=750mm
achromatic objective. Also, the barlow will not correct this SA to the level of it in the 40mm PST achromat. The same with the field curvature and
the astigmatism. See the difference in the performance for the center of the FOV and at the edge of the sweet spot. It is dramatical.
So, if a maximum performance is a goal, then a special correcting lens is required.
2. No, I didn't take in to account the ERF thickness in the beam. It can make things worser in this given case and a bit better (a bit !!!) in the case
of your style of mod.
3. Of course, it is not available. But if a fair number of moders will order it simultaneously with 100% deposit, then we can make one or two runs
of such a corrector barlow. I like the performance of such a lens in my scope. It does not leave much to be desired.
4. Such a corrector has better performance than in the case we will re-design both two lenses in the PST etalon for this 150mm F=750mm telescope!
And this is more flexible solution (corrector 2x) and can be easily used in a 100mm F/5 and 120mm F/5 SW refractors even with better overall
performance sweet spot size and uniformity wise.
Valery.
Last edited by Valery on Mon Jan 12, 2015 12:51 pm, edited 1 time in total.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.
Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
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- marktownley
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Re: One step beyond Stage 1 & 2
Regards point 3 I would be interested.
Mark
Mark
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Re: One step beyond Stage 1 & 2
I did some Zemax simulations as well on the same topic and I agree with Valery : using one singlet plano-concave gives poor result with a strong spherical aberration. To work better, the lens must have two different curvatures (meniscus) or use a cemented doublet.
Note also that the soft spot depends on f/F and therefore is impacted by changing the negative FL.
It is also possible two decrease the field angle but that requires a strong menicus and you can't remove it completly as it would break the thermodynamics laws
Note also that the soft spot depends on f/F and therefore is impacted by changing the negative FL.
It is also possible two decrease the field angle but that requires a strong menicus and you can't remove it completly as it would break the thermodynamics laws
- robert
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Re: One step beyond Stage 1 & 2
Is the original lens in the PST etalon well specified for SA etc? I had imagined it was a rough and ready device to be low cost.
Robert
Robert
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- marktownley
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Re: One step beyond Stage 1 & 2
Interesting Neo, can you post some links for this, I would like to read about it...
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Re: One step beyond Stage 1 & 2
thank you!
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Re: One step beyond Stage 1 & 2
You are right that the manufacturing tolerances are too wide in the case with chinese achromats. My system has been designed for the case of SW 150mm F=750mm objective as "design specs" and it is opitimized for 555nm green light. My own objective has no SA at 546nm. Therefore it works at 100%Neo wrote:Valery, from what I read elsewhere on internet and also Rohr's reports is that the Synta 150 mm f/5 is often a mixed bag, performing horrible at green (Strehl < 0,3) and almost textbook performance at Red with Strehl 0,96. But with the wide manufacturing tolerances, it's a lottery what you get. Is your telescope objective optimized for H-alpha wavelengths?Valery wrote:The results:
3. Spherical aberration at 656nm at the center of the FOV 0.07 wave. Strehl ratio 0.99
Resulting aberrations at 656nm at the edge of the sweet spot (approximate) 0.1 wave Strehl ratio 0.99
as designed.
The case Mr. Rohr told you is almost hypothetical (very rare). Chinese achromats almost always have undecorrection which make things for SA at H-a even worser than
in the case of original design.
Here another samples: http://www.astronomy.ru/forum/index.php ... .2040.html
http://www.astronomy.ru/forum/index.php ... .2100.html
Valery.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.
Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
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- marktownley
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Re: One step beyond Stage 1 & 2
thanks for the links valery, some more reading for me when I get home from work.
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Re: One step beyond Stage 1 & 2
Hi Theo
very interesting. Looking forward to your findings on the star
very interesting. Looking forward to your findings on the star
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE
from 7 am - 7 pm http://www.nanosys.ch
from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa
from 7 am - 7 pm http://www.nanosys.ch
from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa