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New APM 6“ f/6 SD Apo design> need your all input

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78 replies to this topic

#1 APM M.Ludes

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Posted 22 June 2021 - 12:13 AM

Hello Everybody

 

many asked for it and as you know I do my best to fullfill  your wishes

 

we start now with the Zemax design for my new Apo Project, 6“ fast doublet , using FCD 100 and whatever Matches best

 

f/6 means very difficult making. So we need for cheaper serial production a not to sensitive design for optical work shop and assembling

 

A triplet can do all better, no question , but we want something real fast and lightweight 

 

i believe important should be :

fieldcurvative should work well with our XWA eyepieces for widefield

colorcorrection as good as possible with this field correction

imaging field can be only small with correctors, or we need triplet

 

please give me your wishes and input before our designer start first designs

 

thanks

 

 


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#2 Tyson M

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Posted 22 June 2021 - 12:32 AM

Sounds like an intriguing project. 


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#3 AstroPotamus

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Posted 22 June 2021 - 07:14 AM

I'd like to see as much backfocus as possible, since this is a perfect DSO AP rig!
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#4 Phillip Creed

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Posted 22 June 2021 - 08:41 AM

I'm of the opinion that this scope should be optimized for visual and come-what-may for intrepid imagers.  A 6" f/6 doublet, regardless of glass type, just seems way too fast for serious imaging, though I can imagine this might work if the user isn't too picky on blue bloat.

As a VISUAL instrument, though, it would have its place.  You can put a larger focuser on it and allow the use of 3"-eyepieces, for instance.  A 30ES-100 would yield a staggering 3.3°-FOV with 6" of unobstructed aperture; an absolute dream for widefield junkies like myself.

A 6" f/6 ED doublet would at least allow decent higher-powered views of globular cluster, planetary nebula and galaxies without the image degradation that comes with comparable fast achromats.  Chromatic aberration is more damaging to visual image quality on deep-sky objects than many people think.  A 6" f/6 with FPL-53 / FCD100 + Lanthanum would simply blow away either the 150mm f/5 or 152mm f/5.9 achros out there at the eyepiece if using, say, 100X - 150X on galaxies.  I'm not sure, though, that it would be good enough correction of chromatic aberration and spherochromatism for, say, high-powered views of lunar/planets.

Clear Skies,

Phil


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#5 AstroPotamus

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Posted 22 June 2021 - 09:01 AM

Phil, you obviously know more than I do about telescope design!  May I misunderstood what the design ideas were, but I do remember seeing "we need for cheaper serial production a not to sensitive design for optical work shop and assembling."  Which made me think that this would be a great scope in line with the Evostar 150 APO Refractor, list price US$2515, which is also a flexible visual and AP scope.  Given that, my choice would be to use it as an alternative to the triplet APOs (which I cannot afford) for AP, thus my request for as much backfocus as possible.  wink.gif



#6 ihf

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Posted 22 June 2021 - 11:12 AM

I am interested in visual. And I am interesting in a scope that is very portable to bring camping to dark sites. Maybe this scope could be for me? My space and weight capacity is limited. My 100ED f/9 works great around the house, but I feel it is a little long to take with me, but could be done in a pinch. The 4kg weight is about right, but I could go up to at most 8kg. Now I am wondering if 6 inch f/6 is maybe going to be a little bit too long and heavy to throw into the trunk? The Orion f/5 120ST seems about right in size and also weight (about 4kg). But I wouldn't mind a higher quality version of it with a better focusser and better glass. I assume 6 inches f/6 would be around 9-10kg? Plus rings etc. That is heavier than I want to lift and probably would require me to upgrade my mount from a 40mm carbon video tripod plus Stellarvue M002C. (Which I don't see great options for.) So if the 6 inch f/6 was light enough, or could be an f/5, or could arrive with a smaller sibling I would be interested in it. Otherwise it is likely more than I want to handle.


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#7 AstroPotamus

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Posted 22 June 2021 - 11:52 AM

150mm objective with f/6 aperture means 900mm focal length.  That's the same as your 100ED f/9.  Except with 225% more glass.  Which means 225% more weight on the objective.  Don't expect it to be better than your 100ED unless magic happens.


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#8 ihf

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Posted 22 June 2021 - 12:01 PM

I obviously expect it to be worse but with more aperture. And I expect the weight to be more than 225% if the same materials are used, but maybe carbon etc. can help? My statement here is that the proposed 6 inch f/6 may just beyond what I am able to handle. But maybe there are people that are also not into astro photography that feel different?



#9 Phillip Creed

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Posted 22 June 2021 - 12:39 PM

I obviously expect it to be worse but with more aperture. And I expect the weight to be more than 225% if the same materials are used, but maybe carbon etc. can help? My statement here is that the proposed 6 inch f/6 may just beyond what I am able to handle. But maybe there are people that are also not into astro photography that feel different?

When I was strictly visual, a 6" f/6 ED doublet would have been a DREAM scope.  It'd have a fair amount of CA; no getting around that.  But FAR better than any 6" achro, not terribly heavy/long, and would probably put up a nice image on planets if it were stopped down.

And therein lies the rub.  A 6" f/8 ED would cost LESS to make than a 6" f/6 and would have better color correction.  Other than the need for wide fields and a shorter tube, a 6" f/6 ED means you're essentially paying more and getting less...

 

...unless there's a way to tweak the system.  For instance, Markus *ALREADY* makes a short 6" f/6-ish scope.  He just happens to do two at once, in the form of the APM 150 SD binoscope.  At 840mm focal length, that's f/5.6 and all accounts say they've got great color correction.  But they also use a prism assembly.  I'm not the optical expert, but maybe the prisms help tame CA.

So who knows?  Maybe if APM decides just to (figuratively) saw the 150-SD binoscope in half and put some mounting rings on the resulting tube, it could work.

Clear Skies,

Phil


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#10 photomagica

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Posted 22 June 2021 - 05:18 PM

It could be a difficult challenge to distinguish a 150mm f6 Apo Doublet in the market place. Several comments here lead to the concept of making it a systems telescope in order to do this. That is the base OTA plus carefully  matching system components are what distinguishes it and makes it appealing. Possible matching system components:

  • Designed to match a set of eyepieces like the XWA or a 3" ES (Already suggested.)
  • An exceptionally large diameter, long-travel focuser (Already suggested.)
  • A matched imaging corrector that would handle coma, field curvature AND residual chromatic aberration.
  • Or two available matched correctors, one for visual and one for imaging.

What else would transform the suggested base OTA into an exceptional systems telescope?

 

Here are a couple of additional thoughts:

  • Lightweight tube assembly design so that this would work on mounts now comfortably carrying smaller aperture telescopes in the 900 to 1000mm focal length range.
  • Mounting the dovetails directly to the tube, versus using tube rings in order to increase overall rigidity.

Other thoughts?

 

Markus - I wish you well with this design challenge and celebrate you opening up the process to suggestions from the community.

Bill



#11 Phillip Creed

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Posted 22 June 2021 - 06:34 PM

It could be a difficult challenge to distinguish a 150mm f6 Apo Doublet in the market place. Several comments here lead to the concept of making it a systems telescope in order to do this. That is the base OTA plus carefully  matching system components are what distinguishes it and makes it appealing. Possible matching system components:

  • Designed to match a set of eyepieces like the XWA or a 3" ES (Already suggested.)
  • An exceptionally large diameter, long-travel focuser (Already suggested.)
  • A matched imaging corrector that would handle coma, field curvature AND residual chromatic aberration.
  • Or two available matched correctors, one for visual and one for imaging.

What else would transform the suggested base OTA into an exceptional systems telescope?

 

Here are a couple of additional thoughts:

  • Lightweight tube assembly design so that this would work on mounts now comfortably carrying smaller aperture telescopes in the 900 to 1000mm focal length range.
  • Mounting the dovetails directly to the tube, versus using tube rings in order to increase overall rigidity.

Other thoughts?

 

Markus - I wish you well with this design challenge and celebrate you opening up the process to suggestions from the community.

Bill

Agreed.  Throwing conventional designs aside and making a system design might be doable.  All that matters is the ends, not the means, and if it's affordable and works, it'll sell.  The obstacles are daunting, but hopefully Markus can pull it off.

Clear Skies,

Phil


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#12 photomagica

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Posted 22 June 2021 - 08:07 PM

Giving this more thought, still from the systems viewpoint, rather than a corrector that removes the residual chromatic aberration, a more cost effective approach could be to have a matched narrow band filter that would just notch out much of the residual chromaticism. This would be something like the Baader Semi-apo filter for achromats but designed very specifically to just reduce the very small amount of residual chromatic aberration in this specific lens.

A corrector may still be needed for curvature and coma, but simpler than one that would attack residual chromatic aberration as well.

Bill



#13 Moravianus

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Posted 22 June 2021 - 08:30 PM

If the coating technology advanced and there would be a possibility of a gradual front lens coating, it can filter the blue from the outside ring of the lens surface improving the visual perception.



#14 Tyson M

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Posted 23 June 2021 - 01:58 AM

I had a TS152 f5.9 achro and loved it. Color wasn't as noticeable on deep sky as you would think.

 

I'd consider buying an APO version of this scope that cools quicker than a triplet. Easy to handle means more use for grunt and go.



#15 APM M.Ludes

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Posted 23 June 2021 - 05:32 AM

Basically the size would be the same as our APM 140 mm F/7 Apo  , weight would be very similar, maybe 1 kg more

 

but a F/6 needs more then a 2.5" focuser

 

big back focus is no problem, we could use a extension between tube and focuser which can be removed for binoviewer use


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#16 ihf

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Posted 23 June 2021 - 10:02 AM

The APM 140mm f/7 Apo is listed as

 

Length during observing:1030 mm

Transportation length:830 mm
Total weight:9,0 kg ( with Tuberings, handle, 3" dovetail )



#17 Jon Isaacs

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Posted 23 June 2021 - 10:12 AM

My two cents:

 

For visual, I don't see field curvature as being too much of a problem.  It's there at 900 mm focal length but in my 120mm F/7.5, I just don't see much.  The defocused blur would 25% larger in diameter at F/6 but that's not much.. 

 

It would seem this scope would be more expensive than the 150mm F/8 just because of the glass costs.  

 

Jon


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#18 Astrojensen

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Posted 23 June 2021 - 01:25 PM

It will be VERY difficult to tame the Spherochromatism in a 6" f/6 ED doublet... A triplet might actually be the easiest way to do it - perhaps not even heavier - as the curves can be far shallower. 

 

 

Clear skies!

Thomas, Denmark


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#19 ihf

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Posted 24 June 2021 - 10:21 AM

IIRC the trick with the new APM 150mm SD binos was that the lenses were matched with the prisms. Maybe this could be done here as well? It is something that was mentioned for the Borg 125SD doublet

"A subtle red fringing is noticeable instead, manifesting itself especially around brighter orange and red stars. By using a good prism star diagonal, however, the scope almost eliminates the reddish tinge at powers less than about 250x."



#20 russell23

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Posted 24 June 2021 - 10:43 AM

Whatever it comes out to be I would want it to be under 20lbs weight.  The color correction would not have to be at the level of other scopes on the market, just a lot better than the 6" f/5-f/8 achromats on the market.


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#21 t.r.

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Posted 24 June 2021 - 10:53 AM

It will be VERY difficult to tame the Spherochromatism in a 6" f/6 ED doublet... A triplet might actually be the easiest way to do it - perhaps not even heavier - as the curves can be far shallower. 

 

 

Clear skies!

Thomas, Denmark

Agreed! Even in triplet design by RC, he stayed at f/6.6 to keep the correction up on the Stowaway and at f/5.9 the Traveler is said by many to be just barely 1/4 wave across all colors (almost). The stated objective of "we need for cheaper serial production a not to sensitive design" seems to be at odds with reality of f/6 in doublet format.


Edited by t.r., 24 June 2021 - 10:54 AM.

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#22 Phillip Creed

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Posted 24 June 2021 - 11:49 AM

If the coating technology advanced and there would be a possibility of a gradual front lens coating, it can filter the blue from the outside ring of the lens surface improving the visual perception.

Maybe this is the way to go.  If it's sold as a system with a diagonal, what would happen if the same coating was applied to both of the objective's elements and a supplied custom diagonal?  Didn't Stellarvue or Burgess try doing this a while back?

A Petzval design would give great color correction at f/6, but it'll be physically longer than a 6" f/6 doublet and $$$$ OUCH.

Clear Skies,

Phil



#23 Moravianus

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Posted 24 June 2021 - 12:54 PM

No use for diagonal, but yes for the lens at areas contributing to the blue halo most. Advantage against a plain MV filter is better color balance, removal of blue but not shift to yellow.



#24 ihf

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Posted 24 June 2021 - 01:00 PM

So what is the competition to the proposed scope? In my mind

- Orion 120 f/5 achro 3.9kg

- Borg 125SD f/6 doublet 3.5kg (out of production)

- Takahashi TSA 120 triplet 6.7kg

- Tele Vue NP127is f/5.2 Petzval 6.6kg

- APM Doublet ED Apo 152 f/7.9 weight 8.4kg without and 10.7kg with rings 

- APM 120/Oberwerk 127 f5.1 bino 11kg

- 8 inch Dobsonian

 

What is the disadvantage of the proposed scope?

- Not optimal optical quality.

 

What are the advantages of the proposed scope?

- Wider fields?

- Ease of setup?

- Less expensive/lighter weight mount?

- Cooldown?

- Ease of transportation?

- Lower price?



#25 Sarkikos

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Posted 24 June 2021 - 01:07 PM

I can see this as the upgrade from my 150 Omni XLT f/5 achro.

 

Mike


Edited by Sarkikos, 24 June 2021 - 01:08 PM.

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