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

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#26 Sarkikos

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Posted 24 June 2021 - 01:09 PM

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

Any field curvature could be handled by the TSFLAT2.

 

Mike


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#27 Sarkikos

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Posted 24 June 2021 - 01:14 PM

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.

Yes.  :waytogo:

 

Mike



#28 Phillip Creed

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Posted 25 June 2021 - 01:33 PM

Dumb question, maybe, but what about something a bit smaller?  Like, say, a 120mm f/6 FCD100+Lanthanum doublet?  It would be easier to tame the demons of spherochromatism and chromatic aberration in an f/6 at 120mm aperture vs. 150mm.  And it'd be a LOT lighter.

 

120mm is plenty of aperture for most refractor aficionados.  Plus, a 2"-eyepiece with a 46mm field stop (think 41 Pan, 40XW, etc.) would generate a 3.7°-FOV.  A 31T5 could sweep up 3.3°.  That's nothing to sneeze at, particularly under dark skies.  That's the entire Veil Nebula in one gulp.  M31 end-to-end.  The entire Sagittarius Star Cloud.

Drop it to, say, 110mm f/6, and you could get max out at 4.0°, save even more weight and have better color control.

Clear Skies,

Phil


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#29 sanbai

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Posted 25 June 2021 - 02:55 PM

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.

Since this is for visual and Markus wants to make it *affordable*, I would avoid the 3" focuser. The diagonal and the eyepiece in question adds ~$1600. Of course, if a 3" focuser is at a minimal price difference, it does not harm.

 

It's not clear to me what are the constrain limits. "Cheaper serial production" and calling for ideas seems to be a bad combination, or a call to chop features.

 

My personal view on refractors for visual use is that those instruments are for easy setup (no collimation, quick thermal stabilization) and/or for wide fields. After that there are things (high magnification, light gathering ability) that other designs solve better at a similar or better price (but with other compromises, of course).

 

What I would ask for a refractor is a retractable dewshield and a good focuser able to handle a 31T5 with a 2" diagonal. The glass is decided, and it's a good one. I like "whatever matches best", it does not have to be lanthanum. You can't get much better than that, even if expensive fluorite is used. The cost of the optics are then only based on how much time one wants to invest in the journey to perfection.

 

Cost aside, my very bold idea is a tube that you can split in two parts fore easier transport. A bayonet like the one in camera and lenses is an example. The rear tube could be smaller to fit into the front part, with an adapter ring/flange in between. Imagine flying with 6" refractor in your carry-on cabin luggage!

 

I won't ask for a flat field telescope, this is not part of the "affordable" concept. However, a visual-oriented dedicated flatener would be a plus for those that dos not tolerate field curvature. If Jon says he does tolerate a 900 mm FL refractor, I take his word. I can tolerate my 600 mm FL refractor.

 

I certainly would not like having any kind of permanent blue light filtration. Such filtration can be added on demand with a 2" filter with negligible impact in performance, but if I'm in a dark site and want to see galaxies I also want the full spectrum to hit my eyes.


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#30 russell23

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Posted 25 June 2021 - 03:53 PM

What I'm hoping for is something lighter than the 6" f/8 scopes.  At f/6 it will be short enough to allow lighter mounts to handle it, but the weight needs to be under 20 pounds for me to take an interest. I know its not going to have perfect color correction.  The purpose of this scope in my mind would simply be to provide significantly better color correction than a 6" f/5-8 achromat.

 

I agree a 2" or 2.5" focuser makes the most sense.  It helps keep the weight and cost down.



#31 sanbai

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Posted 25 June 2021 - 06:07 PM

Since this is for visual and Markus wants to make it *affordable*, I would avoid the 3" focuser. The diagonal and the eyepiece in question adds ~$1600. Of course, if a 3" focuser is at a minimal price difference, it does not harm.

 

 

Mmm... maybe I shouldn't have said that. I'm the one using a sw 80ED with diagonals and eyepieces costing up to 3.5 times what I paid for the OTA in the classifieds grin.gif banjodance.gif


Edited by sanbai, 25 June 2021 - 06:08 PM.


#32 Moravianus

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Posted 25 June 2021 - 06:51 PM

Dumb question, maybe, but what about something a bit smaller?  ..

I am not following that market closely but at least at slower f ratio, is the market not already saturated ?



#33 Jon Isaacs

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Posted 25 June 2021 - 08:46 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.

 

StellarVue did something like this with their 102D which was 102mm F/7 Achromat with the outer half of the objective coated with minus violet... There was quite a commotion when the coating house that had done the original prototype revealed this fact.. 

 

But I see no reason to incorporate such a filter into the objective, it could be filter that fit over the objective... allowing for the owner to choose if and when they wanted to filter the incoming light. 

 

Jon 


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#34 Jon Isaacs

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Posted 25 June 2021 - 08:49 PM

Dumb question, maybe, but what about something a bit smaller?  Like, say, a 120mm f/6 FCD100+Lanthanum doublet?  It would be easier to tame the demons of spherochromatism and chromatic aberration in an f/6 at 120mm aperture vs. 150mm.  And it'd be a LOT lighter.

 

120mm is plenty of aperture for most refractor aficionados.  Plus, a 2"-eyepiece with a 46mm field stop (think 41 Pan, 40XW, etc.) would generate a 3.7°-FOV.  A 31T5 could sweep up 3.3°.  That's nothing to sneeze at, particularly under dark skies.  That's the entire Veil Nebula in one gulp.  M31 end-to-end.  The entire Sagittarius Star Cloud.

Drop it to, say, 110mm f/6, and you could get max out at 4.0°, save even more weight and have better color control.

Clear Skies,

Phil

 

I don't see much of an advantage over a 115mm F/7 triplet or a 120 mm F/7.5 doublet, you basically gain some field of view but not a lot.  That market is already well populated.

 

Jon



#35 stevew

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Posted 26 June 2021 - 03:05 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

I agree, but a triplet design would be much more expensive, and really only required for imaging.

I'm thinking that a 6 inch F-6 doublet is more geared toward the grab and go, crowd for low to mid power sky sweeping.



#36 AstroPotamus

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Posted 26 June 2021 - 04:02 PM

So let this be your guide:

 

A triplet can do all better, no question , but we want something real fast and lightweight
It sounds like these are the design criteria at this time:
  1. It will be a doublet
  2. It will be FCD 100
  3. It will be 150mm f/6
  4. It needs to be able to be made on a line
  5. It needs to have minimal assembly required

Additional information:

  • Same size as APM 140 mm F/7 Apo
  • Similar weight, maybe 1 kg more
  • F/6 needs more then a 2.5" focuser
  • Possibly an extension between tube and focuser which can be removed for binoviewer use

So if this is the base design spec, my question becomes, at what point is it beyond the current scopes on market in terms of price to build something different?  There are a few similar designs out there already, so is the point to be technically superior at any price (not likely, since the design specs say minimal assembly, and simpler line manfuacture).  Or is the point to be superior on price, in which case compromises need to be made?  If so, then what compromises?

 

I'm not sure who the customer is for this scope, which would greatly help me understand what it should have in terms of design specs and features.


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#37 Astrojensen

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Posted 27 June 2021 - 03:05 AM

Will this scope even make sense, compared to the APM 140/980 ED? It will have only slightly shorter focal length, probably be just as expensive, be even heavier, and have compromised high-power performance. 

 

 

Clear skies!

Thomas, Denmark


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#38 Moravianus

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Posted 27 June 2021 - 12:18 PM

The "half of the150SD" bino with its prism improved color correction seems to be the best option.



#39 zjc26138

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Posted 28 June 2021 - 09:05 AM

I would be interested in this scope.

 

Would considering selling my TMB130SS to get it.



#40 garret

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Posted 29 June 2021 - 03:27 PM

The "half of the150SD" bino with its prism improved color correction seems to be the best option.

A solid diagonal could be the answer



#41 Moravianus

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Posted 29 June 2021 - 05:04 PM

A solid diagonal could be the answer

The APM150SD is designed together with the prism to achieve its correction. A solid mirror diagonal would not guarantee the same result and standard prism, possibly the same.


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#42 Benach

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Posted 02 July 2021 - 12:25 AM

My input:
- make it a triplet so you will also be serving the astrophotographers.
- make it compatible to the existing APM flatteners and reducer/flatteners
- give future customers a choice of their tube material: aluminium or carbon fiber.
- build in a dew heater so you will only have to plug it into an external unit.

#43 Sarkikos

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Posted 02 July 2021 - 11:48 AM

I'd only get a triplet if it actually made a difference for visual astronomy.  Otherwise, for me, the extra weight and cost wouldn't be worth it.

 

Mike


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#44 AstroPotamus

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Posted 02 July 2021 - 11:52 AM

I'm a business owner.  I know who our customers are and what they're willing to pay for our services.  I know where we fit in the world of our competitors. 

 

I do not known who the intended user of this telescope is, what the anticipated price point would be, or how it will differentiate itself from all the other 150mm f/close-to-6 APO scopes that are out there.

 

Markus, now would be a great time for you to say what your thoughts are on all the comments made thus far, and is there is any further direction on this idea or not.


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#45 russell23

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Posted 02 July 2021 - 12:29 PM

I'm a business owner.  I know who our customers are and what they're willing to pay for our services.  I know where we fit in the world of our competitors. 

 

I do not known who the intended user of this telescope is, what the anticipated price point would be, or how it will differentiate itself from all the other 150mm f/close-to-6 APO scopes that are out there.

 

Markus, now would be a great time for you to say what your thoughts are on all the comments made thus far, and is there is any further direction on this idea or not.

 

I'd only get a triplet if it actually made a difference for visual astronomy.  Otherwise, for me, the extra weight and cost wouldn't be worth it.

 

Mike

There are plenty of 6” triplets and 6” f/8 ED doublets out there.  The triplets are super expensive and the doublets f/8 doublets are a bear to mount.  

 

Here is what I envision - the 6” equivalent of the Astrotech 102mm f/7 with fpl51 glass.  What I mean is that there is a parameter space between perfect color correction and a standard fast achromat.  If I get a 6” refractor I don’t need perfect color correction.  I’d like something quite a bit better than what you get with a standard 6” f/6 achromat.  But it doesn’t have to be perfect because this scope would be a deep sky focused instrument.  I want the f/6 for easier mounting than the f/8.  Give me a 2.5” R&P focuser and a weight under 20 pounds and a price similar to the f/8 ED doublet versions and I’m going to be very interested in this scope. 

 

My impression is that Markus doesn’t ask this kind of question unless he has already had quite a few inquiries.

 

Dave


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#46 Sarkikos

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Posted 02 July 2021 - 02:08 PM

There are plenty of 6” triplets and 6” f/8 ED doublets out there.  The triplets are super expensive and the doublets f/8 doublets are a bear to mount.  

 

Here is what I envision - the 6” equivalent of the Astrotech 102mm f/7 with fpl51 glass.  What I mean is that there is a parameter space between perfect color correction and a standard fast achromat.  If I get a 6” refractor I don’t need perfect color correction.  I’d like something quite a bit better than what you get with a standard 6” f/6 achromat.  But it doesn’t have to be perfect because this scope would be a deep sky focused instrument.  I want the f/6 for easier mounting than the f/8.  Give me a 2.5” R&P focuser and a weight under 20 pounds and a price similar to the f/8 ED doublet versions and I’m going to be very interested in this scope. 

 

My impression is that Markus doesn’t ask this kind of question unless he has already had quite a few inquiries.

 

Dave

Some observers do get 6" refractors for planet viewing, where optimal color correction would be important.  

 

Mike



#47 russell23

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Posted 02 July 2021 - 03:01 PM

Some observers do get 6" refractors for planet viewing, where optimal color correction would be important.  

 

Mike

And for that there are triplets and the f/8 ED doublets.  If the cost can be kept down to under $3000 and it has perfect color correction I won’t complain.  My feeling is that there is no 6” f/6 low ($<3000) cost ED doublet option out there for people that just want a fast 6” refractor that has better color correction than an achromat.

 

It takes away the reasons this scope could make sense when demands it be a triplet and have perfect color correction start getting tossed in.  What is needed on the market is a fast 6” refractor doublet that is less than 20 pounds and better corrected than an achromat.  As soon as you make it a triplet the weight is going to be too much.  And demanding perfect color correction means a spike up in the final cost.  Both of those things only serve to put it in competition with what is already on the market.  If it is a doublet with FK-61 both the price and color correction would put it in a unique and still useful place.  If FPL-53 as Marcus suggested then more expensive but better color correction. 


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#48 Astrojensen

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Posted 03 July 2021 - 01:43 PM

 

My feeling is that there is no 6” f/6 low ($<3000) cost ED doublet option out there for people that just want a fast 6” refractor that has better color correction than an achromat.

Better color correction than an achromat is easy enough, but what good is that going to do, if it's hampered by spherochromatism? Will you accept an ED with better color correction than an achromat, but worse spherical aberration? That's almost certainly what you're looking at, in a 6" f/6 doublet ED. 

 

Will you pay $3k for a 6" f/6 with a big, fat first diffraction ring and visible second ring, with smeared, low contrast planetary views? 

 

Going for a triplet isn't to ensure perfect color correction, it's for ensuring much better spherical correction...

 

 

Clear skies!

Thomas, Denmark


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#49 russell23

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Posted 03 July 2021 - 04:39 PM

Better color correction than an achromat is easy enough, but what good is that going to do, if it's hampered by spherochromatism? Will you accept an ED with better color correction than an achromat, but worse spherical aberration? That's almost certainly what you're looking at, in a 6" f/6 doublet ED. 

 

Will you pay $3k for a 6" f/6 with a big, fat first diffraction ring and visible second ring, with smeared, low contrast planetary views? 

 

Going for a triplet isn't to ensure perfect color correction, it's for ensuring much better spherical correction...

 

 

Clear skies!

Thomas, Denmark

Say goodbye to less than $3000 if you want it to be a triplet.  Say goodbye to less than 9 kg if you want it to be a triplet.  The idea is a 6” refractor that is more portable, more easily mounted and less costly than current 6” APO’s and with better color correction than the 6” achromats.  That is the only purpose for this offering IMO.  Otherwise just buy one of the 6” options already on the market. 

 

You have the 6” f/8 ED doublets available - so why go with f/8?

 

You have numerous 6” triplets available - so why add one more?

 

A lot of people buy the 6” f/5 and 6” f/5.9 achromats that are available.  This would simply be an improved version of those that would give you better color correction and better ability to handle higher magnifications with more portability than the 6” ED and triplet APOs available. 


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#50 russell23

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Posted 03 July 2021 - 04:41 PM

I’d actually like to see something different.  How about a 130mm f/9.2 ED doublet? Give me the longer focal length with a weight that is still in the 9 kg range. 

 

Why does everything new have to be a triplet?


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