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Views through large APO Refractor

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#1 sunrag

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Posted 27 September 2020 - 11:56 AM

I keep reading how APO refractor owners like their views through their telescopes because they make the sky look like dark velvet. How does this happen? I have never looked through an APO before so I am curious.

 

Is there something intrinsic in APO refractors that darkens the skies, vs a large 12" reflector? 

 

I can see this darkening effect when I use a zoom eye-piece. The sky does become dark when I change the ep focal length from 24mm to 8mm. So one possible explanation is that APO owners use high power eye-pieces more often than Achro or reflector owners. Also could it be that they use high-end Televue eyepieces?


Edited by sunrag, 27 September 2020 - 11:58 AM.


#2 Dave Mitsky

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Posted 27 September 2020 - 12:12 PM

The largest apochromatic refractor that I've ever used was one of the very rare 206mm (8-inch) f/7.7 Astro-Physics Starfire EDFs.  I've also had the chance to observe through a 10-inch achromat that was equipped with a Chromacor lens system, which reduces chromatic aberration. 

 

http://www.astrosurf...romacor-doc.pdf

I've also had views through 6 and 7-inch Astro-Physics Starfires, 11-inch privately-owned achromats, and a number of large observatory achromatic refractors up to 24 inches in aperture. 

The views through an apochromatic refractor can be stunning.  Aesthetically speaking, they are without equal.

The Strehl ratio, which is the ratio of peak diffraction intensities of an aberrated versus a perfect wave front, of high-end apochromats can be quite close to a perfect 1.0.  

https://www.peak2val...ge_1801026.html

 

http://www.whichtele.../benchmarks.htm

 

So it's a matter of optical and mechanical excellence, not the decrease in exit pupil when increasingly higher magnifications are used, which is true of all telescope designs, that make the images produced by such instruments so pleasing.


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#3 M11Mike

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Posted 27 September 2020 - 12:25 PM

The higher the magnification - the DARKER the sky background - any scope (and all eyepieces), not just APO's.   


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#4 bobzeq25

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Posted 27 September 2020 - 12:39 PM

I keep reading how APO refractor owners like their views through their telescopes because they make the sky look like dark velvet. How does this happen? I have never looked through an APO before so I am curious.

 

Is there something intrinsic in APO refractors that darkens the skies, vs a large 12" reflector? 

 

I can see this darkening effect when I use a zoom eye-piece. The sky does become dark when I change the ep focal length from 24mm to 8mm. So one possible explanation is that APO owners use high power eye-pieces more often than Achro or reflector owners. Also could it be that they use high-end Televue eyepieces?

The central obstructions in reflectors decrease contrast significantly.  So the skies appear darker with regard to the stars.



#5 Sketcher

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Posted 27 September 2020 - 12:43 PM

The higher the magnification - the DARKER the sky background - any scope (and all eyepieces), not just APO's.   

Yes, but . . .

 

A high quality apochromat refractor will put less light (that belongs elsewhere) into the background sky.  This results in higher contrast images and a darker background sky -- all other things being equal.


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#6 KBHornblower

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Posted 27 September 2020 - 12:56 PM

The central obstructions in reflectors decrease contrast significantly.  So the skies appear darker with regard to the stars.

The diffraction from the obstruction does indeed reduce contrast on small scales over the face of a planet, by smearing out the diffraction pattern of each point source.  I am not convinced that this scatters a perceptible amount of light into the background sky that is well separated from the individual stars.  Can you provide a technical reference?

 

If the sky is perceptibly darker in a refractor of equal throughput and magnification, my hunch is that better baffling and perhaps less scatter at the optical surfaces are the cause.  I have read somewhere that a good lens scatters less light than the best mirror coatings, but don't take that as gospel.  My attempts at online searches for topics like this are usually wild goose chases.

 

Perhaps someone could do some side by side imaging with the two scopes and measure the background brightness, to eliminate possible confirmation bias.


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#7 MikiSJ

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Posted 27 September 2020 - 01:02 PM

I had a TAK FS-152 doublet that gave me great views, but then a fellow astronomer put his AMP/TMB 152/1200 TRIPLET together in a side-by-side comparison and I immediately made an offer to buy his scope. The only reason he sold it was he had a 180 on order.

 

I have had the chance to look through the "Great Lick Refractor" a 36" doublet, at M57. I was really disappointed in the contrast. My APM/TMB was far superior in contrast to the Lick.

 

It is a bit like buying a Cayenne S (which I have) or a Chevy Suburban. They will both get you there, but oh, what fun it is in the Porsche.



#8 spaceoddity

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Posted 27 September 2020 - 01:06 PM

I have not noticed a darker sky in my ES127(5" apo) vs. my 10 or 16 inch dobs. If it's there or if contrast is better, it's pretty subtle. I've never done a side by side comparison though. My 10 inch is fully flocked and 16 UTA is flocked across from focuser. That probably helps. I prefer the views in the larger dobs over the 5" refractor. I guess beauty is in the eye of the beholder. My aging eyes aren't very good in the dark anymore so the extra aperture pretty much trumps all for me.


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#9 KBHornblower

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Posted 27 September 2020 - 01:30 PM

The diffraction from the obstruction does indeed reduce contrast on small scales over the face of a planet, by smearing out the diffraction pattern of each point source.  I am not convinced that this scatters a perceptible amount of light into the background sky that is well separated from the individual stars.  Can you provide a technical reference?

 

If the sky is perceptibly darker in a refractor of equal throughput and magnification, my hunch is that better baffling and perhaps less scatter at the optical surfaces are the cause.  I have read somewhere that a good lens scatters less light than the best mirror coatings, but don't take that as gospel.  My attempts at online searches for topics like this are usually wild goose chases.

 

Perhaps someone could do some side by side imaging with the two scopes and measure the background brightness, to eliminate possible confirmation bias.

Addendum:  I just now checked some optical surfaces with a laser beam.  The aluminized mirror surfaces appeared to scatter more light than did the surfaces of my eyepieces.  These mirrors are 35 years old so they may not be in the best shape compared to the much newer eyepieces.


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#10 Dave Mitsky

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Posted 27 September 2020 - 01:42 PM

If the sky is perceptibly darker in a refractor of equal throughput and magnification, my hunch is that better baffling and perhaps less scatter at the optical surfaces are the cause.  I have read somewhere that a good lens scatters less light than the best mirror coatings, but don't take that as gospel.  My attempts at online searches for topics like this are usually wild goose chases.

I seem to recall seeing this somewhere too.  I wonder if it's discussed in Star Ware.



#11 Kent10

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Posted 27 September 2020 - 02:11 PM

I like the dark backgrounds in my refractors too.  But if you are using the same magnification in a smaller refractor and a larger dob, the exit pupil will be larger in the dob and therefore the sky background will appear brighter.  If you have scatter for the mirrors in the dob, you might also have some scatter from the mirrors in the refractor's diagonal, if you are using a mirror diagonal.


Edited by Kent10, 27 September 2020 - 02:12 PM.

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#12 KBHornblower

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Posted 27 September 2020 - 04:34 PM

I like the dark backgrounds in my refractors too.  But if you are using the same magnification in a smaller refractor and a larger dob, the exit pupil will be larger in the dob and therefore the sky background will appear brighter.  If you have scatter for the mirrors in the dob, you might also have some scatter from the mirrors in the refractor's diagonal, if you are using a mirror diagonal.

My bold.  At the risk of sounding pedantic, I will say that I am amused by a choice of words that could make a novice think the greater sky brightness is a result of the larger exit pupil.  My understanding is that the diameter of the exit pupil and the brightness of the background are concurrent results of the optical geometry.



#13 KBHornblower

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Posted 27 September 2020 - 06:24 PM

My bold.  At the risk of sounding pedantic, I will say that I am amused by a choice of words that could make a novice think the greater sky brightness is a result of the larger exit pupil.  My understanding is that the diameter of the exit pupil and the brightness of the background are concurrent results of the optical geometry.

Let me revise this, as in hindsight the "amused" could be on the rude side, for which I apologize.  It could come across as belittling someone for whom the optical principles are not intuitively obvious.  I can now imagine envisioning the exit pupil as analogous to the iris in a camera lens, which does indeed darken the image if stopped down.  That would be an error, and it takes some ray tracing to see what is really happening when we change the magnification of a telescope.  This is easier done with sketches than with words.  I would be happy to post some sketches if you all so wish.



#14 Jon Isaacs

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Posted 27 September 2020 - 06:43 PM

The central obstructions in reflectors decrease contrast significantly.  So the skies appear darker with regard to the stars.

 

The CO decreases fine scale planetary contrast. The central obstruction only affects a region a few airy disk diameters from a star. It does not affect sky brightness. 

 

The smaller aperture has this same effect except more so.

 

My own experience is that waxing estatitic about dark sky background is smaller exit pupils.  At equivalent exit pupils in my apo refractors, the sky glow is of similar brightness to my Reflectors.  

 

Roland Christen of Astro-Physics is known for his pioneering work developing the modern apo triplet refractors. Roland's scopes are among the very best and he's a perfectionist. At times there have been waiting lists of 10 years for a particular model. 

 

This is what Roland once wrote about contrast:

 

"There are two kinds of contrast. One needs to define which contrast you mean. If it's planetary contrast, then baffling plays no part. Planetary contrast is strictly a function of how well the optic is figured.

 

If it's deep sky contrast, then optical perfection plays little or no part, and the main contributer will be how well the tube has been made to exclude stray light (i.e. baffled).

 

Rolando"

 

Jon


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#15 KBHornblower

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Posted 27 September 2020 - 08:34 PM

I had a TAK FS-152 doublet that gave me great views, but then a fellow astronomer put his AMP/TMB 152/1200 TRIPLET together in a side-by-side comparison and I immediately made an offer to buy his scope. The only reason he sold it was he had a 180 on order.

 

I have had the chance to look through the "Great Lick Refractor" a 36" doublet, at M57. I was really disappointed in the contrast. My APM/TMB was far superior in contrast to the Lick.

 

It is a bit like buying a Cayenne S (which I have) or a Chevy Suburban. They will both get you there, but oh, what fun it is in the Porsche.

My bold.  That is because a large doublet inevitably has worse residual chromatic aberration than a smaller one of the same focal ratio.  This scope is at its best in such things as resolving close double stars, where the worst of the color can be suppressed with a yellow filter.  For nebulae such as M57 the Crossley reflector, with the same aperture, would have been better.  No telescope is the best at everything.



#16 sunrag

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Posted 27 September 2020 - 09:06 PM

In their highly enjoyable book "Astronomy Hacks" by Robert B. Thompson & Barbara F. Thompson, the authors describe a hack (Hack #36) to "Eliminate diffraction spikes and increase contrast. Build a $0 aperture mask to turn your $500 dob into a $2000 apo refractor."

 

Basically, it is an off-axis aperture mask that you place in between the vanes on the OTA opening. By doing this, you are covering the vanes and the secondary mirror. The aperture dia would be = (Main Mirror Dia - Secondary Mirror Minor Axis)/2.  The authors say that this will give you APO like views. It is not going to be as good as 4" APO of course because of lack of baffles, but it is supposed to increase contrast and give more pleasing views. T

 

Well, I tried that on my AD12. I made a 4" aperture mask and fitted it between the vanes. Otther than considerably reduced brightness, the view did not seem that much different. I could see no more detail on Jupiter than I could see with the full aperture dob.
 



#17 sunrag

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Posted 27 September 2020 - 09:17 PM

There is a similar topic going on in the Refractor forum:

https://www.cloudyni...e-4?hl= apo vs



#18 treadmarks

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Posted 27 September 2020 - 09:19 PM


 

Well, I tried that on my AD12. I made a 4" aperture mask and fitted it between the vanes. Otther than considerably reduced brightness, the view did not seem that much different. I could see no more detail on Jupiter than I could see with the full aperture dob.
 

There you have it. What you actually tested here is the effect of a central obstruction. Many others have done this experiment and seen the same. This agrees with what most experts will tell you, which is that the effect of the central obstruction is subtle regardless of what the MTF charts say.

 

I've noticed that the people who enjoy expensive APO refractors the most are usually lifelong, experienced astronomers. They know what the CO effect looks like and they are picky, rightfully so if you've been doing this for decades. For us newbies and casuals, we probably shouldn't be worrying about it - there are bigger things to worry about.


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#19 sunrag

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Posted 27 September 2020 - 09:20 PM

To be honest, what made me start this topic was this ad for a large 178mm APO refractor today. It is an easy drive for me to pick it up from KY if I had $3200 to spend.

 

https://www.cloudyni...eade-178-edapo/

 

But after following the discussions, I think I will skip this one. Thanks everyone!


Edited by sunrag, 27 September 2020 - 09:23 PM.


#20 Don W

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Posted 27 September 2020 - 09:35 PM

I have had views through many 6, 7, and 8" Astrophysics scopes over the years and yes, looking at anything but in particular the planets is like they are set on black velvet. There's no gray tint to the background. This is due to the high quality of the optics and not having a central obstruction.

 

That said, I have had similar views through a 10" AP Maksutov, which of course also had excellent optics, but did have a central obstruction. I wish I had checked the seeing conditions at the times of these observations. I'm sure it must have been good too.


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#21 gnowellsct

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Posted 27 September 2020 - 10:37 PM

The higher the magnification - the DARKER the sky background - any scope (and all eyepieces), not just APO's.   

They're not going to make an urban or suburban sky like "black velvet."

They're not going to make a bad transparency sky like "black velvet."

They're not going to make a moonlit sky like "black velvet."

They're not going to make a hazy smoke filled sky like "black velvet."

 

But you get a big apo out to Colorado or NM on a clear moonless night you'll be getting some outstanding views no doubt about it.  

 

Such conditions will also boost the performance of any scope whose owner is lucky enough to be out there.

 

Greg N


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#22 gnowellsct

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Posted 27 September 2020 - 10:54 PM

I had a TAK FS-152 doublet that gave me great views, but then a fellow astronomer put his AMP/TMB 152/1200 TRIPLET together in a side-by-side comparison and I immediately made an offer to buy his scope. The only reason he sold it was he had a 180 on order.

 

I have had the chance to look through the "Great Lick Refractor" a 36" doublet, at M57. I was really disappointed in the contrast. My APM/TMB was far superior in contrast to the Lick.

 

It is a bit like buying a Cayenne S (which I have) or a Chevy Suburban. They will both get you there, but oh, what fun it is in the Porsche.

I have had a similar conversion-to-triplet-apo experience.  I try not to write about it too much because it ticks some people off.  Not that I am that bashful about ticking people off.   There's plenty of enjoyment to be had out of fluorite and ED doublets (I owned an FS128 too once upon a time).  But yes, there is the potential for "conversion" in the matter of top notch triplet apos.  The FS152, along with the FS128 and FS102, are among the finest visual scopes ever made.  But I would not be surprised to learn that top tier apos outperform them even for visual use.   I do think my 92mm and 130mm triplets are in a class of their own.  Still the FS series is so good....it's very subjective.

 

I will say this.  I am unlikely ever to be in a situation where an FS152 and a 152mm APM triplet are side by side.  That implies a very rarefied company.  Vulgar SCT guys don't get invited to such exalted circles.

 

Greg N  



#23 gnowellsct

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Posted 27 September 2020 - 10:58 PM

 

I've noticed that the people who enjoy expensive APO refractors the most are usually lifelong, experienced astronomers. They know what the CO effect looks like and they are picky, rightfully so if you've been doing this for decades. For us newbies and casuals, we probably shouldn't be worrying about it - there are bigger things to worry about.

Lots of people who enjoy expensive APO refractors also enjoy other scopes and don't give a hoot about central obstruction.  


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#24 VNA

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Posted 27 September 2020 - 11:15 PM

Hello: Could the lens /glass be with much less impurities will let light with fewer interference?

Just a suggestion poorly worded.

 

("I know nothing I am from Barcelona") quote from Fawlty Towers!



#25 Tony Flanders

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Posted 28 September 2020 - 05:51 AM

The main reason that refractors in general have better large-scale contrast (including darker backgrounds) than reflectors is that lenses inherently have very little scatter, and the scatter can be reduced near to zero with proper coatings. That means that almost all the light hitting them goes into the image rather than into the background.

 

Mirrors inherently have higher scatter. That's especially true for commercial mass-market mirrors. Part of the scatter comes from the reflective coating, and can be eliminated by using ultra-expensive dielectric coatings. But for mass-market reflectors the biggest part comes from surface roughness -- very small-scale irregularities in the shape of the mirror, which do not affect the sharpness (as in 1/4 wave or 1/8 wave) but do affect the contrast.

 

Top-quality mirrors like the Zambuto mirror in my 12.5-inch Dob are often described as producing "refractor-like images" due to their very smooth surfaces.


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