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Can An Apochromatic Refractor Use More Powerful Eyepieces Than An Achromatic Refractor Of The Same Size?

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#26 Mitrovarr

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Posted 19 December 2017 - 03:38 AM

Also, these comparisons of fast achromats to apos are relevant to my interests. I am seriously considering upgrading from a combination of a 4" F/15 and ST120 to a 5" apo, probably an ES127. I'm hoping it would represent a solid upgrade to the 4" especially.

#27 Wildetelescope

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Posted 19 December 2017 - 07:50 AM

Also, these comparisons of fast achromats to apos are relevant to my interests. I am seriously considering upgrading from a combination of a 4" F/15 and ST120 to a 5" apo, probably an ES127. I'm hoping it would represent a solid upgrade to the 4" especially.

It will!  For sure.

 

jmd



#28 BarrySimon615

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Posted 19 December 2017 - 10:57 AM

My conclusion - "IT ALL DEPENDS".  It all depends upon a variety of factors - objective size of the comparison scopes, focal ratio of the comparison scopes, quality of the objectives of the comparison scopes, etc., etc.

 

Some years ago I did a comparison of three scopes.  The first scope was the University Optics "kit" telescope (you assemble all parts and paint) that only requires assembly and painting.  It is an 80 mm with a 500 mm focal length (f/6.25).  The second telescope was the Onyx 80 mm ED also with a 500 mm focal length (f6.25) and the third telescope was the Takahashi FCT 76 (76 mm objective with 487 mm focal length @ f/6.4).  Resolution was surprisingly similar.  The FCT was best, but the University Optics scope beat the Celestron Onyx.  In addition the limb of the Moon had a greenish tinge with the Celestron Onyx.

 

The Tak FCT 76, which is a triplet was not the equal at magnification/resolution in a comparison with the Orion 100ED (gray tube doublet ED introduced in the summer of 2004).  Many have that scope now in the SkyWatcher Pro ED100 version.  

 

When scopes start to diverge a bit more in focal ratio, all bets are off.  Longer f/ratio achromats (f/10 or slower) are often better at holding magnification and offering better resolution than some ED scopes, particularly doublets with focal ratios near f/7 or faster.

 

Barry Simon

Attached Thumbnails

  • 3 Inch Fast Refractors.jpg
  • Scope Comparison Detail.jpg
  • Focused star comparison.jpg

Edited by BarrySimon615, 19 December 2017 - 11:03 AM.

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#29 Aperture Mask

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Posted 19 December 2017 - 12:35 PM

My conclusion - "IT ALL DEPENDS".  It all depends upon a variety of factors - objective size of the comparison scopes, focal ratio of the comparison scopes, quality of the objectives of the comparison scopes, etc., etc.

 

Some years ago I did a comparison of three scopes.  The first scope was the University Optics "kit" telescope (you assemble all parts and paint) that only requires assembly and painting.  It is an 80 mm with a 500 mm focal length (f/6.25).  The second telescope was the Onyx 80 mm ED also with a 500 mm focal length (f6.25) and the third telescope was the Takahashi FCT 76 (76 mm objective with 487 mm focal length @ f/6.4).  Resolution was surprisingly similar.  The FCT was best, but the University Optics scope beat the Celestron Onyx.  In addition the limb of the Moon had a greenish tinge with the Celestron Onyx.

 

The Tak FCT 76, which is a triplet was not the equal at magnification/resolution in a comparison with the Orion 100ED (gray tube doublet ED introduced in the summer of 2004).  Many have that scope now in the SkyWatcher Pro ED100 version.  

 

When scopes start to diverge a bit more in focal ratio, all bets are off.  Longer f/ratio achromats (f/10 or slower) are often better at holding magnification and offering better resolution than some ED scopes, particularly doublets with focal ratios near f/7 or faster.

 

Barry Simon

Thank you. I snipped the info.



#30 Jeff B

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Posted 19 December 2017 - 04:26 PM

 

As others have said, F ratio is king even with APO's but try to find an F10 APO bigger than 4" that's in ready production.  One saving grace with long FR achromats is the use of filters to isolate specific portions of the spectrum.

Really? I've heard that really excellent apos are equally good for high power work whether fast or slow, which is why it's basically impossible to find slow apos anymore.

 

I've actually experienced it, my TMB 130SS (F7), TEC 140 ED (F7) and my 200ED (F9) scopes for example (though you could say F9 is not fast.   But it certainly is for an FPL-51 design with a 200mm aperture.)

 

So my point is simply this: Especially with APOs,  if you want to try squeeze the absolute best performance possible out of a particular combination of glasses, one resulting in very low levels of longitudinal color correction, spherochromatism, coma and higher order corrections in the deep blue/violet and red, F ratio is a very powerful tool. Longitudinal false color decreases linearly with focal length for a given design, while with spherochromatism the reduction is non-linear. It's strong medicine for the APO in particular.   

 

But the point is well understood by me that going past a certain F ratio does not buy you much more with a give aperture and glass combination in an APO.  For example there is the APM 130 F9.25.  I recall a while ago discussions here concerning the visual differences at high power concerning the 130 F9.25 and its F6 brother.  All I know is that mine was stunning at high power on planetary objects.  Also, CFF makes APOs of the same aperture and glass combinations at several F ratios  and their "slower" focal ratio designs are for those who want the best possible high power views. 

 

Now given the choice between a 5" F9.25 APO and a 5" F9.25 achromat, I would absolutely go with the APO.  However, IME when I start to apply filters to bring out certain and specific details on the planets, a quality achromat is a very effective tool.   

 

Jeff



#31 Mitrovarr

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Posted 19 December 2017 - 04:48 PM

Well, how's this for specificity - of the three cheap 5" apos I could actually get (SW120, ES127 essential, and AT130ED), would you expect them to be substantially better than a very good 4" F/16 achromat?
The SW and ES are F7.5 and the AT130 is F7. Anything else I can think of would probably be too expensive or is unavailable.

#32 Wildetelescope

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Posted 19 December 2017 - 05:02 PM

By my estimation, my tv102 ed doublet is a significant improvement over my very nice 80mm f15. I would guess you will see a similar improvement going from 4 inch f15!achro to a well executed 5 inch ed doublet design.

Jmd

#33 Jeff B

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Posted 19 December 2017 - 05:08 PM

Go for the SW120.  Mine is truly excellent optically and a very nice "package" and was for me a better scope than the excellent 4" F12 achromat I had.  But it was the really good Celestron 100 ED doublet (F9) that actually unseated the achromat.  In smaller apertures and given good performance in the visual range, I'll take a doublet over a triplet, even if the doublet is a little "slower".  I find doublets tend to "behave" better thermally which I find super important around here.

 

Jeff



#34 coopman

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Posted 19 December 2017 - 08:29 PM

I agree with Jeff B's comments.  Don't get too obsessed with magnification.  On any given night there are a LOT of variables to this.  It will do what it will do, and it won't do what it can't. 



#35 Jon Isaacs

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Posted 19 December 2017 - 08:46 PM

I've actually experienced it, my TMB 130SS (F7), TEC 140 ED (F7) and my 200ED (F9) scopes for example (though you could say F9 is not fast.   But it certainly is for an FPL-51 design with a 200mm aperture.)

 

So my point is simply this: Especially with APOs,  if you want to try squeeze the absolute best performance possible out of a particular combination of glasses, one resulting in very low levels of longitudinal color correction, spherochromatism, coma and higher order corrections in the deep blue/violet and red, F ratio is a very powerful tool. Longitudinal false color decreases linearly with focal length for a given design, while with spherochromatism the reduction is non-linear. It's strong medicine for the APO in particular.

 

But the point is well understood by me that going past a certain F ratio does not buy you much more with a give aperture and glass combination in an APO.

 

 

Jeff:

 

Focal ratio is certainly part of the equation as is aperture.  And your point about going beyond a certain focal ratio,  for a given aperture,  design and glass type,  is a good one. 

 

For a 4 inch FPL-53 doublet,  F/9 seems to be plenty.  For an 8 inch,  the equation certain changes and a slower focal ratio could be advantageous..  Assuming one wanted to deal with an even longer tube. 

 

Jon



#36 Bomber Bob

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Posted 19 December 2017 - 09:06 PM

I am seriously considering upgrading from a combination of a 4" F/15 and ST120 to a 5" apo, probably an ES127.

 

I chose an APM ED 152/1200 as my upgrade, and I expect to see a big difference on lunar & planetary.  My Edmund 4" F15 is sharp at 320x (80x / inch), and I expect the same or better from the 6" F8 -- 480x is comparable.  Decades ago, my 1988 D&G 5" F10 was a planet killer at 400x.  Having to wait on Mars, Jupiter, & Saturn to get within reach of the 152 is the hard part now!



#37 photoracer18

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Posted 20 December 2017 - 10:59 AM

The planet killer of all time was Fred Mrozek's 130mm F12 APOMAX (a D. Buchroeder design).

It was built in the short era when FPL-52 was available. Ohara Abbe numbers were:

51= .81

52= .90

53= .94+

 

Roland also used FPL-52 during that era. Cool down time was very long for that scope, in the area of 1.5 hours even from an unheated garage from reports. Don Yeier came by once and showed me one but I never got to look thru one. Unless you like sitting on the ground you needed a mount extension to use it at normal heights although not as bad as my old D&G built Jaegers 6" F15.


Edited by photoracer18, 20 December 2017 - 11:00 AM.


#38 Jeff B

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Posted 20 December 2017 - 03:04 PM

 

I've actually experienced it, my TMB 130SS (F7), TEC 140 ED (F7) and my 200ED (F9) scopes for example (though you could say F9 is not fast.   But it certainly is for an FPL-51 design with a 200mm aperture.)

 

So my point is simply this: Especially with APOs,  if you want to try squeeze the absolute best performance possible out of a particular combination of glasses, one resulting in very low levels of longitudinal color correction, spherochromatism, coma and higher order corrections in the deep blue/violet and red, F ratio is a very powerful tool. Longitudinal false color decreases linearly with focal length for a given design, while with spherochromatism the reduction is non-linear. It's strong medicine for the APO in particular.

 

But the point is well understood by me that going past a certain F ratio does not buy you much more with a give aperture and glass combination in an APO.

 

 

Jeff:

 

Focal ratio is certainly part of the equation as is aperture.  And your point about going beyond a certain focal ratio,  for a given aperture,  design and glass type,  is a good one. 

 

For a 4 inch FPL-53 doublet,  F/9 seems to be plenty.  For an 8 inch,  the equation certain changes and a slower focal ratio could be advantageous..  Assuming one wanted to deal with an even longer tube. 

 

Jon

 

Hey Jon.

 

Amen. 

 

Actually, I've pretty much stopped recommending achromats in the 4" to 5" range because of the easy availability, affordability, and high quality levels of the many 100ED F9's out there as well as the SW 120ED.  The only exceptions are for folks wanting such apertures for strictly low  power viewing who are on a tight budget.   

 

Jeff



#39 Peter Besenbruch

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Posted 20 December 2017 - 10:41 PM

Also, these comparisons of fast achromats to apos are relevant to my interests. I am seriously considering upgrading from a combination of a 4" F/15 and ST120 to a 5" apo, probably an ES127. I'm hoping it would represent a solid upgrade to the 4" especially.

I have had a Vixen made C102, and a Synta made 120ED. The 120 is a solid step up in both resolving power and deepsky reach.


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#40 Bomber Bob

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Posted 20 December 2017 - 10:47 PM

Going from a 4" F15 achromatic to an ED 152 F8 has been eye-boggling!   Absolutely the best view of M42 that I've ever seen in one of my scopes.



#41 Aperture Mask

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Posted 21 December 2017 - 12:08 AM

Going from a 4" F15 achromatic to an ED 152 F8 has been eye-boggling!   Absolutely the best view of M42 that I've ever seen in one of my scopes.

What is the most powerful eyepiece you can use with the Ed 152 F8? Can you go has high as 3mm?




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