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8" SCT versus 5"APO

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#226 A6Q6

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Posted 13 November 2014 - 01:17 PM

An 8" SCT can beat a 5" APO, a 5" APO can beat an 8" SCT, an 8" SCT can match a 5" APO...Location and Seeing are the primary culprits. All the other variables have their effect too.

 

I've had 80, 90, 100, 130 and 140mm ED/APO refractors and 90, 130, 150, 200 and 280mm MAK/SCT's. Compared them head to head many nights. There were nights the refractors won...there were nights the SCT's won. My 4" TV Genesis has beat my C8... My C8 has beat the Genesis. My AP130GT has trumped my C8/C11...My C8/C11 has trumped my AP130GT. My TEC140 whipped my C11...My C11 has whipped my TEC140. To make any other claim or state absolutes about this topic, is simply folly. There are indeed too many variables to account for. In general I find that I need at least good seeing conditions and weather to allow the sct's to strut their stuff, but strut they can. So too can the refractors in average seeing conditions and less. I will always have at least one of each type to compliment, not compete, with each other.

 

 

 

 

I think you need a C5 to round out your SCT's



#227 SpooPoker

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Posted 20 November 2014 - 11:44 PM

I used to believe that mirrors were a better optical device than a lens that splits colors like a rainbow.  I now think that refraction is better than reflection.  Even when I equalized two scopes so both are subject to the same limitations, the refractor still wins. 

 

I have compared a 12" Newt to a 4" APO and the 4" APO trounces it on planetary details when the 12" Newt has an offset mask making it an unobstructed 4" reflector.  For the 12" Newt to beat a 4" APO, it needs to be operating at full aperture. Funny enough, when I obstruct that 4" APO, even with a significant obstruction close to 50% the full aperture, it still outperforms the Newt with 4" mask (with the Newt being the unobstructed scope in this example)!  

 

I do not believe the performance gain of the refractor has anything to do with the obstruction necessary for a reflecting telescope - there are enough experiments to suggest that smallish obstructions have minimal effect on the image.  It is all about the mirror. 

 

I believe the main reason for this is that mirrors tend to be "pragmatic", useable, good enough.  They are never smooth, to a light wave, those pits are abyssic.  Not even the best mirror can approach anything resembling "smooth" to the light path.  I read somewhere that it would take something like two years of constant polishing to make a mirror ostensibly perfect.  The lack of smoothness and the fact that mirrors are typically parabolic makes me think a mirror has no hope of matching an APO objective.  This alone explains why, if I obstruct the APO to a similar extent to an SCT or Newtonian, 99.9% of the time, the APO produces a better image no matter what.  This is even true if the APO is purposely obstructed and the Newtonian is purposely unobstructed by applying an off axis mask! 

 

It is slightly controvercial to suggest mirror telescopes are bad.  They are fine, but just to put into context:  A plate glass objective can focus sunlight to burn wood or paper.  Mirrors cannot do this to the same degree.  This is easy to demonstrate in the field - the mirror just cannot make a small enough spot.



#228 KJL

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Posted 21 November 2014 - 12:06 AM

It is slightly controvercial to suggest mirror telescopes are bad.  They are fine, but just to put into context:  A plate glass objective can focus sunlight to burn wood or paper.  Mirrors cannot do this to the same degree.  This is easy to demonstrate in the field - the mirror just cannot make a small enough spot.

Oh boy, I've stayed out of this thread because I've never directly made the comparison, having never owned an exactly-5" and exactly-8" scope of the respective types. But surely what you say here just this isn't so? Let's see, a bit of Google and ... oh yeah I remember this one, and my goodness me, and oh look actual instructions on how to burn a hole in paper -- I mean, measure the focal length -- with a concave (let's say spherical) telescope mirror.

 

Sorry. That was just too easy. :)

 



#229 leviathan

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Posted 21 November 2014 - 12:07 AM

Well, you should also consider light scattering on 2 mirrors, ~94% for each usually with normal coatings. And again, comparing optics of premium APO to let's say regular chinese cheap 12" is not right, they are never the same. I bet premium optics 12" newtonian with 4" mask should be very close to 4" APO.



#230 JMW

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Posted 21 November 2014 - 12:29 AM

I have the C11 EdgeHD, TEC 140 and a Webster D14 with 14.5" Zambuto f/4.3 mirror representing my 3 most expensive scopes all purchased used. Under the best of conditions the Webster>C11 EdgeHD>TEC 140 but my seeing seldom allows the higher magnifications (300+) possible with the larger aperture. If I want to see all of the Veil complex at once I prefer my shorter SV115T20 with a Pan 41 and UHC filter. I paid $2800 for my used C11 EdgeHD and about $5000 each for the TEC 140 and the Webster D14. The TEC 140 is the easiest to setup and use when on a DM6 mount and always puts up pleasing views. The C11 EdgeHD takes longer to adapt to conditions even with the Tempest fans but is very flexible for imaging at f/10, f/7 or f/2. The Webster is the most work but goes the deepest and puts up the best planetary views on rare perfect nights that I have enjoyed in a scope. 

 

There is no right choice and if you take your time and save no choice needs to be made. Buy quality scopes meet your needs. No point in saying one scope is better for all situations than another. Sometimes you want view wide fields, other times planets, other times extremely faint galaxies. Sometimes you want a lightweight refractor on a light grab and go mount for a quick peek with no fuss. My SVR90T on a FTQ and 3 pound tripod fits that bill.


Edited by JMW, 21 November 2014 - 12:33 AM.


#231 schang

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Posted 21 November 2014 - 07:54 AM

I used to believe that mirrors were a better optical device than a lens that splits colors like a rainbow.  I now think that refraction is better than reflection.  Even when I equalized two scopes so both are subject to the same limitations, the refractor still wins. 

 

I have compared a 12" Newt to a 4" APO and the 4" APO trounces it on planetary details when the 12" Newt has an offset mask making it an unobstructed 4" reflector.  For the 12" Newt to beat a 4" APO, it needs to be operating at full aperture. Funny enough, when I obstruct that 4" APO, even with a significant obstruction close to 50% the full aperture, it still outperforms the Newt with 4" mask (with the Newt being the unobstructed scope in this example)!  

 

I do not believe the performance gain of the refractor has anything to do with the obstruction necessary for a reflecting telescope - there are enough experiments to suggest that smallish obstructions have minimal effect on the image.  It is all about the mirror. 

 

I believe the main reason for this is that mirrors tend to be "pragmatic", useable, good enough.  They are never smooth, to a light wave, those pits are abyssic.  Not even the best mirror can approach anything resembling "smooth" to the light path.  I read somewhere that it would take something like two years of constant polishing to make a mirror ostensibly perfect.  The lack of smoothness and the fact that mirrors are typically parabolic makes me think a mirror has no hope of matching an APO objective.  This alone explains why, if I obstruct the APO to a similar extent to an SCT or Newtonian, 99.9% of the time, the APO produces a better image no matter what.  This is even true if the APO is purposely obstructed and the Newtonian is purposely unobstructed by applying an off axis mask! 

 

It is slightly controvercial to suggest mirror telescopes are bad.  They are fine, but just to put into context:  A plate glass objective can focus sunlight to burn wood or paper.  Mirrors cannot do this to the same degree.  This is easy to demonstrate in the field - the mirror just cannot make a small enough spot.

Just wonder if the obstruction you put on your refractor is on the outside, or in the center, as in CO of SCT or MCT, or Dob.  There is a difference if it is CO or not...

 

(edit) PS: An offset mask for a 12" Dob eliminates the CO effect, but it does not address the thermal situation of the larger mirror in the OTA.  I would think that if those factors are taking care of, the difference would be non-existent, assuming the mirror and the refractor lens has the same Strehl and smoothness.  On top of that, when CA is entering the picture, I'd think the Dob would be better in terms of contrast on bright objects...


Edited by schang, 21 November 2014 - 08:10 AM.


#232 dotnet

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Posted 21 November 2014 - 08:15 AM

And again, comparing optics of premium APO to let's say regular chinese cheap 12" is not right, they are never the same.

 

Indeed. If a tube holder (!) for a 4" FSQ-106 costs as much as a complete 10" GSO Dob setup then the scope that it holds had better be performing miracles.

 

Cheers

Steffen.



#233 GJJim

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Posted 21 November 2014 - 08:49 AM

 

I used to believe that mirrors were a better optical device than a lens that splits colors like a rainbow.  I now think that refraction is better than reflection.  Even when I equalized two scopes so both are subject to the same limitations, the refractor still wins. 

 

I have compared a 12" Newt to a 4" APO and the 4" APO trounces it on planetary details when the 12" Newt has an offset mask making it an unobstructed 4" reflector.  For the 12" Newt to beat a 4" APO, it needs to be operating at full aperture. Funny enough, when I obstruct that 4" APO, even with a significant obstruction close to 50% the full aperture, it still outperforms the Newt with 4" mask (with the Newt being the unobstructed scope in this example)!  

 

I do not believe the performance gain of the refractor has anything to do with the obstruction necessary for a reflecting telescope - there are enough experiments to suggest that smallish obstructions have minimal effect on the image.  It is all about the mirror. 

 

I believe the main reason for this is that mirrors tend to be "pragmatic", useable, good enough.  They are never smooth, to a light wave, those pits are abyssic.  Not even the best mirror can approach anything resembling "smooth" to the light path.  I read somewhere that it would take something like two years of constant polishing to make a mirror ostensibly perfect.  The lack of smoothness and the fact that mirrors are typically parabolic makes me think a mirror has no hope of matching an APO objective.  This alone explains why, if I obstruct the APO to a similar extent to an SCT or Newtonian, 99.9% of the time, the APO produces a better image no matter what.  This is even true if the APO is purposely obstructed and the Newtonian is purposely unobstructed by applying an off axis mask! 

 

It is slightly controvercial to suggest mirror telescopes are bad.  They are fine, but just to put into context:  A plate glass objective can focus sunlight to burn wood or paper.  Mirrors cannot do this to the same degree.  This is easy to demonstrate in the field - the mirror just cannot make a small enough spot.

Just wonder if the obstruction you put on your refractor is on the outside, or in the center, as in CO of SCT or MCT, or Dob.  There is a difference if it is CO or not...

 

(edit) PS: An offset mask for a 12" Dob eliminates the CO effect, but it does not address the thermal situation of the larger mirror in the OTA.  I would think that if those factors are taking care of, the difference would be non-existent, assuming the mirror and the refractor lens has the same Strehl and smoothness.  On top of that, when CA is entering the picture, I'd think the Dob would be better in terms of contrast on bright objects...

 

 

I think I learned a new word today -- abyssic.  :grin:

 

The comparison between reflector and refractive must consider the light path and the stability of the air in that path defined by the telescope tube and optical configuration. It boils down to one pass for refractors versus two or three for Newts and SCTs. Surface scatter of mirror coatings is also a factor to some extent, but how many refractors are used with an aluminized diagonal?

 

In my experience when temperature effects and seeing cooperate, a 14" SCT will show more planetary detail than a 7" apo. However the apo is a more versatile instrument in general use because it is less affected by the external factors that I cannot control. At my location with wide diurnal temperature swings and variable seeing, the SCT can perform at its aperture limit on only a handful of nights each year. The apo is a much better performer on average.



#234 BillP

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Posted 21 November 2014 - 09:34 AM

But what SpooPoker suggests would actually be a great experiment.  Someone with a premium mirror make an off-axis mask, position that mask in a quadrant with the least thermals in the path, then compare with an APO of the same size or masked to the same size. 



#235 GJJim

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Posted 21 November 2014 - 09:58 AM

But what SpooPoker suggests would actually be a great experiment.  Someone with a premium mirror make an off-axis mask, position that mask in a quadrant with the least thermals in the path, then compare with an APO of the same size or masked to the same size. 

 

Over the years several companies have offered off-axis Newts. Where are they now? The design is flawed by asymmetry (fast optic coma + astigmatism) and the folded light path. 



#236 schang

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Posted 21 November 2014 - 10:53 AM

 

But what SpooPoker suggests would actually be a great experiment.  Someone with a premium mirror make an off-axis mask, position that mask in a quadrant with the least thermals in the path, then compare with an APO of the same size or masked to the same size. 

 

Over the years several companies have offered off-axis Newts. Where are they now? The design is flawed by asymmetry (fast optic coma + astigmatism) and the folded light path. 

 

To do this off-axis Newt, one would need to start with a large mirror, then cut it into four mirrors then put the secondary mirror on the edge of the OTA.  It is a hassle and costly to market something like that (IIRC, Orion and other companies have or had products like this), I think...When one uses an off-axis Newt, the effective focal ratio is increased... As far as image quality for a slower Newt (fast refractor has its own issue..) I hope those who own such scope, or large Dob with such an interest, can pitch in...Here are some links on this type of scopes

 

http://www.dgmoptics.com/

 

http://www.dgmoptics.com/

 

http://www.dgmoptics...oa_4_review.htm - one of review by Mike Palermiti, who stated:

 

quote: [This sample 4" F/10 Off-Axis Newtonian, by DGM Optics, stands alone as the finest optical system at 4 inches of aperture, I have seen worldwide].unquote

 

I have not looked through one, although I did have a 3.7" off-axis mask for my 10" dob for double stars viewing for average seeing condition. I do not have a 4"  APO for comparison myself, which I did not intend.  But I did have a C102 achromat.  I prefer viewing planets through my 10" dob instead of 4 inchers though, unless I am lazy to take it out...



#237 GJJim

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Posted 21 November 2014 - 12:11 PM

 one of review by Mike Palermiti, who stated:

 

quote: [This sample 4" F/10 Off-Axis Newtonian, by DGM Optics, stands alone as the finest optical system at 4 inches of aperture, I have seen worldwide].unquote

 

A 4" f/10 apo is easier to produce and will have none of the coma or astigmatism inherent in the off-axis Newt.



#238 KJL

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Posted 21 November 2014 - 12:36 PM

I think I learned a new word today -- abyssic.  :grin:

Lol, that was my first reaction too! Until I found out it's not (yet) a word, so don't feel too bad ....

 

My verbal SAT score wasn't the hottest, but that was a long time ago and today I usually only encounter words I don't know when I read. Anything. Which isn't very often, except for CN of course. :lol:



#239 Swedpat

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Posted 21 November 2014 - 03:01 PM

When it comes to the comparison between an APO and a mirror scope there is one thing I wonder: while APO always is considered as superior to a mirror with equal or slightly larger aperture the comparison usually is between an APO much more expensive than the mirror scope. But what would be the conclusion if the compared mirror scope is at the same price level as the APO(and the mirror scope has the same effective aperture)?


Edited by Swedpat, 21 November 2014 - 03:04 PM.


#240 BillP

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Posted 21 November 2014 - 04:26 PM

Will need to find a premium mirror maker that will actually make a 130mm or 140mm or 152mm mirror for that comparison  :lol:



#241 junomike

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Posted 21 November 2014 - 05:37 PM

Will need to find a premium mirror maker that will actually make a 130mm or 140mm or 152mm mirror for that comparison  :lol:

I did a somewhat similar comparison between a fully acclimatized  Intes M703 (7" 1/8th Wave) and an APM 130mm F6 LZOS (1/9th Wave)

 

Result:  On Jupiter both gave similar views.  The Mak's darker tint due to the Meniscus coatings seemed to help bring out Jupiter's banding better (similar to TV Plossl coffee-tone effect).

The details were about equal in both, but it  just seemed a little easier to see in the Mak.

 

On the Moon it was a different story as, although the Mak's tonal qualities aided in Lunar detail, It could never seem to meet the sharpness of the Apo.  To clarify, the Mak was perfectly collimated at 500X.

 

Mike



#242 BillP

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Posted 21 November 2014 - 09:56 PM

Very interesting.  Were these observations from same night or on two different evenings?



#243 junomike

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Posted 21 November 2014 - 10:42 PM

Very interesting.  Were these observations from same night or on two different evenings?

Bill, not only was it on the same night, but I had both OTA's mounted on the same Mount via "Side by side" plate and pointed at the same object.

I should also note that I kept the magnification equal and not the exit pupil.

 

Mike



#244 leviathan

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Posted 22 November 2014 - 12:30 AM

Very interesting information. Thanks.




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