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How many really good SCT's are out there

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#51 REC

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Posted 23 June 2019 - 12:52 PM

I just did a quick count in my garage and I currently own 16 SCTs from 4" up to 10", and from about 1977 to 2014, being careful to step over the Halley years (from about 1985 to 1988). (I really gotta stay out of the classifieds!) To be honest, there's not a bad scope in the bunch. Some are optically better than others, and anything built this century has been consistently very good, but I would not rate any of my scope any worse than fair, and that's really only one scope, a spotter version of the Meade 2045. I also have an LX3 version of this same scope and it is outstanding!

 

Having bought nearly all of my scopes off of the used market I am usually buying someone else's problem, and I have yet to see an SCT whose problem wasn't the same; collimation and acclimation. The most problematic members of my SCT family have been the Meade f/6.3 Wide Field SCTs. These have a reputation for spotty optics and my first (a 10" f/6.3) seemed to confirm this. Before giving up on it I spent and entire evening zeroing-in the collimation using a camera, and it blossomed into a solid performer. I liked it so much that I bought another 10" f/6.3 for visual and an 8" f/6.3 for EAA, both went through the same teething pains and both ended up with the same result. For these 'fast' SCTs, close is not good enough. But once adjusted and the screws snug, they have remained in great shape for several years.

 

Just for reference; I was an active ATM for 30 years (from about 1968 through 1998), and I built something like 30 scopes from a 4.25" f/8 up to my Green Monster 16.5" f/6.5, so I like to think that I know my way around at least basic optical testing, but I could be wrong about that. smile.gif

 

Soooo, I think that if anyone is in the market for an SCT built in the past 20 years by one of the main manufacturers you have a good chance of getting a good performing SCT. Just take the time to let it acclimate and keep an eye on the collimation.

 

My latest baby...

 

attachicon.gif Lightswitch 8 ACF (6-10-2019)-1.jpg

 

...a Meade LS 8 ACT with a micro-focuser. I had this out last night DSO-hopping from Scorpius northward up through Cygnus (with a stop-over at Jupiter). Loverly!

 

And one for the older members of the family...

 

attachicon.gif Sandcast C8 Setup (3-23-2018)-2.jpg

 

These kinda book-end my SCT gear. smile.gif

 

Enjoy!

You have good taste! I had that older C8 in the 70's and now have the LS-8 ACF like yours. I also have the same chair! I got lazy in my older years and don't want to fuss with alignment. I turn on the switch and 10 minutes later I'm observing! What do you think of it's optical performance?



#52 Eddgie

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Posted 23 June 2019 - 01:10 PM

I have to confess this as well.  I find it frustrating to talk about optics and optical quality when so many people don't even understand the basic optical concepts of linear resolving power (which is the true limit of a telescope's performance, and not the angular resolution that most people use) and Modulation Transfer Function (MTF) which is the way that contrast loss in a system is described.

 

Without knowing these fundamental concepts, people are forced to rely on vague and imprecise terms like "Sharpness."  What is "Sharpness?"   How is "Sharpness" measured?  A scope can be sharp, but the question that is generally of interest in quality conversations is:  "Is the scope is as -Sharp- as it can be?" and this is how linear resolution and MTF are used to describe the contrast behavior of an instrument.  This is what Suiter teaches early in his book.  It teaches you to know how much contrast is being left on the table. 

MTF is all inclusive that it includes the design, quality of manufacture, the aperture, and the focal ratio. and it makes it very easy to compare how one scope of one design, quality of manufacture, aperture, and focal ratio would compare to a telescope that is different in design, quality of manufacture, aperture, and focal ration, and Suiter does this exact thing in his book, showing how a nothing special larger Newtoinan (14" as I recall) in less than perfect alignment can still have better MTF (contrast transfer) than a much smaller perfect optical system (6" unobstructed as I recall). 


Edited by Eddgie, 23 June 2019 - 03:01 PM.


#53 jhayes_tucson

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Posted 23 June 2019 - 01:25 PM

Celestron currently has the ability to interferometrically tests their fully assembled systems and they are producing some very good optics.   Unfortunately, that doesn't mean that all of them that have been produced over the years are of the same high quality.  I've interferometrically tested three C14s with a PhaseCam dynamic interferometer system and after some realignment, two of them looked excellent.  Here's one of the reports:  https://www.cloudyni...f-the-c14-edge/.  This is the scope that I use for all of my imaging and it achieved a measured the on-axis Strehl performance of 0.93, which is outstanding for a scope of this aperture.  As I recall, the design limit for the C14 shows an on-axis Strehl around 0.98.  Keep in mind that the definition of diffraction limited performance is when the imaging performance is driven primarily by diffraction rather than by wavefront errors.  The accepted cross over point occurs at a Strehl of 0.8 so any Strehl value over 0.9 will be virtually indistinguishable from a perfect system--even in a vacuum.  Atmospheric seeing adds another level of wavefront error makes any imaging performance gain over a Strehl of 0.8 very difficult to detect.  It may be possible to detect a very minor improvement in imaging performance by going from a Strehl of 0.8 to 0.9, but in my view, it's complete nonsense to try to rank imaging performance for any systems with Strehl values above 0.9.  You simply won't see any difference.

 

Putting aside the variation in optical quality among individual SCTs, the single biggest parameter that will affect imaging performance is secondary alignment.  On a C14 Edge system, the secondary has to be aligned to within less than 2 arc-minutes of tilt in order to achieve a Strehl better than 0.8 on a perfect system.  That represents a 5 degree turn on any of the secondary alignment screws.  That is a very tight tolerance and pretty challenging to achieve without good feedback on the alignment status.  When I aligned the C14 Edge in double pass on the interferometer, I experienced this sensitivity first hand.  Since the PhaseCam has the ability to read out 3rd order coma in real time, I was able to simply dial it down to a minimum value in real time (about 0.010 waves.)  This adjustment required moving the alignment screw in the smallest possible increments determined by mechanical stiction in the system.  Lubricating the screws might have made this an easier process but I didn't try that.  High surface accuracy is necessary for good imagining performance, but I suspect that for a lot of SCTs in the field, secondary misalignment is the single largest contributor to poor imaging performance.  It does not take very much misalignment to produce a system with a Strehl of 0.6 (or worse.)

 

John

Attached Thumbnails

  • C14 Edge14 secondary alignement sensitiivity.jpg

Edited by jhayes_tucson, 23 June 2019 - 01:26 PM.

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#54 Cotts

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Posted 23 June 2019 - 01:29 PM

Even though I have Suiter's book and I have read it thoroughly, I have never wasted a night with  seeing good enough to effectively and reliably do his testing regimen to actually do the testing.

 

When I get sub-arc second seeing where I can see with my 1989 Celestron Ultima 11 or my Lockwood/Teeter 12.5" f/6.5 dob, the tiny, perfect Airy patterns of two 7th magnitude stars 1 arc second apart I don't care what my Strehl is or my linear resolution or my contrast transfer are.  The last thing I want to do on such a night is to waste it on 'testing'......

 

I have always rated the optics of both of my big scopes as 'superb'.  Due to its relatively tiny 16% central obstruction the 12.5" shows a much more delicate first ring in the Airy pattern and noticeably better contrast on Jupiter than the 11" with its 35% C.O..   

 

But quantified measures of SA, CA, Strehl etc.?  Don't know, don't care....

 

Dave



#55 Astrojedi

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Posted 23 June 2019 - 01:37 PM

Don’t want to turn this thread into a SCT vs refractor but some qualifications are required. Again generalizations need to be addressed.

 

Deep Sky Objects: A good C8 will outperform or easily match any 6” refractor on deep sky.

 

Lunar: My EdgeHD 8 beat the pants of my very high strehl 5” APO on lunar. A good 6” APO (many times the cost of the EdgeHD) may outperform the EdgeHD 8. 

 

Saturn: I would say a 6” APO will likely match a high quality C8

 

Jupiter / Mars: A high quality 6” APO will outperform a EdgeHD 8 in contrast. The C8 will likely show similar detail but the refractor view has an etched high contrast view which is hard to beat.



#56 REC

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Posted 23 June 2019 - 01:47 PM

I have to confess this as well.  I find it frustrating to talk about optics and optical quality when so many people don't even understand the basic optical concepts of linear resolving power (which is the true limit of a telescope's performance, and not the angular resolution that most people use) and Modulation Transfer Function (MTF) which is the way that contrast loss in a system is described.

 

Without knowing these fundamental concepts, people are forced to rely on vague and imprecise terms like "Sharpness."  What is "Sharpness?"   How is "Sharpness" measured?  A scope can be sharp, but the question that is generally of interest in quality conversations is:  "Is the scope is as Sharp as it can be?" and this is how linear resolution and MTF are used to describe the contrast behavior of an instrument.  This is what Suiter teaches early in his book.  It teaches you to know how much contrast is being left on the table. 

MTF is all inclusive that it includes the design, quality of manufacture, the aperture, and the focal ratio. and it makes it very easy to compare how one scope of one design, quality of manufacture, aperture, and focal ratio would compare to a telescope that is different in design, quality of manufacture, aperture, and focal ration, and Suiter does this exact thing in his book, showing how a nothing special larger Newtoinan (14" as I recall) in less than perfect alignment can still have better MTF (contrast transfer) than a much smaller perfect optical system (6" unobstructed as I recall). 

I first hear about MTF curves and charts back in the 80's. I worked in the photo industry for Vivitar and the standard definition for lens performance was a test showing how many line per millimeter the lens would perform. So they used a chart for testing a lens and would show how many lines it resolved and at what f/stop it was. The old good, very good and excellent words where used in the lens performance. We came out with an advanced line of SLR lenses for the Pro's called Series 1. We advertised the lenses in the photo magazines of the time. In the ad they would show the MTF charts as the test results. Not many consumers ever heard of the test before, but that is what we used. Our Series 1 70-210 f/3.5 Macro Focusing zoom lens became an instant hit and we sold a lot of them. It put Vivitar up on top of the list of serious lens companies.


Edited by REC, 23 June 2019 - 01:50 PM.


#57 sg6

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Posted 23 June 2019 - 01:51 PM

In engineering terms they are not easy to produce. Tolerances are on the narrow side.

 

Always said the idea that they match scope and corrector is rubbish. If they did then at the end a number of SCT OTA's and correctors would be just incompatible and the solution is hit them with a big hammer. Which is not going to happen. Skywatcher/Celestron may want to propogate that idea so you return the scope to them for rectification. But in a mass production area they cannot be "matched".

 

Likely any corretor is placed to the best position that can be set in say 2 minutes or less. But I doubt any more then that.

 

So I would suggest that the components are reasonably set up but no more, and that the mechanical and alignment requirements are such that somewhat small deviations reduce the performance of the final artical. So getting a good one means that several slightly critical factors all have to be good. And the probability of that happening in a random sense is accordingly low.



#58 starman876

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Posted 23 June 2019 - 01:59 PM

These technical details about on scope testing and presenting the facts about how critical alignment is what I have seen also.  All you need to be is a hair off on the alignment on an SCT and you lost the resolution the scope is capable of.  This is real easy to see in DPAC.  Just turn on of the alignment screws a small bit the wrong way and you go from 1/4 wave or better to something much worse in a heartbeat.  From almost nice straight bars to very curved bars.  I do not think many people really know just how critical this alignment is.  


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#59 Ballyhoo

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Posted 23 June 2019 - 02:08 PM

These technical details about on scope testing and presenting the facts about how critical alignment is what I have seen also.  All you need to be is a hair off on the alignment on an SCT and you lost the resolution the scope is capable of.  This is real easy to see in DPAC.  Just turn on of the alignment screws a small bit the wrong way and you go from 1/4 wave or better to something much worse in a heartbeat.  From almost nice straight bars to very curved bars.  I do not think many people really know just how critical this alignment is.  

what do you mean by alignment? Collimation?  I presume so and certainly that is the case with collimation, IME.



#60 Astrojedi

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Posted 23 June 2019 - 02:08 PM

Very true. Collimation is critical to performance of all optical Instruments. But the good is that my SCTs hold critical collimation very well for months even years if I fully tighten the screws.



#61 HxPI

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Posted 23 June 2019 - 02:09 PM

$34.  Yes, that is the one.

 

https://www.willbell.com/tm/tm5.htm

 

For whatever odd reason, if a book is out of print (as is the case with the original version of Suiter's book, which is the one I saw on Amazon when I looked) book sellers will often put a stupid high price on it, I guess hoping someone will think that it is the only way to get the book anymore?  I don't know why really, but I have seen it with other out of print books, but in this case, the original was simply superseded by a second (and improved) edition.

 

The first version is really good, but the Second Edition is much better.  In addition to just telling how to star test (and even here, there are improvements) it also goes into quite a bit of telescope design.  For example, Suiter looks at things like the design limits of the inexpensive small, all spherical f/12 MCT design, and goes into comparison between polychromatic Strehl (something that the refractor forum almost completely ignores) and visual weighted Strehl.  

 

It is far more than a book about how to test telescopes.   I  consider it a must read by people that really want to understand how contrast transfer is affected by design and fabrication, and there is a great deal of information about different designs as well. 

 

It is a treasure chest of all things to do with optics and optical quality. 

Excellent! Thanks for sharing.



#62 HxPI

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Posted 23 June 2019 - 02:11 PM

That's the book. I have the original version (which also came from Will-Bell) and have no idea why/how some are charging such a high price for the second edition. Rest assured, Will-Bell is a legit publisher/dealer that has served the scientific community for decades. 

Excellent! Thanks for sharing.



#63 Astrojedi

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Posted 23 June 2019 - 02:18 PM

In engineering terms they are not easy to produce. Tolerances are on the narrow side.

 

Always said the idea that they match scope and corrector is rubbish. If they did then at the end a number of SCT OTA's and correctors would be just incompatible and the solution is hit them with a big hammer. Which is not going to happen. Skywatcher/Celestron may want to propogate that idea so you return the scope to them for rectification. But in a mass production area they cannot be "matched".

 

Likely any corretor is placed to the best position that can be set in say 2 minutes or less. But I doubt any more then that.

 

So I would suggest that the components are reasonably set up but no more, and that the mechanical and alignment requirements are such that somewhat small deviations reduce the performance of the final artical. So getting a good one means that several slightly critical factors all have to be good. And the probability of that happening in a random sense is accordingly low.

Actually yes and no. Your comments are completely unqualified. What level of optical quality / performance are you talk about? Yes, you probably cannot get a 1/10wave system from commercial SCTs consistently. But 1/5-1/6 wave I would argue absolutely.

 

Celestron at least have tightened up the manufacturing enough that individual components match or exceed tolerances especially for  1/5 wave or better from my first hand experience.



#64 CHASLX200

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Posted 23 June 2019 - 02:47 PM

Design and construction of SCT's involves for two major optical compromises;
1 - F/2 Primaries discussed earlier.
2 - Vacuum method of producing correctors. Vacuum method was invented by Celestron and is responsible for making the low cost production of these scopes possible but there is a catch to it, the vacuum method produces a close approximation of highly complex aspheric Schmidt curve, to improve on it hand figuring by highly skilled opticians will be required resulting in unaffordable SCT.
I have looked through good many SCT’s, they all produce relatively soft images compared to high end refractors and Maksutovs, but no need to compare SCT’s to expensive scopes, all one needs to do is to compare SCT’s to Meade 7” Mak to see the difference in image quality.

.

Vahe

My SW150 Mak just blew past any SCT i have ever looked thru. Just my freaky good 1984 C8 came close.



#65 CHASLX200

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Posted 23 June 2019 - 02:52 PM

None of the newer SCTs I have owned have produced soft images once collimated and in thermal equilibrium. You will be surprised how many folks who own SCTs don’t realize how to use one. Thermals in the OTA are almost indistinguishable from SA for practical purposes.

 

Beyond the typical differences between refractors and cats, the contrast and sharpness are a function of optical quality not optical design. My EdgeHD 8 holds its own very well against any 7” Mak and even 6” refractors. The only objects that look better in a high end 6” APO are Jupiter and Mars.

 

If you are ever in San Diego please visit the SDAA dark site. I go there often and do outreach as well. I would be happy to change your mind.

Won't be changing my mind. Still a good Newt will beat them on the planets.



#66 Don W

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Posted 23 June 2019 - 03:00 PM

I have had a dozen or so SCT's since 1981. Had one off brand 4" that had awful optics (Bausch & Lomb?). One C11 was buggered by a previous owner. The rest had acceptable to great optics. I have seen bad optics in some other people's late 80s Meade 8" SCTs. But the Meade 8" and 10" models I had in the latter half of the 90s were very good. No complaints at all.

 

My first 'serious' scope was a 1981 Orange Tube C8. I felt it had good optics. My latest 2018 model C8 is as good or better. I have had a couple other Celestron OTAs from the 90s as well. Also good optics.

 

I'm on my 3rd C11. My first one and my present one were/are Nextstar 11 GPS models. Both had very nice optics. The middle C11 was  an OTA that was treated poorly by the previous owner. Thankfully he took it back and refunded my money. I must say here that the C11 is my all time favorite SCT size. My Nexstar C11s have never disappointed me.

 

At present I have two C5s. One OTA from 1979 with very nice optics and a late 90s Nexstar 5 that also gives very nice views. I keep that one at my girlfriend's place about 60 miles away.

 

As to the member that always badmouths SCTs but has owned a very high number of them, don't you get tired of posting the same anti-SCT crap  over and over and over and over? I know I'm getting pretty tired of hearing it. It's like a broken record!!


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#67 Astrojedi

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Posted 23 June 2019 - 03:04 PM

Won't be changing my mind. Still a good Newt will beat them on the planets.

Prejudice is usually hard to fight even with evidence. Problem is opinions for some members here have been shaped by 20+ year old OTAs. And in my experience many don’t know how to use an SCT.



#68 starman876

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Posted 23 June 2019 - 03:19 PM

I have had a dozen or so SCT's since 1981. Had one off brand 4" that had awful optics (Bausch & Lomb?). One C11 was buggered by a previous owner. The rest had acceptable to great optics. I have seen bad optics in some other people's late 80s Meade 8" SCTs. But the Meade 8" and 10" models I had in the latter half of the 90s were very good. No complaints at all.

 

My first 'serious' scope was a 1981 Orange Tube C8. I felt it had good optics. My latest 2018 model C8 is as good or better. I have had a couple other Celestron OTAs from the 90s as well. Also good optics.

 

I'm on my 3rd C11. My first one and my present one were/are Nextstar 11 GPS models. Both had very nice optics. The middle C11 was  an OTA that was treated poorly by the previous owner. Thankfully he took it back and refunded my money. I must say here that the C11 is my all time favorite SCT size. My Nexstar C11s have never disappointed me.

 

At present I have two C5s. One OTA from 1979 with very nice optics and a late 90s Nexstar 5 that also gives very nice views. I keep that one at my girlfriend's place about 60 miles away.

 

As to the member that always badmouths SCTs but has owned a very high number of them, don't you get tired of posting the same anti-SCT crap  over and over and over and over? I know I'm getting pretty tired of hearing it. It's like a broken record!!

and I thought I was the only one that thought it sounded like a broken record.


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#69 starman876

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Posted 23 June 2019 - 03:25 PM

it is starting to sound like there are a lot more good SCT's out there than there are bad ones.  Maybe my feelings are right that the bad ones are being passed around.  Most likely they are sold for a very good price and I know everyone hopes to get that great scope for a bargain price.    I know my eyes were opened once I got a DPAC set up.  You can tell fairly quick before you even put the scope on a mount if you will be disappointed or not.   Also, checking out scopes that you have had for a while might end up being disappointing.   Now having a DPAC set up I test every scope before I pass it on.  Seems only fair.   I figure there are a enough bad scopes out there no use passing them on.   There should be a scope gave yard  for bad scopes. 



#70 Rac19

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Posted 23 June 2019 - 03:41 PM

I have noticed that SCTs (usually 14”) seem to be used at professional observatories for public outreach and student access. My guess is that the are they come out on top as a good price/performance choice and general all rounder.

 

My Evolution 8”, with HyperStar, gives f options for f/10, f/6.3 and f/8 and is not too difficult to loud out to the backyard in one piece.

 

No doubt there are other options for better optical performance for specific purposes but I have a budget for one ‘scope only.



#71 CHASLX200

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Posted 23 June 2019 - 03:46 PM

I have had a dozen or so SCT's since 1981. Had one off brand 4" that had awful optics (Bausch & Lomb?). One C11 was buggered by a previous owner. The rest had acceptable to great optics. I have seen bad optics in some other people's late 80s Meade 8" SCTs. But the Meade 8" and 10" models I had in the latter half of the 90s were very good. No complaints at all.

 

My first 'serious' scope was a 1981 Orange Tube C8. I felt it had good optics. My latest 2018 model C8 is as good or better. I have had a couple other Celestron OTAs from the 90s as well. Also good optics.

 

I'm on my 3rd C11. My first one and my present one were/are Nextstar 11 GPS models. Both had very nice optics. The middle C11 was  an OTA that was treated poorly by the previous owner. Thankfully he took it back and refunded my money. I must say here that the C11 is my all time favorite SCT size. My Nexstar C11s have never disappointed me.

 

At present I have two C5s. One OTA from 1979 with very nice optics and a late 90s Nexstar 5 that also gives very nice views. I keep that one at my girlfriend's place about 60 miles away.

 

As to the member that always badmouths SCTs but has owned a very high number of them, don't you get tired of posting the same anti-SCT crap  over and over and over and over? I know I'm getting pretty tired of hearing it. It's like a broken record!!

If you mean me you won't have to worry about it anymore. Not going to be around much longer. I think i have also said there are some good 1's out there as well.  


Edited by CHASLX200, 23 June 2019 - 03:46 PM.


#72 Don W

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Posted 23 June 2019 - 04:08 PM

Yes, you have, many, many, many times!! Usually after ripping on SCTs in general.


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#73 rolo

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Posted 23 June 2019 - 04:21 PM

If you mean me you won't have to worry about it anymore. Not going to be around much longer. I think i have also said there are some good 1's out there as well.  

We would like you to be around but take some of our advice sometimes my friend...


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#74 Magnetic Field

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Posted 23 June 2019 - 04:23 PM

My own opinion:  I would say that a very large percentage of telescope owners really don't know how to do an accurate objective analysis of their optics.  

 

After a decade of encouraging people to read Suiter's book on telescope testing, I still hear people that totally dismiss the validity of the star test.  

 

Count me in the group that dismisses the test.

 

I cannot be done without an interferometer.

 

If someone says to me he has tested his telescope to 1/6 ptv (or whatever number): good luck. Also the star test cannot easily give you a rms value or Strehl number.

 

If you look out the window and the sun is shining all you can say: the sun is shining but you wouldn't have any clue about the temperature or humidity. Only real measurements can give you that.

 

The star test is good for testing for coma or collimation etc.

 

People only fool themself if they think the star test gives them any meaningful numbers. Okay, before everyone jumps on me. How about this:

 

1. You test your telescope with the star test and claim you scope is 1/6ptv. Who knows maybe you also feel confident enough to give a rms estimate and Strehl number estimate.

2. If the interferometer test from an authorised source says it is 1/4ptv (and rms and Strehl will also have numbers) you will sell your house become a renter and the proceeds of the house sale go to a charity.



#75 CHASLX200

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Posted 23 June 2019 - 04:34 PM

We would like you to be around but take some of our advice sometimes my friend...

I have every right to say if a scope is bad or not. But i tell ya what since you are not free to say the truth i won't bring it up again.  I will just type they are the greatest thing since the Corvettte.




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