MTF of a telescope

[andrew s]

1 minute ago, jetstream said:

I wonder if the the test results from one would be held in high regard? I'd buy one if I could test a few scopes to pay for it and I could have my own fracs tested by an optician I know, for comparison and reference with regards to accuracy. The exchange rate is in the wrong direction for me...

Does this set up do mirrors?

Have a look here http://www.alcor-system.com/new/SH/Shackscope40x40.html for what it can do. I think it best suited to an optical test lab. You would also need a large high quality  flat unless you had a very long line of sight to a pin hole . Failing that you would be at the mercy of the weather and seeing.

Regards Andrew 

[jetstream]

7 minutes ago, andrew s said:

Have a look here http://www.alcor-system.com/new/SH/Shackscope40x40.html for what it can do. I think it best suited to an optical test lab. You would also need a large high quality  flat unless you had a very long line of sight to a pin hole . Failing that you would be at the mercy of the weather and seeing.

Regards Andrew 

Holy cow! about 17% Strehl on the first scope... this is what Im interested in- the whole system capability.

Well, I was tossing around buying out a dob maker, too complicated here in Canada but I do have his support as well as from a top optician who loves testing optics and supporting the astro community.I'm sure I could be helped along with the flats as well as set up, my numbers, the results etc, to get me going.

The main question would be-would it pay for itself?  Dreams are nice but then there is reality.

Thanks for the info Andrew.

[andrew s]

@jetstream this is their NA agent https://www.innovationsforesight.com/product-category/wave-front-analyzer/

Regards Andrew 

[JeremyS]

34 minutes ago, andrew s said:

Have a look here http://www.alcor-system.com/new/SH/Shackscope40x40.html for what it can do. I think it best suited to an optical test lab. You would also need a large high quality  flat unless you had a very long line of sight to a pin hole . Failing that you would be at the mercy of the weather and seeing.

Regards Andrew 

That’s not MTF, it’s more WTF!
Looks amazing!

[jetstream]

Of course the Shack Hartman device opens up the possibility of buying a bunch of GSO mirrors (16"), testing them and keeping the good ones for the truss dobs.Some of them are really good I hear, but... then theres the rest.

Moreso for the secondaries... I would weed these out too.

Edited February 9, 2021 by jetstream

[Deadlake]

28 minutes ago, jetstream said:

Of course the Shack Hartman device opens up the possibility of buying a bunch of GSO mirrors (16"), testing them and keeping the good ones for the truss dobs.Some of them are really good I hear, but... then theres the rest.

Moreso for the secondaries... I would weed these out too.

Would GSO take them back?

Maybe this would be for a dealer to do QA on products?

[jetstream]

1 minute ago, Deadlake said:

Would GSO take them back?

Maybe this would be for a dealer to do QA on products?

Not sure but most likely not from what I hear-just coffee shop talk though.Same goes for scope vendors I think... this device could be a nightmare for them IMHO.Company 7 tests all the optics they sell and if I remember could have been at odds with some manufacturers. This is a good company.

I could get the poor ones refigured but there is that deal about residual strain in the glass..between shipping and preliminary testing I could go in the hole.

[vlaiv]

1 hour ago, andrew s said:

Have a look here http://www.alcor-system.com/new/SH/Shackscope40x40.html for what it can do. I think it best suited to an optical test lab. You would also need a large high quality  flat unless you had a very long line of sight to a pin hole . Failing that you would be at the mercy of the weather and seeing.

Regards Andrew 

Why is this better than say - Roddier analysis - which is essentially free if you have a camera?

[Deadlake]

9 minutes ago, jetstream said:

Not sure but most likely not from what I hear-just coffee shop talk though.Same goes for scope vendors I think... this device could be a nightmare for them IMHO.Company 7 tests all the optics they sell and if I remember could have been at odds with some manufacturers. This is a good company.

I could get the poor ones refigured but there is that deal about residual strain in the glass..between shipping and preliminary testing I could go in the hole.

Well FLO do this:

 

And Astrograph will star test LZOS scopes along with the lens report. 

If there was  an optical report with the scope then a better insurance against a poor QA process. What happens if the scope cannot cope with temperature changes, e.g with a Petzval where you have four lenses moving together... Hard to mimic all conditions... All down to dealer support.

[jetstream]

@Deadlake you guys are lucky to have FLO as a vendor.

Stellarvue, in the past sorted through (Chinese?) lenses and then star tested them to ensure performance -and it worked very well IMHO. Now they use interferometers etc and still star test.

Williams optics use similar glass (top notch) I think so they should be VG- and any issue is most likely through the assembly process IMHO. If the glass itself was poor I think any vendor would change the line up.

interesting that APM released some doublets years ago with interesting performance. No links but if I remember CN had posts about it. You can rest assured the LZOS lensed scope will be superb.The MTF (and everything else) will be near perfect.

[Deadlake]

1 minute ago, jetstream said:

@Deadlake you guys are lucky to have FLO as a vendor.

Stellarvue, in the past sorted through (Chinese?) lenses and then star tested them to ensure performance -and it worked very well IMHO. Now they use interferometers etc and still star test.

Williams optics use similar glass (top notch) I think so they should be VG- and any issue is most likely through the assembly process IMHO. If the glass itself was poor I think any vendor would change the line up.

interesting that APM released some doublets years ago with interesting performance. No links but if I remember CN had posts about it. You can rest assured the LZOS lensed scope will be superb.The MTF (and everything else) will be near perfect.

APM is moving production of FPL51/53 glass telescope products from Sharpstar to Long-Perng. I'll leave you to wonder why... Was tempted by 4" from APM (Long-Perng model, delayed by pandemic),  however if I can get a flat field refractor in a 4" I'd go with that. One criticism of my Vixen SD103S. 

 

[jetstream]

2 minutes ago, Deadlake said:

APM is moving production of FPL51/53 glass telescope products from Sharpstar to Long-Perng. I'll leave you to wonder why... Was tempted by 4" from APM (Long-Perng model, delayed by pandemic),  however if I can get a flat field refractor in a 4" I'd go with that. One criticism of my Vixen SD103S. 

 

I was going to sell the 90mmSV for a "faster" telescope, imaging wise. Many thanks to @vlaiv for the patience and information regarding sampling rate- I will now try this scope for widefield DSO imaging down the road.

I think Long Perng makes good glass.Yes, I think APM had to do something to reduce sample to sample variation, just my humble opinion.

[Deadlake]

3 minutes ago, jetstream said:

I was going to sell the 90mmSV for a "faster" telescope, imaging wise. Many thanks to @vlaiv for the patience and information regarding sampling rate- I will now try this scope for widefield DSO imaging down the road.

I think Long Perng makes good glass.Yes, I think APM had to do something to reduce sample to sample variation, just my humble opinion.

I'd be interested if I could find out how much a scopes impacted by seeing with aperture. I'd put a Mewlon on my list, however the JetStream might not make it worth it, or reduce the aperture of what's worth having. Thats why I went for a 5" APO first.

[John]

How accurate / useful are the tests that Herr Rohr does considered to be these days ?:

http://r2.astro-foren.com/index.php/de/9-beitraege/01-aeltere-berichte-auf-rohr-aiax-de-alles-ueber-apos/21-tmb-130-780-erfolgreiche-restauration-lzos-020

 

Edited February 9, 2021 by John

[Deadlake]

2 minutes ago, John said:

How accurate are the tests that Herr Rohr does considered to be these days ?:

http://r2.astro-foren.com/index.php/de/9-beitraege/01-aeltere-berichte-auf-rohr-aiax-de-alles-ueber-apos/21-tmb-130-780-erfolgreiche-restauration-lzos-020

 

They are using the correct make of scope.. 😃

[John]

6 minutes ago, Deadlake said:

They are using the correct make of scope.. 😃

He has tested loads of scopes:

http://r2.astro-foren.com/index.php/de/

The tests of the Synta ED120's are interesting, eg:

http://r2.astro-foren.com/index.php/de/10-beitraege/02-ed-optiken-halb-apos-und-frauenhofer-systeme/530-b007-sky-watcher-ed-120-900-halb-apo

Edited February 9, 2021 by John

[Deadlake]

1 minute ago, John said:

He has tested loads of scopes:

http://r2.astro-foren.com/index.php/de/

The tests of the Synta ED120's are interesting.

This one,
http://r2.astro-foren.com/index.php/de/10-beitraege/02-ed-optiken-halb-apos-und-frauenhofer-systeme/530-b007-sky-watcher-ed-120-900-halb-apo

Makes you question why a TSA120?

[vlaiv]

10 minutes ago, John said:

How accurate / useful are the tests that Herr Rohr does considered to be these days ?:

http://r2.astro-foren.com/index.php/de/9-beitraege/01-aeltere-berichte-auf-rohr-aiax-de-alles-ueber-apos/21-tmb-130-780-erfolgreiche-restauration-lzos-020

 

I think that best test of the "test" is repeatability.

Can the same result be obtained by another test and can two independent testers using that test obtain same results.

If so - then I'd say test is valid.

[John]

4 minutes ago, vlaiv said:

I think that best test of the "test" is repeatability.

Can the same result be obtained by another test and can two independent testers using that test obtain same results.

If so - then I'd say test is valid.

He did another ED120:

http://r2.astro-foren.com/index.php/de/10-beitraege/02-ed-optiken-halb-apos-und-frauenhofer-systeme/531-b008-skywatcher-ed-120-900-proseries-halb-apo

 

[vlaiv]

2 minutes ago, John said:

He did another ED120:

http://r2.astro-foren.com/index.php/de/10-beitraege/02-ed-optiken-halb-apos-und-frauenhofer-systeme/531-b008-skywatcher-ed-120-900-proseries-halb-apo

 

That is not quite what I had in mind.

Thing that we measure needs to be the same - but tools used or person doing it can change.

If two unrelated people perform same technique measurement on object and get the same result - I have more confidence that none made an error in doing the technique

If two different measurement techniques are performed on an object and we still get the same result - then I have more confidence that measurement reflect object properties.

[Deadlake]

1 minute ago, vlaiv said:

That is not quite what I had in mind.

Thing that we measure needs to be the same - but tools used or person doing it can change.

If two unrelated people perform same technique measurement on object and get the same result - I have more confidence that none made an error in doing the technique

If two different measurement techniques are performed on an object and we still get the same result - then I have more confidence that measurement reflect object properties.

And we haven't been touched sample size....

[John]

1 minute ago, vlaiv said:

That is not quite what I had in mind.

Thing that we measure needs to be the same - but tools used or person doing it can change.

If two unrelated people perform same technique measurement on object and get the same result - I have more confidence that none made an error in doing the technique

If two different measurement techniques are performed on an object and we still get the same result - then I have more confidence that measurement reflect object properties.

I understand what you mean. In reality though, how likely is that to actually happen in the world of amateur telescopes ?

 

[Deadlake]

2 hours ago, John said:

How accurate / useful are the tests that Herr Rohr does considered to be these days ?:

http://r2.astro-foren.com/index.php/de/9-beitraege/01-aeltere-berichte-auf-rohr-aiax-de-alles-ueber-apos/21-tmb-130-780-erfolgreiche-restauration-lzos-020

 

Does this work in google translate, not working for me...

Oh well, apple translate works, built into the browser...

Edited February 9, 2021 by Deadlake

[vlaiv]

1 minute ago, John said:

I understand what you mean. In reality though, how likely is that to actually happen in the world of amateur telescopes ?

 

Not very likely unless people learn to do their own tests.

Say you had your scope shipped to a professional and you get report back and you do Roddier method on it and you get same result.

Then you are fairly confident that

a) Professional did their job right

b) your scope is as per measurement

If you were to just do Roddier test - you would always wonder if you did it wrong. If you have your friend perform same Roddier test on your scope with their camera - then you would have more confidence that you did it ok - but is it accurate enough? Maybe test itself is not as good as people say?

 

[John]

Just now, Deadlake said:

Does this work in google translate, not working for me...

I didn't try translating it. I just looked at the pretty pictures

Unit to unit variability is an often discussed issue and, in the past, has been used to make the case for buying a premium brand at a premium price.

I suspect that production consistency is pretty good from all the manufacturers these days though.