686 replies to this topic

[Astrowl]

So...   On the bench where the observer's brand bias is neutralized these scopes look remarkably similar with the Askar being slightly worse overall.  Color correction appears to be very close between the two.  This is not surprising as both are similar apertures, focal lengths and designs.  The main difference is the price.  The SVX180 is nearly $20k.   The Askar 185 is less then $5k. 

Hi Paul, were your tests on the SVX also done with double pass like the one of Joe G ?

[turtle86]

The DPAC tests of this Askar scope led me to re-read the thread on the Stellar Vue SVX-180T which was figured in red.  Such a long discussion.  But what strikes me the most is the statement by Rick Runcie, the owner of the scope.  When you read what he says it sounds like it is a somewhat amazing scope with some flaws.  No scopes are perfect.

 

He saw some flaws.  But he also says it put up some incredible views.  I am not going to debate his opinion or opine about the merits of that scope.  Likely if you paid four times the price of the Askar 185 you want perfection. 

 

I didn't expect perfection at one quarter the price.  I hoped the scope would provide "refractor" like views of stuff.  Meaning good on Jupiter, Saturn and the Moon.  And because of the larger aperture, nice refractor-like views of open star clusters, globs, etc.

 

My first report last night was with "good" but not "great" seeing.  Likely it was "great" by most of the country on average nights.  The views were nice.  I was more concerned that this scope, with the lesser non-disclosed glass, would have a lot of purple fringing.  But it seemed closer to an APO than a achromat.

 

It did seem to offer a deeper view of nebulosity and faint stars.  Aperture does matter.

 

Time will tell.  I need to see how it performs in great seeing.  My guess is it will be fine.

 

But it is a large refractor.  These are very expensive.  This breaks a lot of price points in a part of the telescope market that does not have a lot of competition.

 

For sure, I have owned 8" reflectors.  They give a good bang for the buck.  I have a couple of really nice Dobs with premium mirrors.  Under good conditions they perform wonderfully.

 

But this is about large refractors at a reasonable price.

 

Hope to get a night on the moon with stable seeing.

 

This scope isn't cheap.  It is made well.  But it isn't $18,000 plus dollars. 

 

Here is Rick's description of his SV 180.  Siouxsie loves her Stellar Vue 180.  Not sure how the 180s, post the "thread," would DPAC.  Likely we will never know.

 

https://www.cloudyni...0#entry12564112

 

All things considered, I think you got a very nice scope for the money.  The build quality looks and sounds first-rate and the DPAC results indicate that the Askar can hang with scopes costing over three times as much. 

 

I'd be curious to see how much mag it can take for Jupiter on a night with good seeing and if you can spot Sirius B.

 

In any case, I think you're going to have a lot of fun with this scope, and at the end of the day, that's what it's all about.

[bobhen]

As an "imaging" refractor, with today's post processing, digital capabilities, it will do okay. If Paul's estimates are close (and they will be), as a visual refractor with 1/4 wave in the green, I do not think this scope is worth the money, even at $4,799.

 

The reason to buy a "visual" refractor is for a refractor's high contrast, low scatter, high definition, suburb sharpness and the ability to take high magnification and retain those qualities. For visual, one would be better off with an 8" Newtonian of mid FL with a high-quality mirror. The Newtonian would also cost a few thousand dollars less than even this low cost refractor. 

 

Bob

[peleuba]

Paul,

 

The first thing I thought about after running these tests was your test of the infamous SVX 180.  Amazing how close they look with the Askar looking worse.

 

But I remember Rick's description in that thread that I linked to above.  Fortunately the Askar is far less expensive because I doubt Askar is going to change how they test scopes as apparently Stellarvue did after your test results.

 

Given the Askar is not made with FPL53/55 or equivalent glass my expectations were such that it should not be as good as the much more expensive premium scopes.

 

Joe, your tests are excellent.   The veil continues to be lifted.  The Askar is a remarkable bargain when it tests nearly identical to a scope that sells for 4x its retail cost.

  

I don't think Askar has to change anything.   This is exactly the type of scope I would expect at this price point.  My sense is they'll sell a gazillion of them - especially to all of the YouTube "influencers"      I think, when you mate it with the sub aperture compressor/reducer, it will be a very nice imaging telescope with a more neutral (green) null.

 

There are not any FPL53 blanks available at this large of an aperture.  And, FPL55 is not melted with any great regularity.  CFF is the only company I know of that is using FPL55. The Chinese glass company - CDGM - is to filling the gap with their FPL53/FCD100 equivalent:  H-FK95N.  Manufacturers/rebranders will advertise it as FPL53 but its likely H-FK95N.  At the top of the food chain, Roland tells me that for large apertures, Hoya FCD100 is the ED glass of choice.  Moreover, the user would never be able to tell the difference (between FPL53 and H-FK95N) unless the scope was disassembled and the ED glass was sent out for analysis.  And, who would do that? 

 

Nice job.

Edited by peleuba, 31 January 2024 - 11:44 AM.

[SandyHouTex]

Paul,

 

The first thing I thought about after running these tests was your test of the infamous SVX 180.  Amazing how close they look with the Askar looking worse.

 

But I remember Rick's description in that thread that I linked to above.  Fortunately the Askar is far less expensive because I doubt Askar is going to change how they test scopes as apparently Stellarvue did after your test results.

 

Given the Askar is not made with FPL53/55 or equivalent glass my expectations were such that it should not be as good as the much more expensive premium scopes.

Considering the SVX 180 will set you back $18,395, I'll take the Askar.

[drprovi57]

With all of the above testing, visual observations, and imaging information, I am exciting to receive my Askar 185 in February to compare with my SW Esprit 150 F/7 and CFF 200 F6.5 (FPL55 glass tuned for imaging) visually and for imaging.  Unfortunately my skies are on average 2" seeing with rare occasions less than 2" seeing.  I do live in a fairly dark location so should be nice to see the contrast differences visually.  I also have an excellent C9.25 scope that i can compare with as well.  For those with DPAC capabilities, I can offer up my Askar for testing to see how quality control is with Askar - 2024 is shaping up to be fun year for large refractors 

 

 

update:  I also hace a Questar 7 Astro to compare with as well with planets and lunar observing... 

Edited by drprovi57, 31 January 2024 - 11:57 AM.

[SandyHouTex]

<<<perk>>>   Did someone ring?  Infamous?  I am flattered!

 

I will respectfully disagree - there is no aspherization of this lens.  There is absolutely no money in this build for an optician to aspherize this lens.   Hard to say exactly what happened with the hill in the center.   It looks like a valley in DPAC, but in real life its a hill.      

 

This lens looks very much like the SVX180 I tested 10 months ago and published here on CN.  If you recall, during in my comments on the SVX180, I mentioned that these ~180mm class lenses, all of similar design, were the rage in imaging circles in Asia in early 2023.  This scope has same type of correction, but when paired with the sub aperture corrector/reducer, the null is shifted to a more neutral position and becomes a nice imaging scope.   This is just a variation on that VERY same theme.

 

Anyway, this lens is nulled in red.   As with the SVX180, when you null a ~180mm air spaced ED triplet in red, green will be ~¼ wave out and blue about a ~½ wave out.  And, it becomes impossible for blue and red (spherochromatism) to balance one another.  To my eye, this lens looks to be worse then ¼ wave in green and worse then ½ wave in blue.   In red its about ⅙ wave.  An interferometer might give slightly better or slightly worse results.  But my estimates are in the ball park.

So are these wavelength errors after dividing by 2 from the DPAC results, or before.

[Erik Bakker]

Wonderful to see Joe’s reports about his new 185mm Askar. Honesty and realistic expectations shine through them all. That makes for a happy owner of this particular scope I would think 

 

All who have seen and used 7” and bigger APO’s in the flesh now these are not your uncle’s upscaled 4” doublet f/8’s. APO triplets get big fast when aperture increases above 4”. And the difficulties of manufacturing them to high and consistent standards increase dramatically. So do temperature and seeing sensitivity, along with demands on the mechanical tolerances of things like the lens cell.

 

At the price asked, this Askar 185 looks like a great value to me. Hopefully it will keep doing well over time. Mounting it (semi) permanently will help with that.

[peleuba]

Hi Paul, were your tests on the SVX also done with double pass like the one of Joe G ?

 

 

Hi - Yes - double pass.  You can read all about here

 

A couple of general points that always seem to arise as folks question the results...

• Even though the errors are, indeed, doubled, the photos in Joe's example of the Askar185 and my example of the SVX180 are true depictions of what an optic with ~¼ wave of overcorrection in Green; ½ wave of overcorrection in blue actually looks like in a double pass test.  This is why the test is so powerful to the amateur.  In single pass, the bands would have much less curve and would give the the impression that the lens is better then it is.  My Ronchi screen density was 133LPI.   

• The optical flat used in DPAC does not have to be very "flat".  It can be many waves out and not have an appreciable affect on the test results.   The error in the flat is only fractionally additive to the overall wavefront error.  You can read here for the explanation.   If using a flat in interferometry it needs to be REALLY flat.  Mine is 1/20 wave and made from Zerodur.  I use the same one for DPAC.

[saemark30]

So Paul did you also using interferometry on the SVX180?

If so, have you described the process?

[peleuba]

So are these wavelength errors after dividing by 2 from the DPAC results, or before.

 

Yes, good question as it can get confusing.

 

The wavefront error is what it is - in other words its a constant.  For example, the green photo shows what ¼ wave overcorrection looks like in double pass using 133LPI Ronchi screen.   If someone undertakes a single pass Ronchi test of the same lens, the bands would be bowed half as much.  But the amount of bow (seen in single pass vs double pass) does not change the metric - its still a ¼ wave error and remains constant.  The error just looks 2X worse because its traversed the aperture twice.   In other words, its only how the error manifests itself in photos that changes - the error is a constant.  And the driver of the change is the number of times the light passes through the aperture.  

 

Your question highlights one of the strong points of the DPAC test.  It can show sub ¼ wave errors in fast lenses which is something the single pass test could never do.  

Edited by peleuba, 31 January 2024 - 01:24 PM.

[peleuba]

So Paul did you also using interferometry on the SVX180?

If so, have you described the process?

 

Yes, I did.  I am not going to get into that, here.  I shared my IF results verbally with Vic at StellarVue and that they were very close to what his Zygo achieved in red.  When testing on an interferometer, you almost never get precisely the same result as someone else.  You look for agreement + or - some fractional value. 

 

I don't have the time nor the energy to take any more beatings on the SVX180 - hope you understand.  Please know that no one is claiming the the SV interferogram is fake - I proved that it was not.  The argument I pushed forward is this:  when nulling in red, their are negative consequences for visual observers.   And, secondly, when designing a scope to do well at the same wavelength the IF tests in is misleading - especially if its red - as that does not bode well for the performance in the green (visual peak) and the blue in a 180mm F7 ED lens. 

 

Finally, the beauty of DPAC is that no one needs an IF cert to know if the scope is good/mediocre/bad.

 

Best Regards.

[peleuba]

I also have an excellent C9.25 scope that i can compare with as well.  For those with DPAC capabilities, I can offer up my Askar for testing to see how quality control is with Askar - 2024 is shaping up to be fun year for large refractors 

 

 

I am in the Baltimore area.  I can test if our schedules mesh this Spring.  

[peleuba]

If it's undercorrected, a prism can help. 

 

This lens in Joe's Askar185 is overcorrected.

[betacygni]

<<<perk>>> Did someone ring? Infamous? I am flattered!

“Infamous is when you're more than famous! This guy El Paul is not just famous, he's IN-famous!”

Sorry couldn’t resist the Three Amigos movie reference.

On a more serious note thank you to all the DPAC testers out there, our hobby is rising to better levels because of you all.

Edited by betacygni, 31 January 2024 - 01:43 PM.

[Jeff B]

Joe, thanks for publishing this thread.  You are helping to advance our community knowledge and documenting an important test.

 

IMO, yup, like Paul said, I believe this is basically the same objective as that of the SV180.  It sure measures strikingly similar.  

 

Did you see any indication of coma and or astigmatism at high power as 150x may have been enough to start to show it?

 

IME, regarding the visual impact of the polychromatic overcorrection, removing seeing (almost never possible to do in reality), the scope will be identical to a "perfect" one up to a certain magnification, beyond which, the perfect scope will "pull ahead".  Jupiter would be the tough target.  Beyond say, maybe 200x, the perfect scope will still stay sharp, revealing more low contrast structure on the planet's surface that just fades and bleeds out in the "lesser" scope.  Even on the moon, you will see the perfect scope pulling ahead in picking out fine structure, especially in the "rubble" and subtle color shading transitions.  

 

During dropping ambient temperatures, undercorrection from cooling will tend to compensate for the overcorrection.....for a while....giving very nice views.....for a while.....but becoming progressively "softer" as the night progresses.

 

I bet this scope would be complete dynamite in an OTA for deep sky viewing.

 

I suspect Paul is also dead on target in that there was not, and will be, any attempt to apply an aspheric to balance out the lens in blue and red.  It's just too expensive to do that.  Just look at the cost of the CFF 185 F6.5 where Pal does that.  To me, I suspect making a separate, sub-aperture, corrector is a much more pragmatic approach, at least when considering the scope is aimed right at the imaging crowd.  

 

Did you get the corrector?  I'd love to see you retest with that in the food chain.

 

Well done Joe.

 

Jeff

Edited by Jeff B, 31 January 2024 - 02:14 PM.

[stlsoccerfan]

Just curious and confused about glass types and DPAC- would an objective made from FPL-55 show a difference to one with FPL-51 assuming the figuring is identical, and if so, how would that show up?  Or is DPAC all about the the quality of the surfaces and configuration of the individual pieces of glass that make up the objective ? There has been so much discussion of glass types in some threads and DPAC results in others- I'm trying to understand if they meet somewhere. Thanks !

Edited by stlsoccerfan, 31 January 2024 - 02:35 PM.

[peleuba]

Just curious and confused about glass types and DPAC- would an objective made from FPL-55 show a difference to one with FPL-51 assuming the figuring is identical, and if so, how would that show up?  Or is DPAC all about the the quality of the surfaces and configuration of the individual pieces of glass that make up the objective ? There has been so much discussion of glass types in some posts and DPAC results in others- I'm trying to understand if they meet somewhere. Thanks !

 

No.  DPAC is generally agnostic when it comes to glass types.  However, you can tell when its an achromat as the white light bands will have some color fringing.

 

DPAC (Ronchi Autocollimation) is a qualitative test methodology.  Specifically, Ronchi is the test and Autocollimation is the how its executed.  "Autocollimation" is another name for "double pass".     DPAC is used to evaluate the lens or mirror system from an optical correction standpoint.  

 

But you'll not be able to tell Fluorite, from ED nor doublet from a triplet using DPAC.  

Edited by peleuba, 31 January 2024 - 02:45 PM.

[Joe G]

It was a whirlwind of a night as I was rushing around to get two mounts polar aligned with trees in the way before Jupiter moved behind my house.

 

I didn't really notice any significant coma or astigmatism, but I wasn't really looking for it.  My first thought was to look at Jupiter in both scopes.  In the average seeing the Esprit and the Askar looked very similar  I could see details in the bands, but I have seen Jupiter much better in the Esprit with really good seeing.

 

I really wanted to see the double cluster in both scopes but my house got in the way.  Ended up looking at M35, M36, M37, M38, M45, M46, M47, M48 as far as star clusters.  They looked really nice but the power was lower.  I didn't take the time to really concentrate and I kept on switching out different eyepieces.  I used a bunch of different eyepieces including Nagler 31, ES 30, AT 28 ES 25, ES 20, ES 17, ES 14, ES 12, ES 9, AT 7, Nagler 7, ES 5.5, and Nagler 5.  Oh and also The Pentax XW 40, 10 and 7.  Chaos.  Next time out I need to focus a bit more.

 

My general impression is that both scopes were equally sharp but the Askar showed more, as it should.

 

That was very obvious when looking at the nebulosity in M42.  The Askar was much nicer in seeing the gas structures.  The trapezium looked similar.  The E star twinkled in and out a bit.  Certainly was not rock steady due to the seeing.

 

Castor was very nice in both scopes.  Again when I was getting closer to 200x plus the seeing wouldn't support that magnification.  I could split Rigel but not Sirius. 

 

I agree with everything you and Paul have said.  My expectations were for the scope to show a nicer view on globs, star clusters, etc while being decent on the planets and the moon.  Hopefully without too much color.  So far that seems to be the case but I certainly need more time at the eyepiece.

 

I do plan on buying the 1x field flattener.  Agena didn't have it yet.  I am a little burnt out on imaging, but will try it at some point.  I really bought the scope for visual and the "refractor" like view.

 

Of course there are many reflector scopes that offer great views at a much lower price point.  That is a given.  My two dobs when properly cooled put up great pin point stars and are certainly better at deep sky stuff.

 

But the lust over large aperture refractors is a different story.

 

We have rain headed our way for the next couple of weeks.  I can't wait for clear skies and to test the scope out in a more controlled manner.

 

The one thing I need to do is to figure out a better tripod/pier situation to get my rear off the ground.

[stlsoccerfan]

Paul- thanks for the quick and clear response. All you folks that do DPAC are providing great information to us luddites- I for one am really appreciative. Thanks !

Edited by stlsoccerfan, 31 January 2024 - 02:48 PM.

[Astrojensen]

This lens in Joe's Askar185 is overcorrected.

That is unfortunate, as a prism (or a binoviewer) will then only make the situation worse. 

 

On the other hand, you could most likely improve the situation by respacing the objective, and respacing for overcorrection is easier than for undercorrection, because you need to increase the distance between the lenses, and there's no risk of running out of space between the lenses. 

 

 

Clear skies!

Thomas, Denmark

[Jeff Morgan]

I don't think Askar has to change anything.   This is exactly the type of scope I would expect at this price point.  My sense is they'll sell a gazillion of them - especially to all of the YouTube "influencers"      I think, when you mate it with the sub aperture compressor/reducer, it will be a very nice imaging telescope with a more neutral (green) null.

 

Agree. I seriously doubt there is any overlap in TMB/TEC/CFF/AP customer group and the Askar customer group. I can certainly identify with the desire to have a giant refractor, I once owned a 8" f/12 D&G. They will sell many at this price point. And the used prices will be even lower, so they will be around.

 

WRT imaging, I put my Agema SD-150 (1200mm) on the mount last night. With imaging train and dew shield fully extended it fairly well consumes the interior of a SkyShed Pod. I have never tried to fit that scope in there before, the plan is to image remotely with the Agema. I just wanted to leave it set up for a few days while I fine tune a few things.

 

And of course, the Askar is larger yet.

 

For imaging 1200mm (or 1300mm) is good but not a lot. Seeing it "in the flesh" drives home the point that for imaging purposes, this is the top-end for refractors and going larger really requires some form of reflector.

[peleuba]

Here is a similar design to the Askar185 that I mocked up in ATMOS optical design software...  On the right it shows how a high Strehl in red affects blue and green when using common ED glasses.  SD glasses like FPL53/55 and FCD100 would likely offer better performance but at a higher cost per inch of aperture.

 

The amount of bending in the blue DPAC image leads me to believe that the overcorrection is somewhere between ½ to a full wave, with it likely being closer to 1 wave.  As was pointed out to me in a private email from a friend who is far more skilled then I that we don't really know the exact wavelength of the LED's.  That's true and its all a best guess.   But, my sense is that its close enough and probably does not matter much.

 

I don't think this overcorrection really affects color error as noted in the white Ronchigram...  So, not a lot of false color which will make folks happy.   But, it won't be tack sharp in green and blue.  

 

This would be a terrific scope for HA solar and Mars.    

Attached Thumbnails

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Edited by peleuba, 31 January 2024 - 04:57 PM.

[vahe]

How much improvement can be expected in the overall performance of this lens, looking at Jupiter, if its aperture is reduced by about 20mm (Masked off) and if its aperture is reduced what would be the effect of that hill/valley right at the center?

.

Vahe

[drprovi57]

I am in the Baltimore area.  I can test if our schedules mesh this Spring.  

Paul - that will be great.. I will PM you as soon as I receive the Askar 185.. I am about and hour drive time from Baltimore.. DPAC test on another Askar 185 should provide another data point regarding quality control..