160 replies to this topic

[peleuba]

I have been following the chatter on the forums about the 6" class of ED doublets coming from China and offered for sale by SkyWatcher and APM.  A few weeks ago I was able to find an APM 152ED that the seller was willing to ship to me here in Maryland.  I was very curious to get it on my test bench because most of the reviews I've read on the Skywatcher 150 and the APM 152ED having been positive and I wanted to see for myself.

 

When it comes to astronomy equipment reviews - especially telescopes - I trust very few people.  IMO equipment "reviews" invariably show the bias of the reviewer and then often erode into religious and emotional arguments from fanboys on both sides.  Because, after-all, who wants admit (or be told) the expensive APO they just purchased is barely diffraction limited (or worse) I know this, because this happened to me about 7 years ago on a 110mm APO from a boutique maker.  To this end, I try to offer unbiased opinions based on real tests on a bench and under the sky, often backed up with images.

 

In any event, here are the bench test images in double pass autocollimation.  The bench tests dovetail nicely with the supplied interferogram.  In the IF, almost all of the error is concentrated at the very edge and in a few small spots.  This is exactly what the DPAC images show.  So, its nice to know that this Inteferogram is actually worth more then paper its printed on  .

 

I tested in Green, Red and Blue.  The error you see in the tests are doubled so its worse then what actually exists in the glass.  

 

You will not mistake this optic for one by A-P or Tak or even TEC.  But at this price point - especially on the used market - this telescope is a phenomenal bargain.    Its an optically decent telescope with a terrific focuser - this one has the big FT 3" R/P and not the stock APM. 

 

 

 

(Edited to fix spelling errors)

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Edited by peleuba, 26 April 2019 - 11:48 AM.

[peleuba]

 

I tested in Green, Red and Blue.  The error you see in the tests are doubled so its worse then what actually exists in the glass.  

 

 

 

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[Astrojensen]

Looks very, very good. I wonder what the strehl and P/V would be, if it was stopped down to, say, 140mm aperture? 

 

 

Clear skies!
Thomas, Denmark

[junomike]

Looks good to me!

[peleuba]

Looks very, very good. I wonder what the strehl and P/V would be, if it was stopped down to, say, 140mm aperture? 

 

 

 

I plan on doing just that as an experiment.  As you surmise, 10mm will make a big difference.  

[Tnk]

Nice setup.  I am glad you are enjoying the scope.

Tapan

[Jeff B]

Good stuff Paul!     

 

Nothing like unbiased DPAC test results.  I'm glad they match the Wellenform report.

 

Save for regional parts of the edge and the bowl (hill?) across the face, a decent lens.  Figure seems rather smooth.  

 

any star test images?

 

How about a 5.5 or 5" stop?

 

Jeff

[peleuba]

 

How about a 5.5 or 5" stop?

 

 

Exactly.  I think 10mm will make all the diff.

[Jeff B]

Here's a similar situation.

 

A 5" F8 contact triplet.

 

Note the SA in green along with a bit of funniness at the edge.  For this sample the edge issue was very real and present over the whole circumference of the lens.  

 

The outdoor star test was rather horrible, showing the turned edge and the SA on top of it all.  Images at focus were not that sharp, but "color correction" seemed good as well as the smoothness of the figure.

 

Forgive the key-stoning of the lines as this was an early version of my DPAC rig and subject to some alignment issues.

 

Jeff

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[Jeff B]

Here it is after masking to 115-120mm.

 

Much improved edge but the SA in green still there.

 

Visually, the images were much improved though, sharper, but the SA could still be seen in the star test.

 

There is also a small diameter zone right in the middle that you can catch out with the center bar.

 

Jeff

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Edited by Jeff B, 26 April 2019 - 11:58 AM.

[precaud]

Looks good to me!

 

Meh... that's generous, IMO... it looks "ok"

 

Great tests, Paul! 

 

Buying a 150 just to stop it down to 130-140 makes no sense...

Edited by precaud, 26 April 2019 - 12:08 PM.

[peleuba]

Meh... that's generous, IMO... it looks "ok"

 

Great tests, Paul! 

 

Buying a 150 just to stop it down to 130-140 makes no sense...

 

Thanks - Yes - agree not spectacular.  But for the $$$ it represents a phenomenal bargain.  Strehl, according to IF is an even 0.95

 

Also, I purchased a couple of the cameras you recommended in another thread - Canon S30 and A550 - to take better DPAC bench images.  I am using an app on my iPhone 8 that gives me full control over all settings, but want a dedicated camera.  My DSLR is useless for this and my Cannon G15 is not much better.

[precaud]

The S30 is still my fave. Big 1/2" sensor for 3.3MP.gives low-noise images. ISO 50 mode. And RAW for when JPG's eat the detail. If only the CF card door was on the side, it would be perfect.

 

I guess I'm struggling to see the "bargain" aspect of these 6" ED fracs. But that's why I keep reading about 'em 

[jay.i]

Looks very, very good. I wonder what the strehl and P/V would be, if it was stopped down to, say, 140mm aperture? 

 

 

Clear skies!
Thomas, Denmark

Are we really calling sub-quarter-wave "very, very good"? It's decent if you consider the aperture size and ED glass... but nothing more. I don't buy optics to stop them down, but I suppose if you got saddled with less-than-ideal optics, removing the bad edges would bring them up to decent/good territory. 

[G-Tower]

Meh... that's generous, IMO... it looks "ok"

 

Great tests, Paul! 

 

Buying a 150 just to stop it down to 130-140 makes no sense...

Not an expert but you can clearly see a turned edge and a depression or hill toward the central zone and a bit of over/under correction. I agree about stopping down, it kind of defeats the purpose. I would say its what to be expected in this price range and I applaud APM for providing test reports with their scope. Would be nice if the SW150 would come with a test result as well.

[Jeff B]

"My DSLR is useless for this and my Cannon G15 is not much better."

 

Amen Paul.  The camera and settings menu in my LG V30+ Android phone are rather excellent for DPAC work.

 

Jeff

[peleuba]

I guess I'm struggling to see the "bargain" aspect of these 6" ED fracs. But that's why I keep reading about 'em 

 

To get anything in 6" of aperture that's significantly better you'll pay 3 times the price.

[G-Tower]

To get anything in 6" of aperture that's significantly better you'll pay 3 times the price.

Well worth it to me and many others. BTW, I've seen many cheap achromats with better DPACs right here in CN classics section.

[precaud]

No doubt true. I guess I am biased toward the necessity to get the edge right for these incremetally-larger aperture ED's, especially given the mount requirements for a 150 vs 130-ish... To buy real estate that you then need to hide for best results makes no sense.

 

I'd rather have a 5" ED and 7" Mak for about the same $$$.

 

Oh, wait, I DO have that!

[hfjacinto]

No doubt true. I guess I am biased toward the necessity to get the edge right for these incremetally-larger aperture ED's, especially given the mount requirements for a 150 vs 130-ish... To buy real estate that you then need to hide for best results makes no sense.

 

I'd rather have a 5" ED and 7" Mak for about the same $$$.

 

Oh, wait, I DO have that!

You have an ED 127? ES I presume? 

[peleuba]

Not an expert but you can clearly see a turned edge and a depression or hill toward the central zone and a bit of over/under correction. I agree about stopping down, it kind of defeats the purpose. I would say its what to be expected in this price range and I applaud APM for providing test reports with their scope. Would be nice if the SW150 would come with a test result as well.

 

Yes, the central zone is about 1/8 - 1/10 wave high/deep and will not be noticeable in-focus.  Remember - the error is doubled which is what makes this test so powerful.  The edge is really the only issue with some very slight overcorrection in green. 

 

The Interferogram provided is not materially important if one can perform other tests such as DPAC.  In fact, I was a little surprised that it matched the DPAC test.  Having the skillset to perform testing for the purpose of evaluating optical performance makes one less reliant on anecdotal Internet reviews and (sometimes) overly optimistic test reports included as part of the sale.

 

I do have an Interferometer, but the DPAC and indoor star test tells me all I need to know.  I have never once seen an optic perform great in DPAC then be a dog in the field.  But I have seen mirrors with great IF reports have very large turned (~1") edges which swamps high power planetary viewing.

Edited by peleuba, 26 April 2019 - 01:28 PM.

[peleuba]

Well worth it to me and many others. BTW, I've seen many cheap achromats with better DPACs right here in CN classics section.

 

And your point is?   

 

A cheap Acromat at 6" F/8 will only be diffraction limited in a VERY narrow bandwidth...   

 

I own AP's and TAK's as well.

 

Perhaps you should start posting some of your tests reports on your own scopes.  I read in another thread where you have 60 years of experience...  I only have 36 or so...  I am sure we could all learn from you.

Edited by peleuba, 26 April 2019 - 01:18 PM.

[peleuba]

The S30 is still my fave. Big 1/2" sensor for 3.3MP.gives low-noise images. ISO 50 mode. And RAW for when JPG's eat the detail. If only the CF card door was on the side, it would be perfect.

 

 

Good to know.  I have the S30 and an A550 coming from eBay.  Just tired of using the iPhone.  Want something better.

Edited by peleuba, 26 April 2019 - 01:27 PM.

[stevew]

Good for you for getting one of these APM scopes on a test bench.

From your test I'd say that the scope is exactly what is promised by APM.

We live in a great time for refactor fans.

 

Steve

[peleuba]

Are we really calling sub-quarter-wave "very, very good"? It's decent if you consider the aperture size and ED glass... but nothing more. I don't buy optics to stop them down, but I suppose if you got saddled with less-than-ideal optics, removing the bad edges would bring them up to decent/good territory. 

 

Jay - Let's have a teaching moment...   I am not sure of your of level of experience with optical testing, but I have seen a LOT of telescope and the vast majority of optical systems marketed to amateurs regardless of price point is only 1/4 wave.  And this is from some very well known makers.   What this really means is that from the highest peak to the lowest valley on the surface of the glass is 1/4 wavelength of some specific bandwidth - usually green   Take for instance an optic that only has a single high spot a "1/4 wave high" well, by your definition it will only be "a 1/4 wave optic".  When in reality is performance will be much better.  This is where the RMS value and Strehl become more meaningful in describing optical performance.  

 

I don't mean to sound condescending, but your comments really don't tell much of the story.  

 

If you look at the IF I posted, the overwhelming majority of the lens is smooth and regular.  Its the edge, and only in two spots (at the edge) that are the big offenders.

 

I purchased this telescope because I can, was curious after reading tons of reviews and I wanted to test it and post the results.  Not because I thought it would be as nice as my TEC160FL was or a friends AP155.  So, to this end, I stand by my assertion that it represents a remarkable value.

 

And, BTW, this APM 152ED will show brighter/better/more detail on any object that we point it at compared to the A-P Stow that we're both waiting for.  And, the Stow with accessories is nearly double the price I paid.

Edited by peleuba, 26 April 2019 - 02:03 PM.