DPAC venture

[nicoscy]

40 minutes ago, Deadlake said:

The point is SV are the only manufacturer I can think of that tests in red, green is as far as I can see an industry standard. No hindsight then, a red flag. Ho ho ho.

I may be wrong, but I believe that most of the scopes also coming out of China and sold by various rebranders, are also tested in red. 

[skybadger]

Has anyone made the point yet that you can test DPAC using a water bath so everyone everywhere can have a flat of the same accuracy ? 

I use it to test and setup my cassegrain scopes but use a white light led. 

 

[nicoscy]

Someone did over at CN, but now that you are here, and mentioned it, can you please share your methodology with full details? 

I'd certainly like to learn more!

[Deadlake]

54 minutes ago, nicoscy said:

I may be wrong, but I believe that most of the scopes also coming out of China and sold by various rebranders, are also tested in red. 

I think that’s another thread…

[CraigT82]

50 minutes ago, skybadger said:

Has anyone made the point yet that you can test DPAC using a water bath so everyone everywhere can have a flat of the same accuracy ? 

I use it to test and setup my cassegrain scopes but use a white light led. 

 

That’s good, because my next thought about DPAC testing was that it won’t be long before we start seeing threads with people complaining that the flat they’ve bought is not of the advertised quality! 

[John]

1 hour ago, Deadlake said:

I think that’s another thread…

I'd like to see more independent tests of Orion Optics UK optics, but that really is another thread (or two !).

 

[jetstream]

40 minutes ago, John said:

I'd like to see more independent tests of Orion Optics UK optics, but that really is another thread (or two !).

 

Having read a pile of test reports over the years I'm a bit skeptical when every single optic-from whoever- tests with an almost perfect Strehl ratio. It would be nice to see their Focault tests and their shadowgrams -which DPAC can also show I believe. Also an actual IF fringe picture is vg- not just the synthetic image produced from the fringe pictures through a program. I do realize one actual fringe picture does not tell the whole tale, but it goes a long way IMHO....

I really and truly dislike test reports that could potentially be misleading.

There is nothing but good to come out of our enlightenment with respect to DPAC testing I believe.Many thanks to those who brought the test to the forefront through the forums.

[jetstream]

2 hours ago, CraigT82 said:

That’s good, because my next thought about DPAC testing was that it won’t be long before we start seeing threads with people complaining that the flat they’ve bought is not of the advertised quality! 

Brand new, high spec flats are not cheap. Thankfully there appears to be some latitude in the actual specs needed.

[900SL]

8 minutes ago, jetstream said:

Having read a pile of test reports over the years I'm a bit skeptical when every single optic-from whoever- tests with an almost perfect Strehl ratio. It would be nice to see their Focault tests and their shadowgrams -which DPAC can also show I believe. Also an actual IF fringe picture is vg- not just the synthetic image produced from the fringe pictures through a program. I do realize one actual fringe picture does not tell the whole tale, but it goes a long way IMHO....

I really and truly dislike test reports that could potentially be misleading.

There is nothing but good to come out of our enlightenment with respect to DPAC testing I believe.Many thanks to those who brought the test to the forefront through the forums.

It will certainly lead to higher prices, and people returning perfectly useable scopes because their test results were less than ideal.

 

[jetstream]

9 minutes ago, 900SL said:

It will certainly lead to higher prices, and people returning perfectly useable scopes because their test results were less than ideal.

 

I think realistic expectations are in order and what is really needed for good views. Knowing what optical qualities and parameters really degrade these views are a great tid bit of information to know. As an example my 15" tests vg but know where near some of these sky high test reports show. It has an accurately tested .92 Strehl or so IIRC 1/6 PV and very smooth. The IF picture shows all the lines nice and sharp and straight.

It might be good for us to read Peachs vg example of what different levels of optics can really show... perfection is not needed but good average numbers are across the spectrum IMHO

https://www.damianpeach.com/simulation.htm

Edited March 17, 2023 by jetstream

[Astrobits]

3 hours ago, skybadger said:

Has anyone made the point yet that you can test DPAC using a water bath so everyone everywhere can have a flat of the same accuracy ? 

I use it to test and setup my cassegrain scopes but use a white light led. 

 

I was also going to suggest a liquid ( water, oil ) bath as the reference flat. I believe that the National Physical Laboratory in Teddington uses an oil bath as reference. I used the water method when making my reference flats ( 8" dia ) many years ago. I made the flats using the three plate method but just for the hell of it decided to include both sides of the 2nd and 3rd plates making it a 5 surface method😳. The double sided flats were polished to better than 1/4 wave yellow light ( main deviation near the edge ), put into a box and there they have remained untouched for the last 25years or so. The 5th surface was used as my main reference surface when making flats for my telescopes.

Nigel

[jetstream]

2 hours ago, Astrobits said:

put into a box and there they have remained untouched for the last 25years or so.

Maybe you could take that box and ship me one

Edited March 17, 2023 by jetstream

[Captain Scarlet]

22 minutes ago, Astrobits said:

I was also going to suggest a liquid ( water, oil ) bath as the reference flat. I believe that the National Physical Laboratory in Teddington uses an oil bath as reference. …

@PeterW?

[Deadlake]

1 hour ago, 900SL said:

It will certainly lead to higher prices, and people returning perfectly useable scopes because their test results were less than ideal.

 

Most of the premium market do not report Strehl due to cherry picking. 

A scope is rejected as Strehl is 0.96 and not 0.97 for instance.

You could have a lens with a high Strehl however the lens is not smooth, all ripe for some marketing....

[Peter Drew]

I spent much of yesterday reading the CN thread from the start, I could hardly keep up with the expansion!  CN threads of this nature are always entertaining as they expose a lot of "thin skin" but the focus of this thread has important implications for the suppliers of optics to the amateur astronomer.  The telescope at the heart of the controversy is not a lemon, it tested near perfect in red which would be beneficial in some applications.  Where things went wrong was the implication that the telescope was of exceptional quality suitable for the expectation of the purchaser to suit his visual interest which would have been better achieved if tested in green light.

I think that the inevitable rise in the interest in DPAC testing will tend to keep manufacturers on their toes, but like all methods of optical testing it will take experience before stringent results can be obtained.  I suspect that DPAC will be a great tool to differentiate between good and poor for most interested amateurs, finer nuances are likely to be outweighed by real life observing conditions.  My interest has certainly been piqued and I have an 8" master flat.    🙂

Quote

 

  

[JeremyS]

Has anyone on SGL built a DPAC system yet?

[skybadger]

5 hours ago, CraigT82 said:

That’s good, because my next thought about DPAC testing was that it won’t be long before we start seeing threads with people complaining that the flat they’ve bought is not of the advertised quality! 

But surely that's the first thing you do when you get your flat.? I.e. put it in a bath of water under a monochromatic light and look for the fringes ? Not that it's a dpac test but youi mght be interested

[nicoscy]

Peter,

Just to add, there were zones on the lens (see test in white) and some surface roughness. It tested great in red, but poorly in green and extremely poorly in blue. It would be great for anything in red and longer spectrum, but not so great in green and shorter spectrum.

For $18,000, a limited run of 50 and if I am not mistaken, a nearly 2 year waiting time for a flagship product heavily advertised as being top of the tops, the owner was entitled to something much better.

CN can be very judgmental though. Well, people everywhere are judgmental (guilty as charged myself). Vic Maris posted yesterday that he will test the scope and revert. However, unless you are in the market for a Ha optimized scope, I fail to see the reason why one would be happy to invest $18,000 for this scope and why one would be happy with testing in red. 
 

My take is that Stellarvue should drop the marketing lingo, take the punches and reinvent itself, move to green light interferometry (costs about $100k, the run of only these 50 180SXVX scopes retailed $900,000 in the basic configuration and SV says they test all, so would amortize cost rather quickly) as TEC, AP and LZOS do, ensure the polish is on par with the rest and easily become one of the great optical houses of our era.
 

Let the product speak for itself - and not just the “nice” reviews of “tack sharp views” / “sharp as a tack” / “performs great” / “snaps to focus” and all that. The product as tested should consistently be of a high standard. However, it is up to Stellarvue if it will take the free feedback provided and reinvent themselves. All we can do is discuss to no end in various topics

[skybadger]

1 hour ago, JeremyS said:

Has anyone on SGL built a DPAC system yet?

I have used an oil bath and a frame that allowed me to invert the scope over the bath with adjustor feet to get it properly vertical. 

I used an oag to inject the light source through the prism before I made a ronchi eyepiece with the led built in. 

I'll look for some pics.

https://www.skybadger.net/equipment/12inch Cass.shtml contains a dpac arrangement at the  end which was used to acquire a set of ronchi images. It stands on the levelling frame and the box contains the oil pan which is a 20" cake holder pan with veg oil in. 

The image below is a simple oag used to create ronchigrams 

 

 

 

 

Edited March 17, 2023 by skybadger
Update with pic and URLs.

[Peter Drew]

25 minutes ago, nicoscy said:

Peter,

Just to add, there were zones on the lens (see test in white) and some surface roughness. It tested great in red, but poorly in green and extremely poorly in blue. It would be great for anything in red and longer spectrum, but not so great in green and shorter spectrum.

For $18,000, a limited run of 50 and if I am not mistaken, a nearly 2 year waiting time for a flagship product heavily advertised as being top of the tops, the owner was entitled to something much better.

CN can be very judgmental though. Well, people everywhere are judgmental (guilty as charged myself). Vic Maris posted yesterday that he will test the scope and revert. However, unless you are in the market for a Ha optimized scope, I fail to see the reason why one would be happy to invest $18,000 for this scope and why one would be happy with testing in red. 
 

My take is that Stellarvue should drop the marketing lingo, take the punches and reinvent itself, move to green light interferometry (costs about $100k, the run of only these 50 180SXVX scopes retailed $900,000 in the basic configuration and SV says they test all, so would amortize cost rather quickly) as TEC, AP and LZOS do, ensure the polish is on par with the rest and easily become one of the great optical houses of our era.
 

Let the product speak for itself - and not just the “nice” reviews of “tack sharp views” / “sharp as a tack” / “performs great” / “snaps to focus” and all that. The product as tested should consistently be of a high standard. However, it is up to Stellarvue if it will take the free feedback provided and reinvent themselves. All we can do is discuss to no end in various topics

Yes Nicos, I was aware of the sub divisions of disappointment of the objective in question.  I suppose it boils down to what constitutes a "lemon".  In the CN thread, the purchaser reported exceptional planetary performance, besting his Zambuto 10" Newtonian, his concern was fringing at the limb of the planet, that's the sort of lemon I could cope with although like the purchaser I would be miffed by the results provided by the DPAC having paid a high premium for better.  I would love a 180mm triplet with a good figure in red as my main interest these days is Ha solar for which I use a 150mm Istar achromat.

I hope the CN thread can continue to the point of resolution of the issue as this will be of importance to amateur astronomy.    🙂

Edited March 17, 2023 by Peter Drew

[nicoscy]

Agreed Re Istar - they care to a segment of the market with Ha optimized objectives!

Yes, the scope is not a lemon per se. It’s the value proposition and reasonable expectations issue I suppose. Plus the marketing approach which does not match the product.

I do believe people will move on and come back to this later on. This is just one data point, however, as far as I know, there are several SV owners now making private arrangements to have their SVX scopes (of all apertures) DPAC’d. It can get better, if it appears that this scope was a statistical blip, a lot worse if this is the standard to be expected of the SVX range. Time will tell.

[jetstream]

I wouldnt pay $18000 to get fringing on Jupiter - I paid a lot less to get it with my SW120ED.

[davidc135]

7 hours ago, nicoscy said:

Someone did over at CN, but now that you are here, and mentioned it, can you please share your methodology with full details? 

I'd certainly like to learn more!

Re water test, look up Ed Jones's 'testing flats with water' or words to that effect on Youtube. Very good

[davidc135]

As perfect as the liquid DPAC test is, it's hard to beat the convenience of having a decent glass flat. After a couple of Newtonian mirrors, I made a 6'' one back in '78 which has been indispensable in making a number of refractors. I think it causes a slightly (1/8 wave) under-corrected wave front. More recently I bought a professionally made 12'' but the 6'' is handier.

In practice, especially for refractors, the test is simple to set up.

David

Edited March 17, 2023 by davidc135

[John]

2 hours ago, jetstream said:

I wouldnt pay $18000 to get fringing on Jupiter - I paid a lot less to get it with my SW120ED.

You ED120 is clearly at one end of the performance scale for the model and mine must be closer to the other end !