DPAC venture

[jetstream]

I'm slowly assembling some things to start along the path of DPAC testing. It should prove interesting, and from what I understand it is very accurate. Just ordered some transparency film to try printing off the Ronchi screen on my laser printer and hopefully this goes well. If it does I'll order an optical flat, 1/4PV 40-60 D/S or better yet find a cheaper flatter laser flat off E Bay or somewhere.

This testing seems popular on another site- anyone here try DPAC testing refractors?

I'm almost scared to see what this shows up...

Gerry

[jetstream]

Years ago I purchased the TSA120 because of its reputation for well figured optics that tests well across the spectrum and because of how highly regarded they are among very experienced refractor owners.

I plan to test the highly vaunted, and recommended here SW120ED in double pass autocollimation -DPAC- to see where it stands .

I dont believe it necessary to nit pick the fringes seen and one look should tell the tale. Hopefully the TSA120 tests well and can be used to correlate visual observations to the test results seen in the other refractors.

This image is from another site

 

[Captain Scarlet]

I must admit I hadn’t heard of DPAC until you mentioned it, Gerry. Interesting. Is the test something you can perform yourself or will you send it away?

Edited March 10, 2023 by Captain Scarlet

[jetstream]

12 minutes ago, Captain Scarlet said:

I must admit I hadn’t heard of DPAC until you mentioned it, Gerry. Interesting. Is the test something you can perform yourself or will you send it away?

I'll be testing here once I get the needed equipment. A big thank you to member @Sunshine who has provided me with the DPAC EP's where the LED and Ronchi screen will fit. The files were from Moshen on CN who put the 3D printer files available to anyone.

Maybe members here would be interested in printing the EP's to try.

[jetstream]

I'll also test the scopes through different diagonals to see the effects, if any. Ideally I'd like testing through the eyepiece/diagonal/scope as well, but I'm not sure if this is possible.

[davidc135]

Looking forward to results. I'd be surprised if the TSA wasn't excellent and the SW very good.

DPAC or more simply AC, autocollimation, is simple and rewarding to do. Its accuracy does depend on the quality of the flat however. Whilst some slight curvature or departure from a true plane is irrelevant, departure from sphericity eg zones should be less than 1/20 wave on the surface or 1/10 wave front.

Setting up the test is easiest with refractors where there is no vignetting and hardest with small cats. where sometimes only a part of the aperture can be seen. A simpler version of the test is single pass collimation where another scope of known quality is used to provide a beam of collimated with which the optics of the scope under test is tested. But double pass has twice the sensitivity.

I used a separate knife edge to produce this Foucaultgram before my grating arrived. It's of a Tak FC100 after regrinding and polishing but before figuring and shows an outer raised zone that needs flattening.  A Ronchi would show overall correction more simply whereas the Foucault is more sensitive. There are some artefacts from the back of the flat.

Note that with tde the Ronchi lines hook in the opposite direction in lenses to that in mirrors i.e inside focus tde would hook away from the centre. Often diffraction causes the same effect inside focus. It's when you see the edge hooking in the opposite direction outside focus that you know if it's really t.d.e

David

[jetstream]

@davidc135 thank you very much for this needed information.

38 minutes ago, davidc135 said:

eg zones should be less than 1/20 wave on the surface or 1/10 wave front.

Ok, this is where I get different opinions. Some say that because the area used is small in the scheme of things that the surface of the flat is not critical. Others say no it needs to be very flat and actually very smooth.

For what I'm doing a 1/4 pv 40-60 scratch dig was suggested as "OK"- thoughts?

My natural first thought was 1/10 PV and smoother...

What effect will a flat less than 1/10 PV 20-40 have on the fringe results?

Thanks for the help David

[davidc135]

I think scratch/dig surface quality isn't that relevant for auto-collimating flats. The 6'' i used in the photo is covered with decades' worth of scratches.

A slight curvature of the flat will only have the effect of marginally changing the apparent focal length of an objective but the deviation from a plane ,assuming the figure is smooth, needs to be massive before any SA is contributed.

The trouble with quoted specs is that they may not specify overall curvature or smoothness, or whether its the flat surface that is being described or resulting reflected wavefront. It should be a flat's surface whereas for any objective it's the wavefront.

Some flats are descibed by their maximum overall error, ie curvature but also max local error i.e smoothness. For DPAC flats only the latter is important whereas if interference testing say large diagonals both count.

I bet it will be OK but if the flat is so poor as to contribute 1/4 wave SA then plainly it's no good.

I have a professionally made ex British Aerospace 12'' x 2'' flat and would be happy to test your flat assuming that its back is clear. 

David

[davidc135]

If there is any problem printing one's own AWR in the U.K sell mounted Ronchi gratings,120 and 80 lines, each at 9.50 inc p&p which is pretty cheap.

David

[jetstream]

55 minutes ago, davidc135 said:

I have a professionally made ex British Aerospace 12'' x 2'' flat and would be happy to test your flat assuming that its back is clear. 

I would love for you to test it! However we are 6000km apart as Im in the middle of Canada.

Great info again, I'll attach the specs in a bit on the flat I'm thinking from Edmund in the US.

[jetstream]

@davidc135 I'm thinking of the Edmund Optics #11-858 optical window.

https://www.edmundoptics.ca/p/150mm-dia-10mm-thick-uncoated-lambda4-n-bk7-window/41182/

[davidc135]

8 hours ago, jetstream said:

@davidc135 I'm thinking of the Edmund Optics #11-858 optical window.

https://www.edmundoptics.ca/p/150mm-dia-10mm-thick-uncoated-lambda4-n-bk7-window/41182/

You'd be taking too much of a chance. The max 1/4 wave error in transmitted light corresponds to a possible 1/2 wave deviation on the surface, due to refraction. If used as a flat the 1/2wave doubles to a possible, disastrous 1 wave.

David

[davidc135]

I'd look on ebay. There's a 5'' ex soviet flat good to 1/20 wave (red light) local error, going for only $69 US. It's a bit small but still very useful if the optics and flat are displaced slightly to show the edge. 

David

[jetstream]

6 hours ago, davidc135 said:

I'd look on ebay. There's a 5'' ex soviet flat good to 1/20 wave (red light) local error, going for only $69 US. It's a bit small but still very useful if the optics and flat are displaced slightly to show the edge. 

David

I'm not on EBay but can you give me a link please? My sis in law can get it for me

[jetstream]

@davidc135I dont see it there but they are selling like hotcakes rumour has it.

[John]

DPAC can deliver some surprises. Here is one that I don't think many saw coming:

Bench Test of a StellarVue SVX180 Apochromat - Refractors - Cloudy Nights

 

[davidc135]

On 11/03/2023 at 14:46, jetstream said:

@davidc135I dont see it there but they are selling like hotcakes rumour has it.

I couldn't see it again so maybe it went.  David

[jetstream]

2 hours ago, John said:

DPAC can deliver some surprises. Here is one that I don't think many saw coming:

Thats really something... this test is very accurate and I hope to explore it myself. I would think Stellarvue might be re thinking things possibly, just a guess.

[900SL]

On 13/03/2023 at 18:16, John said:

DPAC can deliver some surprises. Here is one that I don't think many saw coming:

Bench Test of a StellarVue SVX180 Apochromat - Refractors - Cloudy Nights

 

That went well I thought

Honestly, the moderation over there is unbelievable. That thread should have been locked down much earlier.

I can only assume they have an axe to grind with SV (notwithstanding the test results, which, assuming that they are correct, would be a concern to anybody with a similar scope).

[jetstream]

19 minutes ago, 900SL said:

That went well I thought

Honestly, the moderation over there is unbelievable. That thread should have been locked down much earlier.

I can only assume they have an axe to grind with SV (notwithstanding the test results, which, assuming that they are correct, would be a concern to anybody with a similar scope).

What I have a hard time understanding is how a company, trying to establish itself in the high end refractor business would allow a scope with those (alleged) issues out for sale - makes no sense to me.

I might assume- with no evidence or experience- that the smaller the aperture the less off in the green and blue they will be. I'll tell you this- every refractor I buy from here on in will be run through DPAC... and maybe a knife edge if I can figure it out. First on my list is my SW120ED.

Edited March 15, 2023 by jetstream
delete the E lol

[Space Hopper]

With respect I think anyone who'd spent $18,000 on a scope and it had tested that poorly would be more than upset about it, and have every right to be.

But fair play to them,(Sv) i believe the scope was returned and the cost refunded.

.....although i'm only about a third of the way through the thread, so there may be more twists and turns.

[jetstream]

3 minutes ago, Space Hopper said:

With respect I think anyone who'd spent $18,000 on a scope and it had tested that poorly would be more than upset about it, and have every right to be.

Yeah no kidding- I'm still bummed out about my SW120ED- for a tiny fraction of the price- which was touted as an excellent quality refractor- DPAC will tell me. It was rumoured to have been tested (all the scopes) from a vendor...

[John]

One of the issues with this, it seems to me, is that a single example of a scope gets a less-than-great DPAC test which gets posted online and then is dissected by dozens of posters of varying degrees of experience, leading, possibly, to the impression that the model is generally poor, which might be completely inaccurate - the test sample being just one of possibly 1000's.

If , say, a dozen randomly selected samples of varying ages had been tested, the results might be slightly more interesting ? 

With the SV 180 I understand that the situation is different with just 50 or less having been made. My point above refers to mass-production instruments.

 

Edited March 15, 2023 by John

[jetstream]

I have a funny feeling there are going to be many samples of one tested as DPAC really catches on...

[John]

1 minute ago, jetstream said:

I have a funny feeling there are going to be many samples of one tested as DPAC really catches on...

I can recall Herr Rohr testing quite a few ED120's (or clones) and the results were really rather better than many expected. My own example seems a cracker 👍