DPAC venture

[John]

2 hours ago, Rusted said:

Has optics become Hifi? Where only subjective nuances of "golden eared reviewers" are the accepted measuring stick?
 

 

That is just where I think things might be heading 😒

[Deadlake]

3 minutes ago, John said:

That is just where I think things might be heading 😒

I hope not, I'd follow my eyes as I think a Strehl test do not cover full metrics required to describe the performance of a lens.

[jetstream]

1 hour ago, Deadlake said:

I hope not, I'd follow my eyes as I think a Strehl test do not cover full metrics required to describe the performance of a lens.

There are many tests out there but one thing is for sure: DPAC testing will show spherochromatism, and much more.

[jetstream]

4 hours ago, Deadlake said:

Well Vic has responded

https://www.stellarvue.com/svx-optics/

For AP null in red and for visual null in green.

Is it as simple as that?

I dont know? how about this? Apparently from SV facebook page? Is this correct- I dont know

[nicoscy]

Some misleading facts by SV.
 

Please note : most color cameras have a RGGB matrix, so color cameras are more sensitive to green wavelengths. However, QE depends on each camera. If you are imaging in visible light, the 533MC is superior to the 585Mc due to high QE in visible light. If you go towards longer wavelengths (red, infrared, ha etc) the 585MC is a lot more sensitive. So, this assertion does not hold. It depends on the emissions you are interested to image in. Here’s a graph I created on some wavelengths - so, it all depends.

SV now states that they will have two types of scopes - visual optimized and imaging optimized. This is wholly unnecessary and is a marketing gimmick again. Very disappointing. AP, TEC, LZOS test in green. AP and APQ Zeiss check polystrehl (AP for Stowaway and AP110 clearly indicated this, APQ states this for their new range). Agema, a newcomer with long focal length fluorite doublets, also provides strehl info on all key wavelengths.

The implications of this - All scopes sold so far by Stellarvue  were supposedly optimized for imaging, so visual amateur astronomers can now start feeling disappointed and that they own an inferior (expensive and soon to depreciate more) product for their intended use. 

The “sweep under the carpet” moment: no discussion of zones, ripples and uneven polish of the lens assembly in question which was tested, which at $18k, only one word should have applied for the money: “flawless”. Which it is not…

I was hoping SV would just come up with a similar policy as the true Master Opticians of our times, stick to it, produce real high end scopes and be considered one of the great ones. Alas, it doesn’t appear so…

Edited March 18, 2023 by nicoscy

[nicoscy]

Relevant new announcement by SV is here.

In case it is revised, copied below:

Just as imaging cameras are most sensitive to red light, the human eye is most sensitive to green light. Users who only observe visually and never plan to image should purchase a telescope that is optimized in green light for all the same reasons that imagers should purchase a telescope that is optimized for red light. We are making a large investment this year in a customized phase shifting laser interferometer that will allow us to figure our objectives in green light creating high strehl optics optimized solely for visual users. These telescopes will be made with a slightly different part number using the prefix: SVX-G. This will differentiate if the telescope optic is nulled in green light or red light. SVX-G optics will be made for discriminating visual observers.

 

And  a new claim to justify the past:
 

Originally, (back in 1998) most of our customers used their telescopes visually. That has really changed over the years and by 2018 90% of our customers were astro-imagers and that percentage continues to grow. The Zygo Phase Shifting Laser Interferometers use a HeNe laser, so it measures in red light. CCD and CMOS cameras are most sensitive to red light. So it makes sense using red light when figuring optics to very high Strehl ratios that are used with electronic cameras. Stellarvue SVX optics are made for discriminating imagers and those who image and observe visually. While they take over a year to make we are in continual production so current waiting times are weeks to months, not years.

 

[Deadlake]

12 minutes ago, nicoscy said:

Some misleading facts: most color cameras have a RGGB matrix, so color cameras are more sensitive to green wavelengths. However, QE depends on each camera. If you are imaging in visible light, the 533MC is superior to the 585Mc due to high QE in visible light. If you go towards longer wavelengths (red, infrared, ha etc) the 585MC is a lot more sensitive. So, this assertion does not hold. It depends on the emissions you are interested to image in. Here’s a graph I created on some wavelengths - so, it all depends.

SV now states that they will have two types of scopes - visual optimized and imaging optimized. This is wholly unnecessary and is a marketing gimmick again. Very disappointing. AP, TEC, LZOS test in green. AP and APQ Zeiss check polystrehl (AP for Stowaway and AP110 clearly indicated this, APQ states this for their new range). Agema, a newcomer with long focal length fluorite doublets, also provides strehl info on all key wavelengths.

The implications of this - All scopes sold so far by Stellarvue  were supposedly optimized for imaging, so visual amateur astronomers can now start feeling disappointed and that they own an inferior (expensive and soon to depreciate more) product for their intended use. 

The “sweep under the carpet” moment: no discussion of zones, ripples and uneven polish of the lens assembly in question which was tested, which at $18k, only one word should have applied for the money: “flawless”. Which it is not…

I was hoping SV would just come up with a similar policy as the true Master Opticians of our times, stick to it, produce real high end scopes and be considered one of the great ones. Alas, it doesn’t appear so…

One last thing is cost. I know APM produced extra high Strehl lens for 105 mm scopes some years ago to celebrate 20 years of telescope production. The cost from moving from 0.95 to 0.99 was an extra $1000 per lens. How much for a poly Strehl lens?

Maybe Vic needs to come up with how much a poly Strehl optimised lens costs, the AP and APQ are really another level in costs compared with Taks etc...

 

[inFINNity Deck]

3 hours ago, CraigT82 said:

It also contains this wording, copied verbatim, which is just plain wrong:

CCD and CMOS cameras are most sensitive to red light

 

Indeed most cameras have their highest QE at around green light, or even in the blue:

Starlight Xpress Trius:

 

QHYCCD QHY533, QHY268, QHY600:

 

ZWO ASI2600, ASI6200:

 

ZWO ASI1600:

 

Only red-sensitive camera I found so far is this ZWO ASI990/991:

 

Nicolàs

[Highburymark]

I read the Suiter book, and have to admit half of it flew over my head. However I understand the basics, and for people like me, the simple star test, done properly, is enough to establish whether a telescope is doing a good enough job to be happy with it. Only one of my refractors has ever shown a near perfect star test - the TSA120. The others have shown some false colour at high powers, and clearly contrasting rings either side of focus. Yet I’ve been happy with their performance under the stars. 
So, for most amateur needs, I agree DPAC results are not needed. But I’m interested in what they can teach me about optics - that’s the attraction rather than trying to establish whether my scopes are perfectly figured. The hi-fi analogy is a good one - I sometimes wonder whether hi-fi buffs are too wrapped up in sound quality to enjoy the music they are playing. 

[Deadlake]

37 minutes ago, jetstream said:

I dont know? how about this? Apparently from SV facebook page? Is this correct- I dont know

Since I posted the url has been updated, I've corrected the original post.

[JeremyS]

6 minutes ago, nicoscy said:

Relevant new announcement by SV is here.

In case it is revised, copied below:

Just as imaging cameras are most sensitive to red light, the human eye is most sensitive to green light. Users who only observe visually and never plan to image should purchase a telescope that is optimized in green light for all the same reasons that imagers should purchase a telescope that is optimized for red light. We are making a large investment this year in a customized phase shifting laser interferometer that will allow us to figure our objectives in green light creating high strehl optics optimized solely for visual users. These telescopes will be made with a slightly different part number using the prefix: SVX-G. This will differentiate if the telescope optic is nulled in green light or red light. SVX-G optics will be made for discriminating visual observers.

 

And  a new claim to justify the past:
 

Originally, (back in 1998) most of our customers used their telescopes visually. That has really changed over the years and by 2018 90% of our customers were astro-imagers and that percentage continues to grow. The Zygo Phase Shifting Laser Interferometers use a HeNe laser, so it measures in red light. CCD and CMOS cameras are most sensitive to red light. So it makes sense using red light when figuring optics to very high Strehl ratios that are used with electronic cameras. Stellarvue SVX optics are made for discriminating imagers and those who image and observe visually. While they take over a year to make we are in continual production so current waiting times are weeks to months, not years.

 

What nonsense! The phrase “when you’re in a hole, stop digging” comes to mind.
And how do they know the percentage of customers doing imaging in any case?

[Deadlake]

4 minutes ago, Highburymark said:

I read the Suiter book, and have to admit half of it flew over my head. However I understand the basics, and for people like me, the simple star test, done properly, is enough to establish whether a telescope is doing a good enough job to be happy with it. Only one of my refractors has ever shown a near perfect star test - the TSA120. The others have shown some false colour at high powers, and clearly contrasting rings either side of focus. Yet I’ve been happy with their performance under the stars. 
So, for most amateur needs, I agree DPAC results are not needed. But I’m interested in what they can teach me about optics - that’s the attraction rather than trying to establish whether my scopes are perfectly figured. The hi-fi analogy is a good one - I sometimes wonder whether hi-fi buffs are too wrapped up in sound quality to enjoy the music they are playing. 

A scope's optics should get out of the way, however if not a change is needed.

[Highburymark]

1 minute ago, Deadlake said:

A scope's optics should get out of the way, however if not a change is needed.

For an $18,000 triplet, I agree. But the vast majority of scopes sold as ‘apos’ have varying degrees of CA and SA. It’s why we can buy pretty good ED doublets for £300+. We can’t expect perfection with such competitive products. The only loser in the long term if we expect too much from cheaper scopes will be the consumer.

[jetstream]

7 minutes ago, Highburymark said:

For an $18,000 triplet, I agree. But the vast majority of scopes sold as ‘apos’ have varying degrees of CA and SA. It’s why we can buy pretty good ED doublets for £300+. We can’t expect perfection with such competitive products. The only loser in the long term if we expect too much from cheaper scopes will be the consumer.

I wouldnt think it to be that hard for the makers at any price point to give whats needed for vg views ie .8 Strehl, 1/4 PV or the equivalent MTF profile of 1/4 wave spherical aberration. This level of scope will provide nice sharp views. However, I think many do not which is a shame . 

If a scope can have an MTF plot like the lower left in the image that includes any optical consideration ie central obstruction etc the scope will be sharp. Not too much to ask  IMHO.

[Deadlake]

1 hour ago, Highburymark said:

For an $18,000 triplet, I agree. But the vast majority of scopes sold as ‘apos’ have varying degrees of CA and SA. It’s why we can buy pretty good ED doublets for £300+. We can’t expect perfection with such competitive products. The only loser in the long term if we expect too much from cheaper scopes will be the consumer.

But isn't this the discussion around marketing. For instance would you take a lower Strehl in green line or a higher Strehl in the red line. 

The lower Strehl in the green line gives better views, so in some way a marketing ploy has occurred.

A higher Strehl in the green costs more to fabricate by quite a bit.
 

Some more colour: Two years ago I was shopping for a fast 130 mm scope. I looked at several makes, I avoided SharpStar as I knew APM had moved production to KUO as they had quality issues with SharpStar. At the time I would of liked to know how Sharpstar where testing their lens.

One final point, some of those Technosky scope are DPAC'ing very well on CN, far better than you would expect for the brand.

Maybe we can eat cake?

Edited March 18, 2023 by Deadlake

[jetstream]

Well Im kinda wore out from all this but I do appreciate all the info provided. I'm going to kick back, reflect on DPAC and its requirements and wait for an affordable but good flat to wander by.

In the meantime I'm going snowmobiling today and then observe in some nice skies- and I dont even really care what scope I use.👍

Gerry

[Highburymark]

I remember reading that energy rejection filters used for solar Ha might make good flats for DPAC testing. I have a 130mm Baader ERF that I use with a Sundancer/Solar Spectrum etalon - and I have a Ronchi eyepiece. Hmmm. Need to look into this further.

[jetstream]

1 hour ago, Highburymark said:

. I have a 130mm Baader ERF that I use with a Sundancer/Solar Spectrum etalon

Hmm, I'm noticing you have some vg equipment Mark- nice! Questions will be coming re solar lol

Great idea and this is from Alpines site:

D-ERF filters use a high grade BK7 optical glass which is free of any internal defects. The thick substrates are polished plane-parallel, smooth, and flat to 1/10 wave p-v over the entire surface. Only by using thick substrates can a high surface accuracy be achieved and held in use (for example, the 90mm ERF is 5.5mm thick. Thinner substrates are not capable of holding such high surface accuracy).

This is a vg idea. Now I'm checking Solar Spectrum out... thanks for the heads up, Gerry

[davidc135]

8 hours ago, jetstream said:

Hmm, I'm noticing you have some vg equipment Mark- nice! Questions will be coming re solar lol

Great idea and this is from Alpines site:

D-ERF filters use a high grade BK7 optical glass which is free of any internal defects. The thick substrates are polished plane-parallel, smooth, and flat to 1/10 wave p-v over the entire surface. Only by using thick substrates can a high surface accuracy be achieved and held in use (for example, the 90mm ERF is 5.5mm thick. Thinner substrates are not capable of holding such high surface accuracy).

This is a vg idea. Now I'm checking Solar Spectrum out... thanks for the heads up, Gerry

They might be OK for flats but 1/10 wave p-v on the surface would be halved on refraction as intended but doubled on reflection.  David

[jetstream]

7 hours ago, davidc135 said:

They might be OK for flats but 1/10 wave p-v on the surface would be halved on refraction as intended but doubled on reflection.  David

Thanks David- so for a DPAC flat do you think it would work then? the "might" is not too reassuring - so the 1/10 would then be 1/5 pv for DPAC? I'm interested in the reason why this would be, thanks Gerry

[Highburymark]

That’s the question I was going to ask too. I found a couple of threads on CN in which people say they’ve used ERFs successfully as flats for DPAC - but they give no more detail. 

[davidc135]

DPAC 'Flats' deviate from flat in two ways. By having overall curvature or power, which for the DPAC test is irrelevant unless extreme and in irregularity. If the 1/10 wave refers to curvature then it's harmless but if it's irregular or local then the 1/10 becomes 1/5 wave which might appear as under or over correction or zones which will then contribute to the total error seen in the test. Errors are cumulative and you wouldn't be able to distinguish between those of the flat and those of the optic under test.

A good flat should not deviate from the nearest sphere by more than 1/20 wave surface error, or 1/10 wave when reflected.

So if the erf was made by a quality manufacturer and had a smooth figure it might well be fine.

Optical flats often have 2 specs, one overall and another local. 

Of course, if you also wanted to use the flat for testing diagonals or other planes, then the total error must be small.

David

Edited March 19, 2023 by davidc135

[Rusted]

One can eliminate some zoning on the optical flat by setting the flat off centre to the test objective.
It does not make the zones go away, of course. It just shows them for what they are.
So the objective, under test, is not blamed for the errors on the flat itself.

[jetstream]

8 minutes ago, Rusted said:

One can eliminate some zoning on the optical flat by setting the flat off centre to the test objective.
It does not make the zones go away, of course. It just shows them for what they are.
So the objective, under test, is not blamed for the errors on the flat itself.

So then the zones would all be in the same place relative to the optic making them identifiable ?

[jetstream]

31 minutes ago, davidc135 said:

So if the erf was made by a quality manufacturer and had a smooth figure it might well be fine.

I was looking at the Baader D ERF , which I could use like @Highburymark on the TSA120 for solar too.