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The specs for the D-ERF series refer to surface accuracy of 1/10 wave...

My question is this; does this generally mean 1/10 wave over the entire 160mm or 180mm surface...or 1/10 wave over some unit of distance?
...such as 1/10 wave over a square cm or square inch?

Thx in advance for helping shed some light.

-SP

I believe that when – generally – this "max lambda P-V" accuracy is given, it is for the whole mirror/lens/filter surface.

Thx for responding...so if I understand correctly, a given filter series with that particular spec would actually become increasing accurate as the size increased...like the Baader, going from 95mm to 180mm represents a huge increase effort and difficulty on their part

Thx again!

Hello,

Here, L/10 means Peak to Valley.

My Baader 160 mm was measured to L/9.6 P-V in red light :



It is really good

Hello Christian

I have realized some ERF using a combination of two photographic filters: B+W 091 (RG630) and a B+W 486 (UV-IR Cut) bought used at cheap on Ebay.
I have also measured the transmission (attached), compared to Baader and Lunt ERF.

Looking into the telescope I do not see the quality difference in sharpness compared to a ERF Baader or Lunt.

Have you tried to measure the quality of photographic filters like B + W?

I have in program the measurement of these photographic filters quality with Zygo, the next autumn, but I'm pry to know if someone has already done this measure.
Thank you

wow, this is very good.

i wonder, has valery posted this for aries d-erf?

interested to see the comparison.

Camera filters are usually not 1/4 wave on transmission. They don't need to be. If you use them at the objective end, the image will be soft. If they are used at the image end(before the image). Then they should work. But it allows extra heat in the scope when they are inside the tube.
Mark W.

mdwmark wrote:Camera filters are usually not 1/4 wave on transmission.
Mark W.

In fact, I have read that they are machined 1/2 wave ... and what does that mean?
I have done many tests with 60 mm and 100 mm lenses and I did not see any difference between the B + W filters and ERF Baader filter, then the difference is very difficult to appreciate.
However, although there was a small difference in quality would be between 1/4 wave and 1/2 wave in a refractor changes little.
But one thing I can not figure out (for I have had the same your objection in Italy). For which reason all use the product Astrosolar, which has a much lower quality (low contrast) compared to a correct optical window 1/4 wave coating for solar visible light and nobody objected nothing?
Instead of using the Astrosolar (to see the sun in the visible), following your reasoning, for a refractor 100 mm, for example, you should buy a Herschel prism or a proper 1/8 wave optical window with coating, which costs 400 euro (less than ERF Baader). In this way, the quality would be greater than that which occurs with Astrosolar.
In summary. I did not see any difference in quality between ERF Baader (which I have) and quality filters such as B + W, while I see the difference between the Astrosolar filter and the Herschel prism. Each of you can think about this.
However I asked only if anyone has ever measured the quality of quality photographic filters, such as B + W, and I have not yet been answered. Without this answer there is no theory and it is important to practice, practice experience, which is what I have done.
If one has money, buy ERF Baader, if one does not have much money can get a good result with photo filters of quality. I got the same result (in the visible). After all, this is the same reasoning  of those who choose the Astrosolar and do not buy a 1/8 wave optical window with a coating for solar visible light (who buys a Astrosolar, he knows that will have a minimum of quality degradation and accepts it.). Sorry for my bad English...

Please, I do not sell filters, not Baader, not B + W, not other ...

antonello wrote: For which reason all use the product Astrosolar, which has a much lower quality (low contrast) compared to a correct optical window 1/4 wave coating for solar visible light and nobody objected nothing?

I must disagree about "much lower contrast" (I have observed the Sun through a both a film and wedge). And as for surface quality itself, it seems the film indeed has very uniform thickness and meets at least the 1/4 lambda P-V criterion. I think I saw a third-party interferometric test somewhere, can't find it now... Baader themselves make quality claims here (Strehl 94.1% at 632 nm) and here ("we have a hard time ourselves to see any difference in performance between AstroSolar™ and 1/20 wave precision optical windows" - I know, not really a precise statement).

As Christian points out, the Baader D-ERF's 1/10 wave is Peak to Valley and measured in Red light (preferabley at 656nm).The are many different ways to quote a fraction like this in terms of accuracy and so mislead people. The RMS value would be higher. If it were measured at a higher wavelength it would also be higher. It also depends if your are talking transmission or surface accuracy so when comparing different products, it is important to know that the figures all relate to the same test.

It also depends where the filter goes. Before an objective requires much greater accuracy than after it.

The first Front ERF's I used were coloured glass types which are normally advertised as 1/4 or 1/5 wave. Compared to an internal 2" UV/IR cut or ha filter I found the front mounted filter definatley less sharp but with consistant contrast across the FOV. As I mainly image, sharpness was more important as the centre of the image was better. Using a Baader D-ERF, this changes. It is much better than an absorbtion type filter and sharper across the entire field of view.

With white light, Astrosolar is much better optically than front mounted glass white light filters. This is because it is very thin so light is not distorted as it passes through, unlike a glass filter where inaccuracies cause refraction. In theory it can have less effect than a herschel wedge but the advantage of a wedge is that it does not block as much light so contrast is better.

All this of course is affected by seeing but with an SSM, you can almost gaurantee you can capture in good seeing so better quality filtering should provide the best quality results.

I was hoping to get a scientific answer to my question and instead I received only praise of Baader products, as if I had written that the Baader products are not good ... But I DO NOT wrote that the Baader products are not good…
There are at least two good scientific answers to my question.

The first is that perhaps the quality photographic filters (as B+W filters) are not as poor as some think (In fact, the B + W 486 by 112 mm UV-IR Cut It has a similar price to Baader ERF 110 mm ...), and for verify this I was interested to see an interferometric analysis of these filters…
The second scientific answer (the most likely) is that I did not see the difference between the Baader ERF filter and the two B + W filters, with the telescopes under test (a 60 mm f:15 Celestron and a Tal 100RS), because this telescopes are, evidently, correct only 1/4 or 1/2 wave.
In this case, the best quality of the optical window of the Baader ERF is not useful (not necessary). For me, It's just a gift of money to the seller.

About what they wrote GrearAttractor and Astrograph…
my experience is different. The optical quality (1/4 wave, for example) is not the only parameter to define the quality of a optical product. There are many others parameter, for example the "roughness". If you take a 150 mm (for example Istar H-alpha 150 mm f: 10) and you compare a Astrosolar film with Herschel prism you will notice the difference.
The Astrosolar is a great product and I know perfectly the datasheet of this product

http://www.baader-planetarium.de/com/pd ... test_e.pdf

What that GreatAttractor as written is taken from this datasheet, where it is written that the product is better than the American optical windows (as Thousand Optics), which are correct, if all goes well, one wave for inch (is something likely) ... but if you compare the Astrosolar with a glass optical window with correction at least 1/4 wave PV the quality of the glass window is higher ("Our Safety Film shows notably more sharpness and contrast than any competitive product we know of, especially including all affordable US-Glass Objective Filters )". On the other hand this is also visible in Datasheet Baader ...
I use the Astrosolar on a Cassegrain-Maksutov Intes Micro 200mm f:10 (1/8 wave PV and 1/40 wave RMS) WITH MUCH SATISFACTION, but in a telescope with this quality the quality with Astrosolar is reduced. This NOT CAN BE disputed (of course this is true when the seeing is good to allow you to see all the resolving power of the telescope ...).

Naturally, the fact that this difference (with Astrosolar e without Astrosolar) is for me Big and for you Small (but not is zero as you say) is a personal matter that I respect.

For Astrograph
If you look at the graph that I have posted, you can see that the filter curve B + W UV-IR Cut 486 is better than that of the Baader ERF ... (the curve , please, not the quality, that still no one yet know.... even if the optical quality of the Baader ERF will be certainly higher.).

Hello to all,

Unfortunately, I have no measurement of photographic filters.

You can find on page 13 transmission measurements for the MCM/AiryLab and Aries ERF fiters.
On page 14, there is a measurement of the optical quality of MCM/Airylab ERF.
http://www.astrosurf.com/viladrich/astr ... lution.pdf

My guess about the difference betwen Astrosolar film and Herschel diagonal is that it may be due to the level of diffuse light. I've made some test images on this. I still have to measure them. This is in the to do list ...

christian viladrich wrote:Hello to all,
Unfortunately, I have no measurement of photographic filters ...

Christian,
many thanks for your answer.
I am a technician of a professional astronomical observatory in Italy …
The Zygo of my Institute is set, from several months, for the control of a 300 mm diameter camera for a satellite and then I could not do until now the photographic filters control quality with the Zygo. Maybe next autumn will changed the setup and I will be do the check. However, the curiosity was so great that I have not been able to wait until next autumn and I did a lot of measurements with a 50 mm quartz surface reference with 1/20 wave correction. This work is very interesting, but now I can not write about it. Among the filters inspected there is also a ERF of a Lunt for Double 60mm stack (LS60FHA), of which I can now write something.
I was very surprised by the results, even for the fact that you declared for this filter a correction of 1/4 wave.
As you can see the error was 1/2-1.5 wave in one side and also 2 wave in the other side !!! (see A and B in the picture attached).
Since Lunt Solar Sistems is certainly a serious company, I have insight that the bad result in my test was determined by improper assembly of the filter.
I have discovered that the ring that stops the filter was very narrow, so after breaking black silicone that blocks the ring, I loosened the same ring (so that the filter could move slightly in the ring ) and I made a new test.
This time the result was very different and in complete compliance with the declared correction (better 1/4 wave).
You can see in C and D the result.
It is evident that I have used, in the past, with my telescope, this Lunt filter with a tensioned ERF filter.

Thank you for your attention.

Well this has turned into an interesting thread. Thanks guys!

...and there is Chinese production standards in a nutshell. Could be good, could be bad and definitely could be ugly

An important detail : when considering a window, we don't care much about the surface quality. What is important is parallelism.
When ordering a window from a supplier, I take great care to mention a transmitted wavefront error
That's why the astroslar is quite good : because its thickness is quite constant, and even more because its refractive index is close from air's.