I want to buy a solar filter, which one?

[johninderby]

Not myth but fact. Thousand Oaks glass filters don’t use true optically flat glass. Optically flat glass is far more difficult to make than say an SCT corrector with it’s curved surface.

Zeiss made great glass solar filters with proper optically flat glass but were so expensive they didn’t sell well.

Edited July 6, 2021 by johninderby

[michael.h.f.wilkinson]

1 hour ago, johninderby said:

Not myth but fact. Thousand Oaks glass filters don’t use true optically flat glass. Optically flat glass is far more difficult to make than say an SCT corrector with it’s curved surface.

Zeiss made great glass solar filters with proper optically flat glass but were so expensive they didn’t sell well.

So what is my 105 mm diameter protection filter in front of my 150-600 mm zoom made of? Optically flat glass is not that hard to make (check out the Edmund Optics Catalogue)

The original Schmidt corrector plates were made by pulling a partial vacuum beneath a suitably supported glass plate, and polishing it flat, so after release of the pressure the surface had the suitable curved shape (from my MSc course in optical astronomy at the Kapteyn Astronomical Institute). A colleague over at  Space Research across the road can confirm this (he makes optics for research instruments for a living)

Other examples are Baader ERF for H-alpha use: huge diameters do not cost the earth (their high price mainly being down to the coatings, not the flat optical surface)

 

[Tiny Clanger]

On 04/07/2021 at 13:29, Carbon Brush said:

Everything said in the above posts about filter choices is valid advice.

However, I would add a caution about purchasing.

By all means by a Thousand Oaks or Baader filter, but buy from an astronomy retailer.
That way there is no doubt about the provenance of the product.

You don't get two chances looking through a telescope at the sun. Is your sight worth the risk?
Stay with a local astronomy retailer. You have no risk of buying the 'almost' product.
By supporting these people, you help ensure they are there when you need advice on other astro kit.

HTH, David.

I agree entirely with this. Buy from a reputable source. FLO sent mine out in a card sleeve which handily provided enough corrugated card for most of my filter frames 🙂

While doing some speculative amazon searching for astro oddments a short time ago,, I tripped over a whole slew of Far East solar filter material which was definitely not Baader film,  it included the words 'baader film' in the description, note the lower case 'b' . I'd have given the seller the benefit of the doubt, and accepted they might think 'baader film' was a name for a class of thing, like 'hoover'  , and not a trade name, except for them naming their stuff something like Bayder solar film . Hmm. 

Doubting Amazon would do anything if I complained to them, I sent a link to Baader UK, and had an email back thanking me for bringing it to their attention, they have taken it up with amazon. Don't take risks, the proper stuff is only about £20.

Heather

[saac]

4 hours ago, Stu1smartcookie said:

agree 100% ... my glass solar filter cost £70 but i am delighted with it . Yes , i know if i drop it , it will break ... but , thats down to me , nothing to do with the construction of it . Its an expensive option in an expensive hobby . I for one am happy with the advice i received from a very reputable Astronomy company . I really do get why people wax lyrical regarding Baader film. But i am a bit perplexed at the hostility ( probably the wrong word to use ) of glass filters . 

I am pleased for @Lotinsh that his question has provided such a great response . A lesson for all , if in doubt , ask ... then sit back and enjoy the fireworks  

Similar experience with mine, been using it now  approaching 10 years and never had any problems.  I think at the end of the day it really comes down to personal choice but the glass filter has worked very well for me. 

Jim

[DarkAntimatter]

How important is it for glass solar white light filters to be optically flat?  What are the effects of their not being optically flat? 

 

[saac]

42 minutes ago, DarkAntimatter said:

How important is it for glass solar white light filters to be optically flat?  What are the effects of their not being optically flat? 

 

I was wondering exactly that  - I mean the mylar type film filters (Baader etc) are obviously not optically flat!   Given atmospheric distortion, thermal currents etc anyway I doubt anything the glass does would be a limiting factor. Put it this way, I've used my filter for both visual and electronic capture and I cannot say I have ever had reason to be dissatisfied.  I also doubt that the glass would be that far from optical flat.  Modern float glass is produced under quite remarkable tolerance, not claiming it to be optically flat, but for our purpose it's more than adequate.  

Jim 

Edited July 6, 2021 by saac

[Tiny Clanger]

2 minutes ago, saac said:

I was wondering exactly that  - I mean the mylar type film filters (Bader etc) are obviously not optically flat!   Given atmospheric distortion, thermal currents etc anyway I doubt anything the glass does would be a limiting factor. Put it this way, I've used my filter for both visual and electronic capture and I cannot say I have ever had reason to be dissatisfied.  I also doubt that the glass would be that far from optical flat.  Modern float glass is produced under quite remarkable tolerance, not claiming it to be optically flat, but for our purpose it's more than adequate.  

Jim 

In the instruction sheet on how to make your own Baader film filter they explicitly say it should be wrinkled, not flat. I'd guess this is to avoid any mechanical damage done to the carrier layer by stretching it. 

Heather

[Stu]

6 minutes ago, saac said:

I was wondering exactly that  - I mean the mylar type film filters (Baader etc) are obviously not optically flat!   Given atmospheric distortion, thermal currents etc anyway I doubt anything the glass does would be a limiting factor. Put it this way, I've used my filter for both visual and electronic capture and I cannot say I have ever had reason to be dissatisfied.  I also doubt that the glass would be that far from optical flat.  Modern float glass is produced under quite remarkable tolerance, not claiming it to be optically flat, but for our purpose it's more than adequate.  

Jim 

Hands up, I’ve always held negative views of the glass filters on the assumption that they are not optically flat, and therefore not as good quality as the films which quote 95% Strehl ratios generally.

Do I know this for a fact? No. I’ve not done a side by side comparison which is what would be needed to tell the difference. No disrespect meant to anyone but it’s also not that informative to say you are very happy with the glass filter without having compared it directly with other film filters or a Herschel wedge for example, you just won’t know what the views could be like. If you’ve compared and prefer the glass, then that would be interesting to hear.

I suspect in most instances though, people are not pushing these filters very hard ie generally using low to medium magnifications so the quality differences may not be seen. At high power under good seeing conditions you may see the difference in fine detail and granulation.

It would be interesting to see a side by side comparison under these circumstances so we get to know the true situation. Glass may well be just as good, I just don’t know. I use a wedge which I know from comparison gives better performance than the films.

[JamesF]

2 minutes ago, Tiny Clanger said:

In the instruction sheet on how to make your own Baader film filter they explicitly say it should be wrinkled, not flat.

I generally use a wedge these days, but my Baader film filters are fairly flat, though they're not under tension.  I just lay the filter on the table and then gently placed the carrier on top to fit it, as far as I recall.

I am wondering though if the reason flatness doesn't matter for the film is that it's only a few wavelengths of light thick (about 10µm, I believe, compared with about 0.5µm for green light), so refraction effects in the medium of the film are small, or perhaps the film just has a refractive index close to 1.  I've not ever had a glass filter in my hands, but I assume they're several millimetres thick and probably have a refractive index somewhere in the region of 1.5, meaning greater distortion of the final image due to refraction if the surfaces are not flat?

James

[saac]

37 minutes ago, Stu said:

Hands up, I’ve always held negative views of the glass filters on the assumption that they are not optically flat, and therefore not as good quality as the films which quote 95% Strehl ratios generally.

Do I know this for a fact? No. I’ve not done a side by side comparison which is what would be needed to tell the difference. No disrespect meant to anyone but it’s also not that informative to say you are very happy with the glass filter without having compared it directly with other film filters or a Herschel wedge for example, you just won’t know what the views could be like. If you’ve compared and prefer the glass, then that would be interesting to hear.

I suspect in most instances though, people are not pushing these filters very hard ie generally using low to medium magnifications so the quality differences may not be seen. At high power under good seeing conditions you may see the difference in fine detail and granulation.

It would be interesting to see a side by side comparison under these circumstances so we get to know the true situation. Glass may well be just as good, I just don’t know. I use a wedge which I know from comparison gives better performance than the films.

Spot on Stu ,  I can only really comment on the glass type filter it's the only one I've ever used.  All I can say is that I have never been disappointed by the image it produces , no discernable flaws that I can detect;  sharp image and good contrast.  Bear in mind that my bi focal non glass lens spectacles add yet another layer of potential aberration before my brain forms the image    Anyway you have piqued my interest, I can't spring for a Herschel Wedge at the moment but I'll order some solar film and make a comparison.  It will be a nice wee experiment to break the darth of astro activity at the moment . If we ever see the sun again in the next few months I'll upload what I find .   

 

 

Edited July 6, 2021 by saac

[Starslayer]

Making no apology for silly question time as this is firmly planted in the beginner section -    You guys are ‘banging on’ (no offence meant) about all these glass filters etc - where are these placed.  Clearly mine attaches over the entire end of the Tube, job done but I get the feeling you are talking about things that are nearer the eyepiece.  As a BIG novice I was very much warned against this type of thing.  Am I misunderstanding this. . 

[Stu1smartcookie]

8 minutes ago, Starslayer said:

Making no apology for silly question time as this is firmly planted in the beginner section -    You guys are ‘banging on’ (no offence meant) about all these glass filters etc - where are these placed.  Clearly mine attaches over the entire end of the Tube, job done but I get the feeling you are talking about things that are nearer the eyepiece.  As a BIG novice I was very much warned against this type of thing.  Am I misunderstanding this. . 

Basically it’s filters that  fix over the aperture . 

[Ags]

2 hours ago, Starslayer said:

Making no apology for silly question time as this is firmly planted in the beginner section -    You guys are ‘banging on’ (no offence meant) about all these glass filters etc - where are these placed.  Clearly mine attaches over the entire end of the Tube, job done but I get the feeling you are talking about things that are nearer the eyepiece.  As a BIG novice I was very much warned against this type of thing.  Am I misunderstanding this. . 

Small glass eyepiece filters get very hot, crack and can result in dreadful injuries. There are a few eyepiece side filters that can be safely used though. The most common one is a Herschel Wedge, which looks like a telescope diagonal, but it only reflects about one twentieth of the light to the eyepiece (which is still too bright, so there is an additonal ND5 filter to take the light down to safe levels):

https://www.teleskop-express.de/shop/product_info.php/info/p9409_TS-Optics-1-25--Sonnenprisma-mit-Polarisationsfilter-und-Schmalbandfilter.html

I am very pleased with mine. I used to worry the film mask I made would somehow come off while viewing, no worries with a Herschel Wedge.

Edited July 6, 2021 by Ags

[Starslayer]

27 minutes ago, Stu1smartcookie said:

Basically it’s filters that  fix over the aperture . 

The aperture being the front of then scope yes? So the glass filters being referred to would in my case with a Celestron 6” sct be a 6” job that connects to the very front and covers the whole thing?  Sorry to be so basic but I want to get this right and have found here on several occasions that some members refer to the same thing in different ways. 😉

[DarkAntimatter]

I have a glass solar filter that fits over the aperature, not one at the eyepiece end.  I'm happy with it, but have never compared with any other type.  Now I'm also curoius about a comparison of glass, film, and wedges.  

Regarding wedges, is there any danger to the scope in having the filter at the eyepiced end?  With an aperature filter, the light intensity is attenuated before it gets into the scope at all.  With filters at the other end, do we need to worry about heating of secondary mirrors, etc?

[JamesF]

4 minutes ago, DarkAntimatter said:

Regarding wedges, is there any danger to the scope in having the filter at the eyepiced end?  With an aperature filter, the light intensity is attenuated before it gets into the scope at all.  With filters at the other end, do we need to worry about heating of secondary mirrors, etc?

As far as I'm aware, you can't use a wedge with a reflector (which is why I still have some film filters for use with my 127 Mak) for precisely those reasons.

I believe the greatest danger whilst using a wedge is setting fire to one's tie (you do wear one whilst observing, don't you?) because it dangles down into the path of the majority of the heat/light that the wedge is rejecting.

James

[Stu]

10 minutes ago, DarkAntimatter said:

I have a glass solar filter that fits over the aperature, not one at the eyepiece end.  I'm happy with it, but have never compared with any other type.  Now I'm also curoius about a comparison of glass, film, and wedges.  

Regarding wedges, is there any danger to the scope in having the filter at the eyepiced end?  With an aperature filter, the light intensity is attenuated before it gets into the scope at all.  With filters at the other end, do we need to worry about heating of secondary mirrors, etc?

You can only use a wedge with a refractor, with an SCT or Mak there is a risk of overheating the secondary and at best melting the glue holding it in place and at worse cracking it. A front aperture filter is the way to go with a these scopes.

[Stu]

1 hour ago, saac said:

Spot on Stu ,  I can only really comment on the glass type filter it's the only one I've ever used.  All I can say is that I have never been disappointed by the image it produces , no discernable flaws that I can detect;  sharp image and good contrast.  Bear in mind that my bi focal non glass lens spectacles add yet another layer of potential aberration before my brain forms the image    Anyway you have piqued my interest, I can't spring for a Herschel Wedge at the moment but I'll order some solar film and make a comparison.  It will be a nice wee experiment to break the darth of astro activity at the moment . If we ever see the sun again in the next few months I'll upload what I find .   

 

 

Good stuff Jim. I’ll happily eat humble pie if the glass proves to be as good/better. Give it a good run out though, try to view with some excellent seeing conditions and ramp the power up a bit. What scope will you use?

The Lunt 1.25” wedges are excellent value for money, not that expensive really and give very good performance. I use a Baader Coolwedge which does have an additional edge for high power detail in comparison I found, but a lot more expensive. The Lacerta  wedges also get excellent write ups but I’ve not used one so can’t comment.

[saac]

1 hour ago, Starslayer said:

Making no apology for silly question time as this is firmly planted in the beginner section -    You guys are ‘banging on’ (no offence meant) about all these glass filters etc - where are these placed.  Clearly mine attaches over the entire end of the Tube, job done but I get the feeling you are talking about things that are nearer the eyepiece.  As a BIG novice I was very much warned against this type of thing.  Am I misunderstanding this. . 

Here you go, as Stu1smartcookie said it fixes over the aperture of the scope. Filters out 99.99% of the light and then some rendering an eye safe image at the eyepiece - no thermal transmission to speak of.   Pre check the filter each time to identify any pin holes in the coating to ensure it is safe. 

Jim 

[saac]

11 minutes ago, Stu said:

Good stuff Jim. I’ll happily eat humble pie if the glass proves to be as good/better. Give it a good run out though, try to view with some excellent seeing conditions and ramp the power up a bit. What scope will you use?

The Lunt 1.25” wedges are excellent value for money, not that expensive really and give very good performance. I use a Baader Coolwedge which does have an additional edge for high power detail in comparison I found, but a lot more expensive. The Lacerta  wedges also get excellent write ups but I’ve not used one so can’t comment.

I'll happily buy you one from our local bakers Stu if it comes to that , they are quite famous in Fife - their apple or rhubarb pies are absolutely divine     I'll be using my ED80 -  do you know I'm actually looking forward to it, I've always wondered myself what the difference is. I do know that they do give a different contrast - I think I have heard that the Baader film is more likely to give a hint of granulation than the glass type.  High power may be difficult , I've always struggled to get good focus when looking at sun spots  but I will do my best.  I think a wedge may be a potential purchase for next year - whenever I use the solar filter at school safety is obviously a major concern. Of late I have taken to restricting its use there to through the camera only. A wedge may be the way to go there . 

Jim 

[Stu]

3 minutes ago, saac said:

I'll happily buy you one from our local bakers Stu if it comes to that , they are quite famous in Fife - their apple or rhubarb pies are absolutely divine     I'll be using my ED80 -  do you know I'm actually looking forward to it, I've always wondered myself what the difference is. I do know that they do give a different contrast - I think I have heard that the Baader film is more likely to give a hint of granulation than the glass type.  High power may be difficult , I've always struggled to get good focus when looking at sun spots  but I will do my best.  I think a wedge may be a potential purchase for next year - whenever I use the solar filter at school safety is obviously a major concern. Of late I have taken to restricting its use there to through the camera only. A wedge may be the way to go there . 

Jim 

Oooh, a nice apple pie sounds delicious! Also partial to rhubarb so I’m not fussy 🤣🤣

I think fine detail in the penumbra of active regions and granulation will be the areas to focus on to pull out the differences. These tend to need higher powers and good to excellent seeing conditions to pick out.

I find that focusing is critical to pulling out the fine detail, so value good dual speed focusers on scopes used for white light solar. A small tweak on the fine focuser can make all the difference.

80mm is at the lower end of where granulation becomes clear, but hopefully you will still get a meaningful comparison out of it.

[Tiny Clanger]

2 hours ago, JamesF said:

I generally use a wedge these days, but my Baader film filters are fairly flat, though they're not under tension.  I just lay the filter on the table and then gently placed the carrier on top to fit it, as far as I recall.

I am wondering though if the reason flatness doesn't matter for the film is that it's only a few wavelengths of light thick (about 10µm, I believe, compared with about 0.5µm for green light), so refraction effects in the medium of the film are small, or perhaps the film just has a refractive index close to 1.  I've not ever had a glass filter in my hands, but I assume they're several millimetres thick and probably have a refractive index somewhere in the region of 1.5, meaning greater distortion of the final image due to refraction if the surfaces are not flat?

James

That would be my assumption on the thickness factor & refraction. The Baader instructions say to drop the film vertically on to the prepared half of your cardboard frame sandwich , if I recall correctly they even specify a height to  release if from 🙂 . I've spent some time trying to find the 'ripples are a good thing' statement, and was beginning to think I'd imagined it, but tracked it down at last, here it is :

"Baader Solar Filters provide a professional and secure solution to observe and image the Sun using AstroSolar^® Safety Film. Many ready made filters do stretch the filter material like a drumhead in order to make it look like a piece of glass – this absolutely does destroy the image quality, rendering the film useless for high magnification work. AstroSolar^® must be mounted entirely stress free in order to perform like a high precision planeoptical window. It sounds like a contradiction with traditional perception of quality, but AstroSolar must show slight ripples! Only when the film is being put under stress by an improper cell mount it performs just like any single side coated glass-filter made of untreated floatglass. "

It's from this page , which is actually for their filter frames : https://astrosolar.com/en/information/news-en/baader-solar-filter/

[Tiny Clanger]

47 minutes ago, Stu said:

Good stuff Jim. I’ll happily eat humble pie if the glass proves to be as good/better.

I'd check out the recipes for humble (or umble) pie first ,  they sound ... offal

Heather

[Starslayer]

53 minutes ago, saac said:

Here you go, as Stu1smartcookie said it fixes over the aperture of the scope. Filters out 99.99% of the light and then some rendering an eye safe image at the eyepiece - no thermal transmission to speak of.   Pre check the filter each time to identify any pin holes in the coating to ensure it is safe. 

Jim 

Thank you for that and as I said I do have one already and it works fine and I am confident in its safety etc.  It was just that earlier posts indicated other types of filter that do not go over the entire aperture end.  

[saac]

15 minutes ago, Tiny Clanger said:

That would be my assumption on the thickness factor & refraction. The Baader instructions say to drop the film vertically on to the prepared half of your cardboard frame sandwich , if I recall correctly they even specify a height to  release if from 🙂 . I've spent some time trying to find the 'ripples are a good thing' statement, and was beginning to think I'd imagined it, but tracked it down at last, here it is :

"Baader Solar Filters provide a professional and secure solution to observe and image the Sun using AstroSolar^® Safety Film. Many ready made filters do stretch the filter material like a drumhead in order to make it look like a piece of glass – this absolutely does destroy the image quality, rendering the film useless for high magnification work. AstroSolar^® must be mounted entirely stress free in order to perform like a high precision planeoptical window. It sounds like a contradiction with traditional perception of quality, but AstroSolar must show slight ripples! Only when the film is being put under stress by an improper cell mount it performs just like any single side coated glass-filter made of untreated floatglass. "

It's from this page , which is actually for their filter frames : https://astrosolar.com/en/information/news-en/baader-solar-filter/

I wonder if the ripple thing is to ensure you don't stress the film by trying to have a totally flat installment.  Stressing it mechanically would introduce the possibility of damage .  Im not convinced on the sensitivity of the glass filters to any refraction effect - bear in mine these are aperture filters providing light prior to collection by the objective and subsequent magnification.  I'd also consider,  as somebody also commented, most white light filters offer an inexpensive introduction to solar imaging and so their output is not going to be pushed to produce images that would be at the limiting end of the optics - other factors such as seeing would be at play.  Again, all I can comment on confidently is from my own use,  I have never had cause to complain of any optical aberration or otherwise from the glass solar filter , it has performed very well. 

Jim