About that Baader Continuum Filter - Part 1

[Cschur]

Hi all, this weekend is the culmination of a months long project to determine the best green filter for white light imaging. We tried a half dozen top contenders both in the field and on the lab bench, and found some surprising results. I am posting this here FIRST because this group has the largest percentage of very serious solar imagers of any list I know of. The final contenders of the list were: 1. The Custom Scientific G filter from the LRGB filter set, 2. The Baader Continuum Filter, 3. A standard Wratten 58, 4: The Meade DSI G filter, and finally 5: the Lumicon OIII filter.

Each filter was first tested on a visible light and then second a infra red light spectrographs, followed by IR imaging with a solar analog lamp and IR only camera, and then finally - which is why it took so darn long - the daytime field testing. (Lots of clouds this time of year!) At first, and for several months the best filter by far was the CS-G filter. It is really broad band, and even includes some yellow and blue, but performed fantastically on enhancing the faculae, granulation and sunspots contrasts. Nothing would come close. We then purchased a pair of Baader Continuum filters, one 1.25" and another 48mm. Everything you have read on this filter promotes super clarity, unheard of sharpness, and superior contrast. Thats the Advertisments. An extensive survey of visual observers reported quite another thing. Most saw no improvement at all, or barely noticeable. the best seemed to report around a 20% improvement in contrast - really nothing. At the research lab we all compared white light views with the filter in, and well, it DID turn the sun a stunning lime green. Contrast? Couldnt tell.

All hope then was placed on the imaging. I used two different DMK cameras the 31 and 51 and got similar results. Focal ratios from f/6 to f/30 with my 80mm f6 zeiss apochromat were tested. The first results - HORRIBLE. We couldnt believe our eyes. When we went with the filter wheel from CS-G filter back and forth with the baader, the contrast totally vanished, it was all washed out and the view had terrible seeing. When we flipped back to the CS g filter, Sharp, crisp and a 10x improvement in the fuzzy seeing. Jet black sunspots. Granulation like a gravel pit. Rock soid. What was going on here? This was very puzzling because SOME imagers were getting great results with it by itself. (AH)

Back at the lab we extensively tested the baader vs the G filters on the spectrographs. And what a shocker we discovered! While the CS-G , DSI-G filters had a clearly visible green passband surrounded by black on both sides in the spectrum, the Continuum had a HUGE red leak. Not only that, when we examined it with both the IR spectrograph and IR camera, the filter was nearly transparent in IR. This is called the "Red Leak" in the optical industry. Now it was all making sense. Time to go back to the telescope, this time - with an IR blocking filter in hand!

As you may have guessed, it worked very well with the IR blocker in series. The difference in exposure with and without the clear (to the eye) IR blocker was 100x or more. The blinding light if the IR was not only leaking over and blowing away the tiny narrow band green signal, but it was not the same focus either and put a big bright mushy light over the green image, making a big mess of it. And why doesnt the CS-G fitlter do this? No IR leak because the glass they use is Schott glass instead of pyrex or something similar and schott glass blocks ir naturally. And why do a few imagers scopes NOT have the IR problem with this filter when most do? Because their objective lenses are better corrected in IR so that it comes to focus much closer to the visible focus. (my Zeiss obviously sucks in this regard)

Now here is the good news. With an IR blocker such as the Baader IR/UV blocker, Astronomics, Orion, etc. you can make this filter totally useable, and even if your refractor is well corrected, it can still be improved a bit. The DMK series mono, does not have a built in IR blocker. The CCD is most sensitive to IR, and well there you have it.

Final results. So how DID the continuum filter + IR do compared with the CS-G filter anyway? Twice the contrast on both granulation, faculae and sunspots. Seeing? It changes it in the following way. In poor seeing, in the G filter, you see a pulsing throbing image moving in and out of focus but more or less sitting in one spot. With the narrow band continuum filter, you see a sharp image that is constantly distorting and moving around, but doesnt get fuzzy. Which is better for imaging has yet to be determined, but the overall impression on the computer screen this morning of the large sunspot was a slowly rolling distortion with sharp details in the granulation and penubral filaments. So pull that continuum filter back out of the dresser drawer, get an IR blocker and be ready to take the imaging ride of your life!

Images below:

For part 1, Im going to post these full size 1600 x 1200 DMK51 shots. Three images here, (look at the file name for description). One is the CS-G filter, second Continuum no blocker, and the third continuum + Meade IR blocker. Compare the faculae, and granulation and especially the limb darkening contrasts. Thanks for looking. There is more to come on this subject in the works...

[marktownley]

Well, this is a very comprehensive and eye opening report Chris. When the sun finally returns higher in the sky it looks like next year is going to have a whole load of experiments going on... Thanks very much for all your time and effort with this! B)

Mark

[marktownley]

It would be great to see TiO over the continuum

I've done this and found the TiO to be considerably better - certainly for my consistently poor seeing conditions. Will be interesting to try the continuum with a IR block with it...

[Pedro]

Hi Chris:

Very interesting results. I always use my Baader Solar Continuum together with a Baader ND 3.0. I guess this should also have a UV/IR correction since I get good results with the Solar Continuum.

According to Baader the Solar Continuum should be used with a UV/IR filter:

http://www.alpineastro.com/Solar_Observ ... _Continuum

The new Baader Solar Continuum filter is designed to enhance the visibility of solar granulation and sunspot details. By transmitting a specific spectral region around 540nm, free of emission and absorption lines, the Solar Continuum filter is able to boost contrast and reduce the effects of atmospheric turbulence. With the Solar Continuum filter in place, images snap to focus, and granulation becomes regularly visible. Details at the limit of visibility become easier to hold, and image motion reduced.

The Solar Continuum filter works well in all types of telescopes, for both visual and imaging. Users of SCTs and achromatic refractors will find it particularly beneficial, as it completely excludes the red and blue wavelengths, and centers on the peak visual wavelengths where the telescope optics are sharpest and free from chromatic aberrations. For digital imaging, we also recommend the Continuum filter be combined with our UV/IR Cut Filter (to completely cut the defocused far infrared wavelengths).

The Solar Continuum filter also makes the perfect star test filter. It's narrow bandpass and complete blockage of longer and shorter wavelengths make it superior to standard green colored filters for testing refractors at their center design wavelength (and peak visual wavelength). With this filter, more accurate appraisals of optical quality can be made, without the effects of spherochromatism or secondary color error. Due to the narrow 10nm bandpass, star testing through the Solar Continuum filter will require a bright test star.

http://www.alpineastro.com/Solar_Observ ... tinuum.gif

Thanks for sharing

best regards

[marktownley]

According to Baader the Solar Continuum should be used with a UV/IR filter:

There you go, there's a new one on me!!!

[Pedro]

It ia new to me too. I just checked the Alpine Astro website.

best

[Cschur]

Baader metions on thier english web site very little on this, and I found it mentioned in german, but it suggested that performance might be improved, not essential. I do not think the baader ND filters do much for the IR. I tested the ND1.8 and found it transmitted all of it minus of course the overall attenuation factor. Anyway, the image looks like crap in my baader wedge I just got with the continuum and ND filters in it, unless you put a IR block in the camera nose piece. The wratten 58 I tested today with the IR block actually did very well too. Im going to order a baader UV/IR block when the christmas rush is over with. Everyone should have one I think! :cheer:

[Pedro]

Thanks Chris:

I will try to use a UV/IR to see if it improves the quality of the image. However I get decent results with the Solar Continuum + Baader ND 3.0:

http://re.apaaweb.com/sun_20111030_dmk41.jpg
SUN (20111030), AR11330. Takahashi FS128, F/8.1 (X2 Barlow), 2" Lunt Solar Wedge, Baader Solar Continum filter, DMK41, Stack of 1000 images.

http://re.apaaweb.com/sun_20111030_dmk41_mosaic.jpg
SUN (20111030). Takahashi FS128, F/8.1, 2" Lunt Solar Wedge, Baader Solar Continum filter, DMK41, four-panel mosaic (stack of 1000 images each panel).

best

[earl]

This is great stuff and thanks for posting your results. I have always used the UV/IR with my continuum. I found that without it the images were blown out and ghostly. In fact, I just ordered a new 2" continuum and 2" UV/IR for my new eyepieces. Finally broke down and boght a 2" eyepiece for CBSAP events. Man ... those things are heavy.

[Cschur]

Pedro, I first tried the 3.0ND, and the image was too dim for the 5x. I then went to the 1.8nD and I was getting around 1/333 seconds for proper disk exposure. I think I might try the .9ND and see if going around 1/1000 sec might freeze the seeing better. I was not able to get my camera in focus at prime focus, the tube would have to be an inch shorter!

[Pedro]

Hi Chris:

I also change ND filters from time to time. When I image in the Calcium region I use only the 1.8 or 0.9 with the Baader CaK filter. My results are decent without using a UV/IR cut filter. This is not the same for others in this list. Now that you mentioned it I will try using more filters.

best regards

[Montana]

Thanks Chris! that was a very beautifully written and comprehensive guide. I was also conned by the above graph, you think if they aren't showing it, there is nothing to see.

I've never had a problem with fuzzy seeing with my telescope and this filter (Continuum) always wins above all the others I have tried. However I have noticed (and never understood) that the lower the ND filter I put in, the worse my image gets. Now technically it should get better because of shorter exposure but I noticed the camera got hotter, I got more noise and the image was worse and next to impossible to process. My sweet spot seemed to be the ND1.8 + 0.6. I am very excited that I got a UV/IR filter for Christmas, I never guessed I would be using it for solar but this might just clear up why I get this phenomenon. I am extremely excited, anything to see and improve gorgeous granulation is essential. I have had my eyes opened.

Many thanks Chris,
Alexandra

[solarchat]

Im not sure why people use a CaK filter after a white light wedge anyway. Wishful thinking?

[Cschur]

Stephen, I agree, I would think the image would be way too dim. Mine is about right with my Lunt back filter, who would want to dim it? Maybe with a custom or that not too great Baader CaK filter.

> I am extremely excited, anything to see and improve gorgeous granulation is essential. I have had my eyes opened.

I think granulation is gorgeous too!

We also testted with the IR spectrograph all the baader ND filters, and they passed all the IR minus the normal w hite light attenuation factor. I think the reason some of you can get away with no IR blocker is that your object lenses correct the IR better to match the same focus as the white. Most refractor lens makers optimise the correction for yellow/green light. What happens in IR is usually irrelivant since the eye cant see it anyway. But we can sure image it!

[Merlin66]

I don't have the curves with me but I think the Baader Solar film effectively blocks the IR.
(I remember there were some resonance bands....)
So, the visual combination of the Continuum with the Solar film would be a good combo?
The Uv-IR would be required for Herschel wedge users.....

[marktownley]

I don't have the curves with me but I think the Baader Solar film effectively blocks the IR.
(I remember there were some resonance bands....)
So, the visual combination of the Continuum with the Solar film would be a good combo?
The Uv-IR would be required for Herschel wedge users.....

Here's a curve for the ND5

http://www.threehillsobservatory.co.uk/ ... tra_23.htm

[Cschur]

Merlin, the Astro solar film blasts the sensors with tons of IR, so much so that we must use IR blockers when shooting leaky filters like the Continuum. The wedge is a different story - I tested mine today for IR, and the combination of coatings on both the wedge and ND filters blocks MOST of the IR, but not all. Thats why some people can get away with out an IR blocker with the wedge. BUT by adding the IR blocker - the light cut in half, and the contrast doubled. You get a good benefit when using the unfiltered DMK cameras and an IR blocker.

[Pedro]

Hi Chris:

Thanks for that. I will try it tomorrow.

best regards

[marktownley]

That's a great comparison Neo, thanks!

I wonder how much altitude asl affects the amount of IR leak...

[Pedro]

WITH UV/IR filter



WITHOUT UV/IR filter

This morning I did a series of pics with and without the UV/IR filter. The conditions were more or less the same in terms of seeing (sun low in the sky - about 20 degrees). I used exactly the same wavelets parameters in Registax 6 and the same functions in PS CS 5 (unsharp mask maily). Here are the results.

The Baader Continnum filter together with the UV/IR filter in my setup is slightly better in terms of contrast. The images with and without the UV/IR are not very different. I will recommend using the UV/IR filter just in case.

My Taka FS128 is very well corrected in the near IR region. This might be a possible explanation for these results.

LINKS

http://re.apaaweb.com/sun_20111228_FS12 ... mosaic.jpg
SUN (20111228) DISK. Takahashi FS128, F/8.1, 2" Lunt Solar Wedge, Baader Solar Continuum filter, DMK41, six-panel mosaic (stack of 250 images each panel).

http://re.apaaweb.com/sun_20111228_FS128_DISK_UV_IR.jpg
SUN (20111228) DISK. Takahashi FS128, F/8.1, 2" Lunt Solar Wedge, Baader Solar Continuum filter + UV/IR filter, DMK41, six-panel mosaic (stack of 500 images each panel).



best regards

[Cschur]

Pedro, the image with the IR blocker shows the faculae a bit better. Again, most people do not have a wedge like us, they use either a glass or baader film. These have a horrific amount of IR that comes through as far as imaging. While the ND filters do little to block the IR, I think on the wedge the coatings on the prism maybe cut it down a lot. I would be interested in seeing the comparison say with a glass thousand oaks filter with and without the IR block! Moral of the story: USE the IR block for all white light imaging for optimal results.

[Pedro]

I agree with you Chris. The moral of the story should be to use the UV/IR filter at all times. I tried two different UV/IR filters today. One from astronomik and another from Brighstar (a Portuguese dealer). The results were similar. Overall slightly better contrast with the UV/IR filter on. I always image using solar wedges but I an try with the Thousand Oaks Glass filter and Astrosolar as well. Maybe tomorrow (the weather here is always fine...)

best

[marktownley]

What an informative thread - thanks all

[marktownley]

I found a transmission curve for the continuum filter, here it is...

[Cschur]

Egads Mark! Where did you find that little gem? Now THATS the one Baader should be advertising on thier web site for selling the filter. Unfortunately then no one would buy it!

Ill add that to my gree filter write up. Good find!

[solarchat]

you guys are incredible...

[Pedro]

Very interesting Mark. The mistery is solved. My guess is that it should be easy to correct the IR leak.

best

[Merlin66]

Chris, et al,
I think for visual observing with a Baader Solar Film it works very well....
The response of the CCD chips falls off pretty quickly in the NIR; ie ICX098BL = 50% at 700nm and only 10% at 900nm....
(I do agree for maximum performance a UV-IR should be fitted)

[Cschur]

>I think for visual observing with a Baader Solar Film it works very well....

Yeah, sure, if you can stand a lime green sun! I cant, it harks back to the old days of the tasco screw in solar filter that colored the sun green, and then exploded blinding you permanently. If you can stand the green, I have a lot of respect for you... :dry:

[Pedro]

I also like to observe the sun with the Baader Astrosolar. I can't stand a green or a yellow (thousnad oaks) sun. Faculae are better seen in a white sun. Juts my two cents.

I also had the terrible experience of breaking an eyepiece sun filter using a 100 mm F/10 reflector (around 1972). No one was looking through the telescope...

best