Review of Baader AstroSolar
go...@megsinet.net
filter material.
In 1981, with a newly purchased full aperture solar filter, I turned
my C8 telescope to the Sun for the first time. I was absolutely amazed
that one could actually view the fiery surface of our daytime star. I
was also amazed at how quickly it changed. It only took a year to add
a hydrogen-alpha solar filter to my observing arsenal. I was hooked
forever on solar observing.
Years have come and gone and new solar observing devices have been
introduced into the amateur market. One leader in my opinion at making
quality solar observing equipment available to the amateur astronomer
has been Baader Planetarium near Munich, Germany. Over the past ten
years they have produced superb full aperture glass solar filters,
energy rejection filters for H-alpha filters and an outstanding
two-inch Herschel wedge prism. All are highly coveted by experienced
solar observers. They have also produced a H-alpha Coronograph for
viewing solar prominences (tested by Don Trombino, S&T, June 1994). I
was surprised during a recent visit to Astro-Physics in Rockford,
Illinois when owner Roland Christen informed me that Baader
Planetarium had developed a very low cost foil type material for solar
viewing. My initial response was less than enthusiastic. Many have
tried to equal mylar solar filters sold by Roger Tuthill and up until
that point none had succeeded. Roland insisted though that I accompany
him to his observatory to view through a 5-inch refractor that he had
prepared a solar filter for with the new Baader AstroSolar Safety
Film. Well, seeing is believing the new Baader solar filter produced a
very sharp image of the Sun with excellent contrast and little
scatter. The sky adjacent to the solar limb was jet black. Roland gave
me a sheet of the new filter material. It was of a photograph density
(ND 3.8). The next day I contacted Astro-Physics to see about
purchasing some AstroSolar film with a visual neutral density. I was
pleasantly surprised to hear that Thomas Baader had a agreed to send
me some of the material in a visual neutral density.
The first thing I must point out is that the Baader AstroSolar Safety
Film is provided in unmounted sheet form. Instructions are provided
for making a homemade cell to attach the filter to your telescope. The
Sun is very dangerous to observe. Please do not under estimate this
fact. When attaching your solar filter for the first time make sure it
is securely attached and cannot fall off or be blown off the telescope
by a gust of wind.
The manufacturer provided the following information on AstroSolar. The
film is patent pending in all 19 European countries. It also has been
tested by the German National Bureau of Standards for eye-safety. The
substrate is not mylar. There are several pre-treatment processes used
before the film is coated. The film is changed to a dark grey color by
an ionizing process. This coloring minimizes internal reflections..
After the coloring, the film is first annealed similar to optical
glass. The substrate material is heated almost to the melting point.
During the heating process the film shrinks and internal stress is
released greatly improving its optical performance. Lastly, the film
is then coated on both sides with a metallic coating. The double
coating prevents pinholes from appearing. It would be interesting to
know more about the composition of the substrate material and
coatings, however, Mr. Baader is not talking. He did indicate that he
tested several hundred types of films on a Zeiss double pass
autocollimator before finding the right material that would withstand
the treatment process.
My first test of the material I received from Baader came on an
evening shortly before sunset. I mounted a piece of the AstroSolar in
a cardboard cell and went outside. I first checked the material by
holding it up to the Sun. The Sun could be seen through the film. I
judged it to be about neutral density 5. The image was white in color
with little scatter and no pinholes apparent. I then mounted it
securely to my 5.1" f/8 Astro-Physics refractor and pointed the scope
towards the Sun. I next viewed the Sun through the telescope with a
star diagonal, but no eyepiece. I again examined the filter for
scatter and pinholes. Again, there appeared to be little scatter and
what appeared to be one small pinhole in one side of the coating. This
small imperfection was faint and not needing to be darkened with a
felt tip pen because of the two-sided coating process. I then placed a
19 mm Panoptic eyepiece in the star diagonal and brought the Sun into
focus. At 53x the Sun's image was an off white color with a slight
blue-grey tint simulating what you would see through a Herschel Wedge
Prism with neutral density filters. The image was sharp with black sky
adjacent to the solar limb. Although seeing wasn't particularly good
this late in the day, granulation was easily seeing as were sharp
boundaries between the sunspot's umbra and penumbra. Faculae were
easily seen well into the disk. The image was bright, however, at low
magnification the image should be fairly bright so that when higher
magnifications are used the image is not too dim.
I again had the opportunity to test this material several days later
on a day were I estimated the seeing to be about two arc-seconds with
10 % cloud cover and a slight haze. I first observed the Sun with a 10
mm Orthoscopic eyepiece with my 5.1" f/8 refractor at just over 100 x.
The image was very sharp with little scatter along the Sun's limb.
Some scatter was seen from the slightly hazy condition that day. The
sky adjacent to the limb was very dark. Faculae were again easily seen
well in from the Sun's limb. I next switched to a 4 mm Orthoscopic
eyepiece. At just over 250 x the image was still sharp. Granulation
was easily seen as well as detail in both the umbra and penumbra. I
next examined Active Region 8611. I counted a total of 53 sunspots in
the group. Later examining the NOAA Space Environment Center Internet
site I saw that the NOAA count was 39 for AR 8611. The image at 250 x
was slightly dim. A head cover to cut ambient light would have further
enhanced contrast.
In summary, I found the new Baader AstroSolar Safety Film to be of
excellent quality for solar observing. Judging from the solar image at
250x, it appears that AstroSolar film is diffraction limited. Test
results supplied by the manufacturer showed a Strehl ratio of 0.941 at
632.8 nanometers (uncoated film tested on a Zygo Interferometer). The
test results can also be found on the Internet by visiting Adirondack
Video Astronomy's home page at: www.astrovid.com. Follow the links to
Baader AstroSolar.
The solar filter material will be offered in both a visual and
photographic neutral density (ND 5 and ND 3.8). Astro-Physics, Inc.
will be the sole U.S. distributor for individuals and dealers. Baader
Planetarium is also offering this material in a clear uncoated format
called TurboFilm to close open tubes and protect lens surfaces. The
price of an unmounted sheet of AstroSolar film large enough to cover
an eight-inch aperture will be $20 (U.S.). At this price and solar
maximum being within a year away, it should introduce many more
observers to solar observing. Again, be very careful when observing
the Sun. Make sure your solar filter is securely mounted to your
telescope and inspect it before every use.
I also had the opportunity to test the substrate material for the
AstroSolar filter. Baader Planetarium is marketing the clear substrate
material as "TurboFilm." I first looked through the clear film in
broad daylight without a telescope. The TurboFilm showed no distortion
and only a very slight but noticeable dimming.
As daylight faded to twilight I turned my refractor towards a three
day old crescent moon. At 58x the Moon appeared very sharp. There was
no noticeable difference in image quality with or without the
TurboFilm. There was no scatter noticeable from the TurboFilm.
Although, again, I noticed a very slight dimming with the TurboFilm. I
then switched to a 10 mm Orthoscopic eyepiece. Again, I could not
notice a difference in the image other than a very slight dimming. The
image sharpness with and without the filter was confirmed by other
members of my astronomy club. I noticed no color shifting or added
scatter caused by the film.
My next test was to conduct a star test with and without the
TurboFilm. My telescope has a very slight over correction. The star
test showed the same fresnel ring pattern both in a out of focus. I
did notice a slight diffraction spike with the filter material in
place. Closer examination of the material showed a slight crease or
fold mark. I confirmed that the crease was causing the diffraction
spike by rotation the TurboFilm and then reexaming the image. My
TurboFilm sample may have been folded somehow in shipment. This
material is cheap enough to by a roll and then select the best pieces
for use.
Gordon Garcia
Ron Wodaski
I appreciate the effort that went into it. I will definitely be making up a
solar filter using the film for my own use. My only concern is about getting
the film flat when I make up the filter holder, but I understand there are
printed instructions that come with the film. Do they deal with this issue?
I've heard that creases and folds "shouldn't matter," but your comment about
the crease showing a diffraction spike makes me think otherwise.
Ron
<go...@megsinet.net> wrote in message
news:38161f0d...@news.megsinet.net...
JMc
Thanks for your excellent review. I've used the Baader film for a couple
of months and optically, it's a definite improvement over the other
mylar and glass products. The only drawback is the safety issue when
people make their own filter holders. This film is _very_ light weight
and is easily pulled loose or blown by the wind. Your warning about
checking the filter frame before each use cannot be over emphasized.
IMO, the film should be glued or taped in place on the edges of the
filter frame, and it should not be used if there is any chance of wind
gusts dislodging it.
Jim McSheehy
Chuck Hancock
>solar filter using the film for my own use. My only concern is about
getting
> >the film flat when I make up the filter holder, but I understand there are
> >printed instructions that come with the film. Do they deal with this issue?
The Astro-Physics web site has detailed, illustrated instructions for
the construction of home-made filter cells. These are the same
instructions that ship with the film. I would recommend getting the 1/2
meter size, and sharing with a friend or two. You can be as neat you
wish, but practice helps, so do your cheapest scope first. Also, do
your finder scopes, for extra cell-making practice. It's really very
easy.
I would not recommend hot glue. I have seen it fail after only a few
years of service. Since regular wood glue never seems to fail over
time, why not use it?
Here's my experience:
I ordered AstroSolar from A-P by phone last Wednesday morning. When I
got home from work on Thursday the very sturdy package containing the
material was waiting on me. (Thanks A-P and UPS, for the prompt
service). That night I made the cell for my FS80WA. The next day, I
was out there, but only to 50x.
I have only observed the Sun once before, and I have no idea what filter
was used. But I would rate the AstroSolar view excellent. I can't wait
to try the C8+. I'll make a full aperature filter, them try different
aperature masking configurations.
Bright skies (!) and...
Best regards,
Chuck Hancock
go...@megsinet.net wrote:
>
> On Tue, 26 Oct 1999 15:08:24 -0700, "Ron Wodaski" <ro...@nwlink.com>
> wrote:
>
> >Gordon, this is an excellent, detailed review with lots of hard information.
> >I appreciate the effort that went into it. I will definitely be making up a
>
> >I've heard that creases and folds "shouldn't matter," but your comment about
> >the crease showing a diffraction spike makes me think otherwise.
> >
> >Ron
> >
> The piece of TurboFilm I received must have been creased in packing or
> shipment. For the AstroSolar I made a cardboard ring with double stick
> tape around the inside of the ring. I dropped the ring on the film and
> trimmed off the excess. This seemed to work okay. Maybe some other SAA
> readers could supply their techniques. There were instructions
> supplied. I usually read the instructions after I screw up a few
> times.
>
> Gordon
go...@megsinet.net
wrote:
>Gordon, this is an excellent, detailed review with lots of hard information.
>I appreciate the effort that went into it. I will definitely be making up a
>solar filter using the film for my own use. My only concern is about getting
>the film flat when I make up the filter holder, but I understand there are
>printed instructions that come with the film. Do they deal with this issue?
>I've heard that creases and folds "shouldn't matter," but your comment about
>the crease showing a diffraction spike makes me think otherwise.
>
>Ron
>