Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Mylar solar filter

  • Please log in to reply
8 replies to this topic

#1 BillHarris

BillHarris

    Messenger

  • -----
  • topic starter
  • Posts: 464
  • Joined: 29 Nov 2017
  • Loc: northwest Alabama

Posted 30 May 2019 - 09:39 AM

I just came across a piece of that double-coated aluminized Mylar solar filter material that used to be popular years ago. I wonder how that would work on a Q3.5? Get an old dust cap, open a 3" hole in it, a stretch the mylar over it.
  • Matt Looby likes this

#2 JGass

JGass

    Viking 1

  • *****
  • Posts: 537
  • Joined: 13 Aug 2009
  • Loc: MD

Posted 30 May 2019 - 10:13 AM

Bill,

 

I believe that the current standard for filters for direct observation of the Sun was issued in 2015.  Filters and filter material produced prior to that may not meet the current standard.

 

Also, solar filter materials come in different densities - especially on a mylar substrate.  A less dense material is sold for photography only, and that is not eye safe.  It is designed to protect the camera sensor, while distorting fine surface features less than the eye safe material.

 

In principle, using mylar solar filter material for a telescope aperture filter is fine.  But, you shoule be certain that the sample you have is undamaged and filters fully enough for safe, comfortable viewing.

 

BTW, in fabric and craft stores, embroidary hoops can be found that can be used to clamp the material in place.  Fitting one of these securely on the Questar may take some thought and experimentation.

 

Just my thoughts on the subject.



#3 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9128
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 30 May 2019 - 10:21 AM

 If the material your talking about is the Tuthill Solar Screen then the issue with the two piece Mylar filter material is that the Mylar is stretched in one direction when it is made. So you get distortion caused by this. It is also a two piece system so 2x the scatter. 

  You'll get a much nicer image from Baader film. It is not stretched like Mylar and has much better optical properties  and uses a single coating so much less scatter. 

 

                     - Dave 



#4 BillHarris

BillHarris

    Messenger

  • -----
  • topic starter
  • Posts: 464
  • Joined: 29 Nov 2017
  • Loc: northwest Alabama

Posted 30 May 2019 - 11:25 AM

This was a piece I had left over from 1981 when I set my 8" up for a solar eclipse. I have no idea how well that scrap would work.

#5 Gregory Gross

Gregory Gross

    Mariner 2

  • -----
  • Posts: 266
  • Joined: 13 May 2017
  • Loc: Pacific Northwest

Posted 30 May 2019 - 12:00 PM

From what I understand, any kind of mylar-like material whether it be Baader film or otherwise should *NOT* be stretched in any way across whatever cell is holding it. Refer to Baader's how-to page for details on building your own custom filter cell with their material.
 
What's more, I definitely recall that my paper eclipse glasses that I had during the 2017 total eclipse indicated that there was a limited shelf life for the filter material. If your material dates from 1981, I would *definitely* throw it out.
 
The attitude I generally take with solar observing is that my precious eyesight is worth far, far more than whatever cost I'd incur by replacing any solar observing gear whose safety I have even the slightest doubts about.

#6 RobertPettengill

RobertPettengill

    Messenger

  • -----
  • Posts: 416
  • Joined: 06 Jan 2013
  • Loc: Austin, Texas

Posted 30 May 2019 - 12:47 PM

I don't believe that any of the available solar filter for telescopes are certified under the standards used for consumer eclipse glasses.  Telescopes would have to be certified for specific telescope apertures, focal lengths, and eyepieces - an impractical burden.  Fortunately astronomers and telescope makers have lots of experience observing the sun. Used as intended telescope solar filters are safe. 

 

Most astronomical filters and films are rated using the ND rating.  The percentage transmission of a ND filter is defined as:

Percent_Transmission = 10^(-OD) * 100

There are other rating systems used as well and similar numbers in different systems can give very different results.

 

Most telescope filters use ND 5.0 which is 0.01% transmission.

 

Baader is the gold standard for high quality film filters.  As pointed out by Gregory the material should never be stretched as this will result in a distorted image.  Counterintuitively wrinkles and creases have no effect on the image because of the extreme thinness of the material.

 

In addition Baader sells filters rated at ND 3.8 intended for use with cameras.  The brighter image allows shorter exposures which capture less seeing distortion.  NDs lower than 3.8 may allow camera sensors to be damaged.

 

Questar glass filters are specified at ND 4.5.  At shorter focal lengths, I find viewing the full solar disk quite uncomfortable with some of the Questar filters.  Other's report no problems.  I don't know if this is because of variations in the Questar filters or choice of eyepiece focal length.

 

Beyond about 4 or 5 inches of aperture, sub aperture filters are often used to limit the intensity of the image.

 

There are lots of different filter cell designs that you can make or purchase for the Baader film.  After the 2017 total eclipse I've grown particularly fond of the Baader's own wide collar style of filter cells for observing total solar eclipses:

 

* The wide collar shades your telescope and camera from direct sunlight, minimizing heat buildup in a long session.

 

* The external three rubber bumper mounting is easy to adjust to allow ventilation to minimize heat build up between it and the telescope/lens.  The Questar cell for its glass filter has little vents for this purpose

 

* It is easy to remove and replace the filter quickly without disturbing your aim at the beginning and ending of totality. 

 

I have a note with images of some different filter styles showing the resolution and colors of solar images taken on the same day:

http://astronomy.rob...ghtFilters.html


  • Matt Looby likes this

#7 spereira

spereira

    Apollo

  • *****
  • Moderators
  • Posts: 1335
  • Joined: 21 Apr 2017
  • Loc: Bedford, NH

Posted 30 May 2019 - 01:40 PM

... I find viewing the full solar disk quite uncomfortable with some of the Questar filters.  Other's report no problems.  I don't know if this is because of variations in the Questar filters or choice of eyepiece focal length. ...

After I purchased my Q last year, I found that the Thousand Oaks front mounting full aperture white light solar filter I had for my TV-85 fit the front of my Q quite well also.  I set up with it to do some white light observing, and I found the Sun's image was quite too intense for me, and I went back to the Questar Solar Filter for a much more comfortable experience.

 

In hindsight, I now wonder about why the view of the Sun I get through the TV-85 does not seem quite so intense as to cause similar discomfort?  The TV-85 is 85mm aperture vs the Q's 89mm aperture.  The difference is that the TV-85 is f/7 and the Q is f/13 or f/14.  Is that what makes the difference of intensity of the view?

 

smp


Edited by spereira, 30 May 2019 - 01:40 PM.


#8 Loren Gibson

Loren Gibson

    Mariner 2

  • -----
  • Posts: 205
  • Joined: 04 Jun 2013
  • Loc: Northern Florida, USA

Posted 30 May 2019 - 06:44 PM

After I purchased my Q last year, I found that the Thousand Oaks front mounting full aperture white light solar filter I had for my TV-85 fit the front of my Q quite well also.  I set up with it to do some white light observing, and I found the Sun's image was quite too intense for me, and I went back to the Questar Solar Filter for a much more comfortable experience.

 

In hindsight, I now wonder about why the view of the Sun I get through the TV-85 does not seem quite so intense as to cause similar discomfort?  The TV-85 is 85mm aperture vs the Q's 89mm aperture.  The difference is that the TV-85 is f/7 and the Q is f/13 or f/14.  Is that what makes the difference of intensity of the view?

 

smp

 

Are you comparing your perception of the apparent intensity in the two scopes at equal magnifications?

 

Loren



#9 spereira

spereira

    Apollo

  • *****
  • Moderators
  • Posts: 1335
  • Joined: 21 Apr 2017
  • Loc: Bedford, NH

Posted 30 May 2019 - 08:01 PM

Are you comparing your perception of the apparent intensity in the two scopes at equal magnifications?

 

Loren

No, probably not.  Never had the two for a side-by-side comparison.  Both scopes can provide a full disk image, and, as I recall, I usually try to get a reasonably large full disk image.  I generally use the white light view to get my TV-85 aligned before transitioning to my Quark.  IIRC, with a “medium size” full disk image on my Q, I was experiencing discomfort from the image intensity.  That image size would be something I could use for centering and alignment on the TV-85, but who knows?

 

Not really a big deal for me.  My surprise was with the discomfort.  I’ve done a fair amount of Solar observing across scopes from 8” down to 3”.  I’ve never before had discomfort from an image that was too intense.

 

smp


Edited by spereira, 30 May 2019 - 08:02 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics