Dismiss Notice
New Cookie Policy
On May 24, 2018, we published revised versions of our Terms and Rules and Cookie Policy. Your use of AstronomyConnect.com’s services is subject to these revised terms.

Measuring Double Stars

Discussion in 'General Astronomy Chat' started by Seer, Jan 1, 2019.

Measuring Double Stars

Started by Seer on Jan 1, 2019 at 7:05 PM

7 Replies 444 Views 0 Likes

Reply to Thread Post New Thread
  1. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA

    I was just reading about measuring double stars in PRACTICAL ASTRONOMY by STORM DUNLOP. It mentions a few devices and processes that are used such as filar micrometer, binocular micrometer, diffraction micrometer, and it describes a special mask that can be used with reflecting telescopes to modify the diffraction pattern to help reveal close companions, and also using spectra. I wanted to learn more and found this video on YouTube. I would like to learn more about the mask that the book mentions and describes. From the description it is not a Bahtinov Focus Mask. The book says it typically has 6 or 8 straight sides.
     
  2. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA
    Last edited: Jan 2, 2019
  3. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA
    I found the authors email address and just sent him an email asking him about the mask.
     
  4. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA
    I just got a reply from Storm Dunlop answering my question. I am waiting for his approval to post it here.
     
  5. Gabby76

    Gabby76 Well-Known Member

    Joined:
    Dec 16, 2016
    Posts:
    449
    Trophy Points:
    43
    Location:
    Slovakia/ Canada
  6. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA
    WOW, that is very expensive!
    Celestron use to sell one and I think Meade still does.
    I would like to do Astrometrics.
     
  7. Seer

    Seer Well-Known Member

    Joined:
    Oct 29, 2018
    Posts:
    341
    Trophy Points:
    28
    Location:
    Johnstown, Pennsylvania USA
    I got permission to use the email and here it is.

    Your email eventually reached me. It had been shuffled aside as 'spam' by my ISP. The passage in question was written so many years ago that I have completely forgotten where the information came from.

    I do know that the mask used for double-star work is nothing like a Hartmann mask. The latter uses two circular apertures to help to determine the focal length of an objective and also for figuring mirrors.

    There are two types of device for improving the visibility of the fainter companion in double stars: apodizing screens and aperture masks. Apodizing screens modify the Airy disk so that the companion is not obscured by the bright rings that surround the central image of the bright star.

    Aperture masks modify the shape of the telescope's aperture from circular to hexagonal or octagonal. In doing so, they either introduce diffraction spikes (in a refractor) or modify the spikes caused by the spider holding the secondary (in reflectors). By turning the mask, the location of the spikes may be altered so that the comes appears between the main diffraction spikes.

    The only reference that I can find immediately to hand is the Webb Society Deep-Sky Observer's Handbook: Vol.1: Double Stars, p.18 (Enslow, New Jersey / Lutterworth, London 1979). This epecifically mentions a hexagonal mask to be used over the aperture. Offhand, I cannot remember (or find) the reference to an octagonal mask.

    I contacted an experienced double-star observer about this, and this is what he had to say:

    > The hexagonal mask is usually used on refractors and operates as you
    > suggest. I'm not sure what it would do to the diffraction pattern of a
    > reflector. I've not heard of octagonal masks - that would make finding
    > stars like Sirius B more tricky I suspect as you'd get 8 diffraction
    > spikes. I believe some of the space imaging people use square masks
    > and lentil-shaped apertures were used with the 24-inch at Lembang to
    > photographically image Procyon B - much more difficult than Sirius B.

    Although it makes no difference to the query, where are you located? I assume that you are somewhere in North America, because you had a Firefly edition of the book. (I'd even forgotten that they did a reprint.)

    Yours faithfully,

    Storm Dunlop
     
    Gabby76 likes this.
  8. Gabby76

    Gabby76 Well-Known Member

    Joined:
    Dec 16, 2016
    Posts:
    449
    Trophy Points:
    43
    Location:
    Slovakia/ Canada
    The Celestron is a rebranded Baader. Unfortunately they only offer the 12.5mm version now as they used to offer a 9mm also.
    It can still be found on the used market occasionally.
    I am not sure how well the apodizing mask would work with your refractors, I use mine with the 150mm and it does a reasonable job.
    If you do end up getting a reticle eyepiece, the AAVSO has a section of "forgotten doubles" these are double stars that have not been re-visited in quite some time.
    You can find out how they want the report written up and then do some observations for them :)
     

Share This Page