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

Does it matter if your Bahtinov mask fits poorly and is off center?

  • Please log in to reply
8 replies to this topic

#1 Borodog

Borodog

    Viking 1

  • -----
  • topic starter
  • Posts: 945
  • Joined: 26 Oct 2020

Posted 01 December 2020 - 07:32 PM

I ordered a Bahtinov mask for my C8. I specifically ordered one that was for older C8s. The one I was sent is marked that it is for a scope with "Hyperstar" (whatever that is) and it does not fit well at all. The center cutout for the secondary mirror is at least half an inch too large and hence the mask hangs off center and substantial light is let in around the secondary. Will this affect the functioning of the mask, i.e. the position of the central line that shifts with focus?

 



#2 petert913

petert913

    Skylab

  • *****
  • Posts: 4,457
  • Joined: 27 May 2013
  • Loc: Silverton, OR

Posted 01 December 2020 - 07:47 PM

I've often wondered that , too.  I have certainly had my mask off-center and the spikes show up just fine.  So...... ?


  • Borodog likes this

#3 Borodog

Borodog

    Viking 1

  • -----
  • topic starter
  • Posts: 945
  • Joined: 26 Oct 2020

Posted 01 December 2020 - 08:11 PM

Yeah, I see the spikes. I'm just wondering if the off center mask will *also* shift the central line, which would defeat the purpose.



#4 Sacred Heart

Sacred Heart

    Messenger

  • *****
  • Posts: 410
  • Joined: 16 Aug 2020

Posted 01 December 2020 - 10:43 PM

I would think so longs as you have the pattern on screen, see the cross and straight line, you should be good to go. There is a guy on youtube that made his mask and a few lines broke. They were missing, a bigger gap, did not matter much.

    I'm thinking if you are off center, it will still work, your X or straight line may be shorter.  An off axis solar filter does not show the Sun in one corner or another - it is in the center.  Your pattern may not be in the center but should still focus.

               Joe


  • Borodog likes this

#5 dcaponeii

dcaponeii

    Viking 1

  • -----
  • Posts: 689
  • Joined: 01 Sep 2019

Posted 03 December 2020 - 06:23 PM

I would err on the side of needing it centered. You can add “fingers” to hold it in position or a spacer that fits around the secondary mount. Since the mask works by generating a symmetrical diffraction pattern when in focus, the symmetry may be affected by having the mask off center.
  • Borodog likes this

#6 kathyastro

kathyastro

    Mercury-Atlas

  • *****
  • Posts: 2,621
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 03 December 2020 - 06:49 PM

The diffraction pattern depends on the angles between the three segments of lines.  Those angles won't change if it is off-centre. 

 

However, the light leakage around the secondary will create another diffraction pattern to each star.  Probably not enough to worry about.  Your best bet is to make a new centre for the mask that fits better and tape it to the mask.


  • Borodog likes this

#7 Borodog

Borodog

    Viking 1

  • -----
  • topic starter
  • Posts: 945
  • Joined: 26 Oct 2020

Posted 03 December 2020 - 07:10 PM

The diffraction pattern depends on the angles between the three segments of lines. Those angles won't change if it is off-centre.

However, the light leakage around the secondary will create another diffraction pattern to each star. Probably not enough to worry about. Your best bet is to make a new centre for the mask that fits better and tape it to the mask.

Exactly what I’ve done (except glued).

Edited by Borodog, 04 December 2020 - 06:41 AM.

  • kathyastro and dcaponeii like this

#8 Zippysky13752

Zippysky13752

    Lift Off

  • *****
  • Posts: 13
  • Joined: 25 Nov 2019
  • Loc: Florida @ 30 Degrees Latitude

Posted 03 December 2020 - 09:13 PM

I have a long dew shield and have held the mask up to focus stars to avoid bumping the setup.  I have never had a problem with the mask not being perfectly on-center or having gap around the secondary

 

My opinion - think you will be ok.



#9 Kokatha man

Kokatha man

    Hubble

  • *****
  • Posts: 15,072
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 04 December 2020 - 01:01 AM

I have a long dew shield and have held the mask up to focus stars to avoid bumping the setup.  I have never had a problem with the mask not being perfectly on-center or having gap around the secondary

 

My opinion - think you will be ok.

My opinion - ditch that mask...I'm unaware of any hi-res imagers who use such devices to focus any planet..! ;)


  • John Boudreau, KiwiRay and dcaponeii like this


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