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.


NPB filter on guide scope?

  • Please log in to reply
2 replies to this topic

#1 David Ridge

David Ridge

    Vostok 1

  • -----
  • topic starter
  • Posts: 109
  • Joined: 13 Oct 2010
  • Loc: 39.96 -105.25

Posted 15 April 2024 - 02:44 PM

I have been struggling with calibration of my rig using PHD2 with multiple star lost messages per calibration session. I live with a Bortle 5-6 sky, a Meade RCX400, QHY183C, QHYOAG-S and a Lodestar using the Starlight SX-AO (purchased to overcome the limitations of the RCX400 mount). A few months ago, I purchased the DGM Optics NPB filters - 2" and 1.25".


In my battle with calibrating the Starlight SX-AO, I noticed last night that when I used a bright star for calibration (Spica in this case), the background of the Lodestar camera image turned dark, as opposed to my usual calibration position of 0* @ the meridian -- in that case, I couldn't see any stars although PHD2 highlighted several potential guide stars in the "grey mass" of the background. In the last several weeks I encountered advice on one of these forums saying that the filter wheel needs to be BEFORE the OAG to give the guide scope more contrast. But the SX-AO needs to be as close to the OAG as possible, per Starlight's strongly stated advice. 


Has anyone placed a narrow pass band filter JUST on the guide scope to reduce light pollution and increase star contrast? If so, what did you use and what effects did you experience?


Thanks for your insights?

#2 bbasiaga



  • *****
  • Posts: 3,768
  • Joined: 10 May 2006

Posted 15 April 2024 - 03:08 PM

I image in b7 and LP is not a limit for my guide scope. Filters will only reduce the light coming in. Sounds like you need to bump the gain up on the guide cam u til you can see stars in the preview window.

When I do that I see a great background and some white dots...can e ugly visually but the guide software sees them just fine.
  • David Ridge likes this

#3 ButterFly


    Fly Me to the Moon

  • *****
  • Freeware Developers
  • Posts: 6,722
  • Joined: 07 Sep 2018

Posted 15 April 2024 - 03:18 PM

A UV/IR cut does best for me under my Bortle 5 skies.  I don't notice much difference when the moon is out.  You'll need some fairly bright stars for narrower band filters to work well, especially with an OAG with a smaller true field.


The benefit of any filtering is that is makes the bloat smaller and stars tighter.  Guidescopes aren't well known for their excellent color correction, and many astro cameras are sensitive well into near-IR.  My UV/IR cut does a great job of cutting down the bloat by a lot.  The prism in your OAG could also be bloating, so - only one way to find out!


As always, experiment with your setup under your skies.  Simply put, that which works best for you is: that which works best for you.

  • David Ridge likes 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