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Filter dilemma - need advice

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#1 lakeorion

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Posted 10 October 2019 - 08:08 PM

I wanted to stuff the filter wheel with the highest quality glass.  But...

 

  • I'm limited to 1.25" filters.
  • Chroma 1.25" filters don't fit. https://www.cloudyni...ma#entry9693265
  • Astrodon filters have a blue cut on lower wavelength than I'm currently using (Baader UV-IR @ 426 nm) and I'm worried about bloated blue stars. https://www.cloudyni...ma#entry9410246
  • If I poke around at specifications, the Astronomik Deep Sky RGB appear to fit the bill, but they are 1mm thick substrate.

 

So the heart of the question is, if I go ahead and get the Astronomik Deep Sky RGB, and I use an OAG, am I now limited to 1mm thick filters for narrowband also?  The Astronomiks are 6 nm bandpass - am I excluding myself from the Astrodon 3 nm bandpass (3 mm substrate) because the filter thickness will change my focus enough to throw off my OAG?

 

Kit currently consists of WO Star 71, QHY OAG-s, QHY CFW3S 7x1.25, QHY 183M.  Usually imaging from an Orange / Yellow light pollution zone north of Detroit.

 

Does 'parfocal' REALLY matter?  I can refocus on filter swap but the OAG is stuck at it's focus and that's it.

 

On the plus side, the cost difference between Chroma/Astrodon and Astronomik for a full RGB IR Ha OIII SII set is almost my next telescope.   lol.gif

 

Thanks in advance for your experience and opinions.



#2 OldManSky

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Posted 10 October 2019 - 10:33 PM

And this is why I don't use an OAG with my refractor...;)

 

But seriously, if you're using PHD for guiding, it may not matter -- it's really good at still finding centroids on slightly out-of-focus stars.  My 60mm guide scope changes focus a bit as the night cools, but I don't refocus it -- and distinctly fuzzy stars guide just as well (according to the guide logs) as the sharply focused ones earlier in the evening.

 

I know how you can test before you invest in the filters, of course -- focus as normal, then move your focuser intentionally 2mm out of focus.  Guide through the now out-of-focus OAG stars, and see how it goes.  If the results are good (and I suspect they will be), fire away with the filters of your choice!



#3 lakeorion

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Posted 11 October 2019 - 11:33 AM

And this is why I don't use an OAG with my refractor...wink.gif

Understood, but the OAG was a significant weight savings over the 60mm guide scope.  When you have a 71mm refractor and a $1300 mount, extra scope weight matters, along with the power savings of an additional dew heater.  Returning to a separate guide scope is an option I hadn't thought of.

 

I know how you can test before you invest in the filters, of course -- focus as normal, then move your focuser intentionally 2mm out of focus.  Guide through the now out-of-focus OAG stars, and see how it goes.  If the results are good (and I suspect they will be), fire away with the filters of your choice!

That is an excellent idea.  Thanks.  Of course it's Friday night so the clouds come rolling in to prevent testing.




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