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Barlow/Powermate in front of a blocking filter?

Started by edkess, 04/08/2010 04:36PM
Posted 04/08/2010 04:36PM Opening Post
Is it safe to use a barlow or Powermate in front of a blocking filter for Ha viewing or photography?

I'd like to increase the focal length of my system for higher resolution photography but using the barlow in the diagonal and then adding a camera makes things a bit awkward.

Ed Kessler
Posted 04/09/2010 02:32AM #1

I can't think of any reason why a barlow would compromise safety when used as you suggest. The vast majority of energy rejection is done up front in the etalon.

One issue to be aware of is using a barlow increases the size of the Sun's disk as it passes through the blocking filter. Using a 2x barlow effectivly doubles you scope's focal length, so if your scope has a 700mm focal length and the blocking filter is sized for a maximum 750mm focal length, it will now be dealing with a virtual l400mm scope which will cause vignetting.

One physical work-around is to purchase a shorty barlow so the camera doesn't sit so high above the diagonal. One device I've found handy is a multi power barlow wheel made by Seibert Optics - Mag Wheel Barlow.

Mine gives me three different powers so I can experiment with which one works best without having to remove the camera. It's handy for night work too.

Back to your origonal question, go ahead and try your barlow ahead of your blocking filter and see how it works. It certainly won't hurt anything.

Clear, sunny skies.
Tom Masterson
Coronado-Lunt-Daystar Solar Filters forum Moderator