Tuesday, April 20, 2010

Issues with Contrast

I have long known that my 12.5" telescope suffers from contrast issues.  I would constantly compare my scope to others and see a noticeable difference.  The 16" also suffers from this malady.  Contrast issues can be one of two things, overall contrast (i.e., grey background skies) and small contrast (i.e., fine planetary detail).  My problem is with overall contrast where the faint wisps of galaxies can be lost in the grey background sky.  This type of contrast problem boils down to unwanted light sneaking somehow into the optical path.

I went searching on the net on ways to solve it and I found a post on www.cloudynights.com that discusses how you can identify where that light is coming from.  Mike Jones wrote an article in Astronomy Technology today that describes that technique.  Essentially, you use a long focal length eyepiece and a another eyepiece stacked on top of it.  Mike says in his forum post: "The second eyepiece must have a real focal plane such that you can use it like a loupe, like an orthoscopic, Plossl, etc."

I used a 35mm University Optics Konig and an 18mm University Optics Orthoscopic.

When you look into the top eyepiece, this is the view you get:

As you can see, in my 12.5" scope, the white screws and the pink of the dew heater reflects a lot of light.  You would think that it would not be a problem, but it does contribute to the overall brightening of the background sky.  Any light scattering is a bad thing!  When I did this on my 16", I noticed that there is light reflecting off of the secondary shroud (the piece of metal that holds the secondary in place).  It is painted flat black, but there is enough to cause problems.  I added some flocking paper to the secondary shroud and part of the spider hub:

This noticeably darkened the view at the eyepiece.  I redid the test recently when I was observing at a darker sky and it showed a considerable darkness around the primary mirror.  In other words, the primary mirror appears to float in blackness.

For my 12.5", I plan to do something to block the reflected light from the "stuff" above the secondary mirror.  I also plan to replace the spider and the secondary sometime soon with a Protostar.  But that comes after the 8" is done.

1 comment:

Anonymous said...


Very intriguing post, I may be flocking my secondary holder as well.