University Lowbrow Astronomers

Adventures in Flocking.

by Nathan Murphy
Printed in Reflections: April, 2005.

I recently flocked the tube of my 8” Orion XT. What is “flocking?” Flocking paper itself is nothing more than velvet or felt-like fuzzy stuff backed with paper. Some is self-adhesive, some not. I bought a roll of the non-self-adhesive stuff from Duchek Consulting Services. Flocking is simply the act of installing flocking paper.

At first, I was going to flock the entire tube, but two things kept me from trying this:

  1. He would perform the flashlight test to dispel grumbling from shroud-lovers: An observer would look in the focuser tube, and have to state whether the flashlight (pointing obliquely into the tube) was on or off. Needless to say, they could not discern when it was on. Mr. Bartels referred to flocked wood-box dobs as Conestoga Wagons, a comparison I felt apt - what say you, John Causland? This leads me to think that maybe rather than go to the trouble of flocking an entire 4’ tube, I should just flock the top and bottom.
  2. Spray-on adhesive is cumbersome, messy, sticky, awful, wretched, evil and unfortunately, infinitely useful stuff. The less I had to deal with it, the better.

So, I flocked the inside of the tube opposite the focuser (about 8” in from the edge, half the circumference) and the entire tube about 8” up from the bottom (where the primary sits). If you want the gory play-by-play on my adventures with 3M spray adhesive, let me know....

Before the flocking, I stuck my head in the top of the tube and tried to block as much light coming in as I could, to see how “dark” the tube was. I noticed a faint glow coming up from around the primary cell. I believe this was reflected light scattered by the primary, as that end was resting on the carpet, and no light (more or less) should come up from there.

After I flocked it, the entire tube was dark - really dark. The flocking ate up all stray light reflected from the primary, and even the little bit of light coming in from around my head and arms covering the top of tube. Wow. Since I had both the primary cell and the spider/secondary assembly out of the tube, I figured I would blacken the edges of my mirrors. They do that for eyepiece lenses, so it couldn’t hurt my mirrors. So the edges of both the secondary and primary are now black from a permanent marker. I’ve no idea if the blackened edge of the primary prevents light scatter down at the bottom, but now aligning the secondary and focuser are quite difficult, as the tube is very black, and the edge of the secondary is invisible against the flocking! Hopefully, this will translate to improved contrast at the eyepiece.

Observing from my front porch I noticed some interesting developments:

I was looking at M81/M82 which involved pointing the scope just above an orange globe streetlight across from my house. This is typically very bad for night vision, and worse for galaxies. Without dark-adaptation and horrible seeing and transparency, I was able to get a dust lane in M82 with averted vision at 75x. Not a big deal, but considering the conditions, I was very surprised. At higher magnifications, it seemed that the sky was a bit darker than I was used to seeing in my eyepieces. So it seems, at first, that the contrast has improved - but a long night at Peach Mountain looking at the same objects that I saw at the 5 March open house will be the true before-and-after test.

As for scattered light rejection, when the scope was pointed just over the streetlight, there was orange light entering the tube at about 30° from the top, casting light over the top of the OTA and almost into the focuser tube. Ouch. That should ruin my image and nearly blind me, right? I looked in the eyepiece and there was no orange glow (other than the Ann Arbor skyglow on very un-transparent skies). I removed the eyepiece and looked down the drawtube, and there was no light coming in. The tube was totally dark.

Orion paints the inside of their dobsonian tubes flat black, but they still reflect a lot of light. By flocking the inside of the tube opposite the focuser, I’ve eliminated a considerable source of contrast-robbing reflected stray light. Flocking the bottom of the tube has prevented the stray light which comes from in front of or behind the primary from entering the focal path.

After doing some Cheshire tube collimation and checking my focuser/secondary alignment, I noticed that sometimes I get light coming up into my focuser tube. Tsk Tsk! The focuser has gone back to Moonlite Telescopes for a drawtube upgrade, and when it comes back, it will get flocking in the drawtube.

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This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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