If you have a Dobsonian or Newtonian reflector, flocking is a great way to reduce stray light and improve the contrast. What is flocking? It simply means lining or painting the inside of your telescope with something that absorbs light—ideally something that absorbs close to 100% of visible light. Most telescope insides are painted flat black, but that usually doesn’t do a particularly good job of absorbing incident light. If you don’t believe me, look down the inside of your telescope tube in the daylight: is it truly pitch black or just dark gray?

Flocking is particularly helpful in light-polluted environments because there is so much more stray light to eliminate. Think of it this way: if you are in a really, really dark site, flocking doesn’t matter much because there is essentially no stray light. In fact, in really dark skies, you don’t even need a light shroud around a truss-tube telescope—there is just no light to leak into your light path. In contrast, your typical urban or sub-urban environment has plenty of stray light sources: the neighbor’s porch light or bedroom light, the nearby street light, the sky-glow from downtown, and even the light-gray background of the sky itself. If any of this light gets into your eyepiece, it contributes to brightening the background and killing the contrast.

Now flocking is pretty easy and one of the secrets is that you only need to flock relatively small portions of your telescope to get essentially all of the benefits...

What to flock

There is nothing wrong with flocking the entire inside of your telescope tube. You’ll have to disassemble the scope to do that, and you’ll need a fair amount of flocking material. But you really only need to flock a much smaller area: just look through your eyepiece tube (without any eyepieces) and whatever parts of the inside of your telescope you can see, those are the parts you need to flock. This will generally be four areas:

• the areas of the telescope tube opposite your focuser (the areas that appear to be “behind” the secondary mirror)
• the portions of the tube closest to your primary mirror
• some portions of the secondary holder
• the top of the clips that hold the primary

The diagram below depicts what you see looking through the eyepiece tube and the four areas you need to flock are labeled in red. I’ll refer to these as the “key” flocking areas. You can click on the image for a larger version.

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The reason you only need to flock these areas is that any stray light entering your eyepiece has to reflect off of one of these four areas. So if you cover these with flocking material that absorbs 99% of visible light, you’ve just killed 99% of the stray light. You get 99% of the benefit for a small fraction of the work by only flocking these key areas.

Now flocking whole tube does help because light can get to the key areas by multiple reflections from other parts of the tube. By flocking those other parts, they absorb 99% of the stray light and only send 1% on to the key areas. The key areas then absorb 99% of that 1%, for a total absorption of 99.99% in those cases. So if you want to do the additional work to get that extra 0.99% improvement, go ahead.

What to flock with

If you are going to go through the trouble of flocking, you might as well do it with material that does a really good job of absorbing visible light. It’s not worth going through the trouble of flocking to just get 80% or even 90% absorption. You want better than 95% absorption of visible light. Cloudy Nights has a great review [3] that measures the relative light absorbency of the various products. I ended up using Protostar [4] which absorbs over 99.6% of visible light at an incident angle of 0 degrees, and 99.3% at 80 degrees. It comes in peel-and-stick sheets that you cut to your needs. Another flocking product that gets good reviews is sold by ScopeStuff [5].

While not particularly expensive, the Protostar and the ScopeStuff flocking materials aren’t cheap either. If you want to flock your whole tube, you’ll need quite a bit, probably $50 to $100 worth for an 8 to 12 inch telescope. But if you only flock the key areas, you can get by with $20 to $30 of flocking material.

Flocking

The peel-and-stick flocking paper is easy to apply. Don’t try to apply it in large sheets—it’s not only too hard, but you shouldn’t do it in any case because the difference in thermal expansion and contraction between the tube and the paper can cause it to peel. Instead cut rectangular strips and apply them length-wise. Wipe the inside of the tube with a little alcohol, let it dry, peel and stick. I found Protostar sticks very reliably to a clean metal telescope tube. It does not, however, stick well to itself, so don’t overlap Protostar over itself. Here are some pictures of my 10 inch Dobsonian and 4.25 inch Newtonian showing the flocking. The flocking material is quite a bit darker than it appears in the photos.

You can see that for the front of both telescopes I wrapped the flocking material nearly around to the focuser. It was easy to do to and by doing that I covered the key area opposite the focuser as well as the areas near the focuser that are most likely to reflect light onto the key area (getting me a big part of the last 0.99% of absorption). I didn’t bother to custom cut the flocking material around the focuser opening (again, I’m satisfied getting 99% of effect with only 10% of the work). If you look closely at the 10 inch picture, you might also be able to see flocking material on the portions of the secondary holder that are visible from the eyepiece tube.

I also flocked the portion of the tube around the primary mirror, as you can see in the photograph below. Looking at the flocking near the primary, you get a better idea of how black the flocking material really is.  If you look carefully, you'll also see that I didn't flock the top of the mirror clips; I decided I didn't want to block access to the screw heads.

Results

When I flocked my 10 inch Dob a couple of years ago, I made a point of doing it during a two-day period of steady clear weather so I could compare the views before and after. I observed the first night, flock the next day and observed again that second night. The conditions were pretty consistent across both nights, so I think the comparisons are reasonably valid. My test targets were M98 and NGC 3628. Before flocking both of these were very hard to see in my urban skies, requiring averted vision and consulting detailed 9th magnitude star charts to look exactly in the right place. In short, knowing precisely where they were, I was able to see them, but I wouldn’t have seen them otherwise. After flocking, both objects still required averted vision, but I was able to pick them out without any reference to detailed star charts. I just used my finder scope to get them in or near my field of view, then scanned around using averted vision and I was able to see them right away. My estimate is that flocking gained me about half a magnitude against faint diffuse objects such as galaxies. So I’m pretty satisfied with the benefit I got from flocking.