Going to the Dark Side

Darkness is what matters in Astronomy. Stars shining brightly, painted on the black canvas of space.


Orions Belt- Alnitak, Alnilam, and Mintaka

And there's nothing spoils the view more than having a grey or washed out view through your telescope.

One form of light pollution is of course the overhead washing out of the sky due to street lights en-masse shining up into the atmosphere, which is then reflected back down to earth by water-vapour in the air. Filters are used to help astronomers with this kind of light pollution, and when it is impractical to visit a location that is away from the source of the lights. When the full Moon shines high in the sky, you instead choose brighter objects to observe, such as the planets and more dominant stars and constellations. Of course you can take time instead to appreciate the beauty of the Moon herself.

The other form of light pollution that causes bad contrast in telescopes which we do have a little more control over, is stray, unwanted light coming into the end of the telescope at an angle from nearby domestic lighting; be that from streetlights, decorative garden or patio lighting, house lights, the next door neighbours pesky football pitch floodlight, or the gentler but still invasive light from the Moon.

Well at least the Moon has every right to be there.

You can close curtains or turn off un-necessary house lights, garden lights, and have a friendly word with the neighbour. You can even write to the local council to see if they will do something about the nearest streetlamps. Or you can purchase a "dark-cap" or "dew-cap" to wrap around the end of the telescope to help prevent strong side-lighting. I often use a large patio-table umbrella opened up and laid on its side to shield me from a particularly annoying streetlight.

The main issue here is that the interior surface of the telescope tube, even when painted matt black by the manufacturer, still has a degree of reflectivity that causes the light to scatter and bounce its way down the tube, and into the eyepiece, where it adds a subtle lightening of the background, and this reduces the resolution and contrast, and therefore the enjoyment of observing stars, and can make viewing deep-sky objects far more difficult.

A very effective and permanent solution to employ with open tube telescopes such as Newtonians, is the addition of materials with low light reflectivity to the inside surface of the telescope tube, and this procedure is known as "flocking".

Photon-Eating Felts

There are a number of different flocking materials (paints, baffling and the amusingly named "photon-eating felts") available, but the type I found a UK supplier for that is highly recommended is called ProtoStar Hi-Tack Flocking Sheets. On that site they say:-

Image contrast suffers when stray light reaches the focal plane. The best flat black paint still reflects about 5% of incident light, and even more when viewed at high angles of incidence (AOI). ProtoStar flocked light trap material is specifically engineered to be a light absorbing surface, and is equally effective at all angles of incidence. Unlike "decorative" flocking grades, the fibers don’t shed on your optical surfaces. The material is 100% synthetic based, so it's not damaged by moisture.

On bright planets like Jupiter and Venus, you should be able to easily see an improvement in field glare surrounding the planet (assuming your original condition was a simple painted tube wall). Contrast on the Moon is also noticeably improved. The improvement is more subtle on deep space objects, though the flocked paper does a good job at reducing glare from nearby bright objects like streetlights, house lights, etc.

For most tubes or structural parts it is not difficult to install. Small diameter (under 8" ID), and long tubes are the most difficult to install the flocked paper in. It is recommended to install the material in manageable sections, starting from the middle of the tube and working out to the ends. Don't worry too much about small imperfections in your installation job, as they will be invisible when the job is done.

For my XT10 I purchased 4 of the 20" x 28" sheets (£7.95 per sheet) to go all the way around the interior 12 inch diameter tube of my (10inch mirror) XT10i. The circumference is Pi x Diameter = 3.14 x 12 = 37.68inches. So two sheets width gives about 3 inches extra for overlapping the sheets, and 2x28"=56 inches, is plenty long enough to do the full length of the XT10.

Preparing to Flock

You will have to dismantle all major removeable parts from the OTA, except for the altitude bearing mounts which help stop the tube from rolling about while you work.

Wash your hands before you start! You may need to grasp the Secondary mirror support stub while removing it, but if you accidentally touch the mirror surface itself you do not want any grease from your hands getting on it. This applies equally for removing the Primary mirror.

Try to avoid touching the mirrors entirely because even after washing, your fingers will still make imprints. Note: If you do accidentally make finger marks on the mirrors, leave them alone - DO NOT TOUCH THEM! Trust me, unless you plan to fully and properly clean the mirrors while they are out, you will make more of a mess by doing half a job, and you run risk of scratching the mirrors.

ALWAYS POSITION THE TELESCOPE HORIZONTALLY WHEN REMOVING THE SECONDARY, because if you accidentally drop tools, or the secondary onto the primary mirror it could have disastrous consequences!!

I prefer to do this in the safest way by leaving the OTA mounted in the dobsonian base, and rotating the tube down past horizontal, and using a bunjee cord to strap it in its most downward position.

First undo the four thumbscrews holding the secondary spider vane in place, making sure you have a good grip on the stub of the mirror, which itself is surprisingly heavy.

Then I undo the six screws securing the primary mirror, and very carefully slide the primary mirror cell out of the tube.

Be prepared to take the weight, the mirror cell is very heavy! [This photo shows the rubber clamps removed and me lifting the mirror simply to show how thick the glass is. You only need to remove the mirror from the cell if you plan to clean it.]

I also removed the black knob at the top-end of the OTA tube used for positioning the scope when observing, but I left the top-end metal rim fixed in the tube, and also left the crayford focuser in place. Next I laid the tube out on a worktop at a convenient working height, with the seam of the tube at the bottom side against the table. You will start flocking from the straight edge of the seam.

Cutting and Installing the Flocking

Before installing the flocking I recommend you read the ProtoStar installation instructions (PDF document, see Useful Resources below) which are very good and offer some useful tips.

You might prefer to use your own method if you have researched elsewhere about flocking a scope, or if you have a smaller sized scope. Anyway this is how I did the job for my XT10 which has a reasonable 12 inch diameter to get your arms into and move about while working inside the tube.

First inspect the sheets for any lint or paper backing particles leftover from the manufacturing process that might be sticking to the surface of the black material, and remove them with a lint-brush or some adhesive tape (Sellotape).

Using a long ruler mark 2 of the sheets lengthwise into 5 inch wide strips (e.g. 5inch x 28inch). Installing in narrow strips like this allows the material to contract or expand inside the tube, without wrinkling or becoming unstuck from the tube.

When cut this gives 4 strips per 20" wide sheet, which gives 8 strips for the top end of the telescope. Note: Do not cut the second two sheets yet - see "Bottom End" below.

Origami Technique

Taking the first strip, peel back the backing strip from the contact adhesive along the entire length of one edge by only about 15-20mm, and fold the backing paper over completely, giving it a crease. This will spring back a bit, and helps to keep the strip lifted up and off the metal surface until you're ready to stick it down. The folded back piece is also easy to grasp and peel off completely as you stick it down.

The idea here is to expose only a very narrow strip of contact adhesive. This stuff is VERY tacky, and once it grips on the tube interior its quite difficult to lift off again, and woe betide you if the strip accidentally folds over and sticks to itself because it's damn nigh impossible to part.

Next grasp each end of the sheet along the folded back edge, and pull your hands apart so as to stretch/tension the strip so that it is held straight, with the sticky side down, and the peeled back edge oriented so that you can line it up against the tube seam. Keeping it stretched out, reach into the tube as far as you can, and line it up with the seam edge - you can very carefully place the strip into the tube relying on the folded paper backing to keep the exposed adhesive edge lifted away and off the metal surface until you are ready to stick it down. Do NOT stick yet!

Now I found the best method of lining the strip up before sticking down, was to line up the end of the strip with the top edge of the telescope tube just below the metal rim next to the seam, and when positioned correctly, press down firmly only about one thumbs width so the starting point of the strip becomes securely fastened.

Now let go and move to the other end of the tube (the folded backing paper should keep the adhesive off the metal), reach in and grasp the other end of the strip. You can lift this end and pull it quite tightly now because the top end is stuck nicely, but it means you can really tension it well to pull it straight against the seam and then when satisfied you've lined it up properly, stick this lower end down.

Now you can return to the top end of the scope and press down the exposed portion of the strip all the way along the seam. At this point you have still only stuck a narrow section about 15mm wide.

 

 

Next, with the whole length of the strip nice and straight and begun, you can grasp the folded backing strip and start pulling it out from under the flocking material exposing a bit more of the adhesive, and pressing down as you go all along the full length of the strip. It's best to do this in stages along the full length rather than peeling the lot off and sticking down from top to bottom, as it ensures you can get rid of any wrinkles as you stick it down, and it will stay lined up.

Overlaps And Glue

Now repeat the process for the next strip in similar fashion except this time you will overlap the new strip over the first strip by about 5-10mm maximum so as to ensure all metal surfaces are covered.

It is important to get the amount of overlap just right, and use some strong glue if necessary, e.g. epoxy resin (rapid setting Araldite).

What I found is that the flocking material does not stick to itself too well. Of course this is probably to be expected, but over a few days the material curls up and away from the overlaps. Where I used narrow overlaps (e.g. 5-10mm) it's not too much of a problem, it cannot curl up too far, so there's no need for additional glue. But where a larger overlap occurs (e.g. 15mm or more), because I didn’t place it properly but was committed because the adhesive is so strong, the curling lifts the corners or edges of the strips a bit too far for my liking, to the point where it could start encroaching on the field of view.

Also I learnt that it is a mistake to add small cut-off pieces of flocking to cover bolt/nut heads (at the altitude bearing mount points, or the focuser) as these soon un-stick and fall off and down onto the primary mirror!

Another piece 2inches x 6inches which I had cut to stick over a long narrow triangular section of bare metal (that badly placed piece again!) also came un-stuck after a few days, and it wasn’t until I set the scope up and looked through that I thought, “What the heck is that oblong shadow?”, and discovered the strip laying across the mirror, which I had to shake the scope horizontally to get it to fall off the mirror, then gingerly remove with one of those long flexible grab tools. Thankfully the contact adhesive had not stuck to the mirror!

So the moral is to use some strong resin glue (i.e. Araldite) to help glue down corners and wide overlaps when you do the job the first time round!! I’m going to have to dismantle the scope again to do the job properly. You’ve been warned!

Working Around Obstacles

Fixing the strips around the focuser is a little more tricky, and I found it better to measure, mark and cut the strip into smaller sections before removing the backing paper.

After completing the whole scope I cut some smaller pieces to cover around the focuser more closely. As long as a decent area of the adhesive goes directly onto the metal you will be ok. The same comment applies as earlier however, that it does not stick well to itself or nut-heads, and is in danger of unsticking and falling off, so some extra glue may be prudent.

The photo below was taken at a later date, and shows my new Low-Profile Crayford Focuser installed, with the flocking paper stuck around it. Unfortunately I only had some green foam available to seal the gap around the focuser to prevent dust getting into the scope. Black foam would have been better!

Additionally where the material passes over the altitude bearing cylinders fixing nuts at the middle of the tube, I cut slits into the material so it could be pressed down and around the nuts neatly.

Bottom End

After working all the way around the top end of the scope, you can repeat the process for the bottom. This is much easier and quicker to do because you don't have the focuser to contend with, but do not rush it or lose concentration, or you may make a mistake and align the strips poorly.

At the bottom of the scope measure the length required to cover from the bottoms of the top strips down (plus overlap) but falling short of the position of the primary mirror cell when installed. The mirror cell is a very snug fit into the OTA tube, so the strips MUST be cut short. The bottom end strips will be considerably shorter than the top end strips. This means you can cut across the width of the 2 remaining sheets before cutting the strips, giving a couple of excess pieces about 4 inches wide, and these can be used to cut narrow strips which will line the inside of the metal rim at the top end of the tube [this is why I said earlier not to cut the second two sheets yet].

The additional flocking on the rim interior, not only prevents reflections, but also makes a much better tight fit when the "dust-lid" is put onto the top end of the scope. Previously this had always been a bit loose, and would fall off the scope if it was tipped too far downwards (e.g. when I put a shower-cap dust cover over the bottom end after placing the lid on the top end!).


Dust-lid after a frosty nights viewing

Black as the Ace of Spades

You can see in the photo below, taken when only a quarter of the tube has been done, just how effective the ProtoStar Flocking material is at reducing internal reflections.

Secondary Darkening

Since I had the Secondary Mirror out, it made sense to apply some matt-black paint to the edges and back of the mirror and mount, as this will also help reduce light reflections from occurring inside the telescope.

First Darkness

Finally I re-assembled everything in reverse-order and re-collimated the mirrors as precisely as I could. When all was done I took the scope outside to see what improvement the flocking had given. On a fairly clear night, with the occasional cloud passing over, the Pleiades were visible so I lined up on them and checked it out.

Now my rear garden has a white light (mercury) street lamp to the North side, and I have always noticed when pointing the scope to the North or East, that the light entering the end of the scope tube (unless I get our large patio table umbrella out to shield the light) always gives a lightening or greyness to the view.  But with the new flocking there is a considerable difference, with a very much darker view, which does indeed give better contrast. Certainly looking down into the tube from the top it looks very dark inside.

The flocking was well worth the effort and seems to help on nights with full moon also. Its only when the scope is pointing almost directly at the street lamp that it affects the view, so I’m quite pleased.

Useful Resources