Why not flock reflector spider vanes?
Posted 18 November 2013 - 07:40 AM
Emissivities of materials
Posted 18 November 2013 - 07:55 AM
BarabinoJr, I suspect the increase of contrast you noticed to be caused by the flocking of the inside tube. However flocking the spider vanes is not a good choice. See the first answers of this topic.
Posted 18 November 2013 - 07:55 AM
Posted 18 November 2013 - 08:06 AM
Here is a picture of my "gold plated" spider vanes, though not yet tested due to uninterrupted bad weather - that is why I asked the question...
The secondary mirror can be heated against dew (so the thin cable visible in the vane under the blanket).
I can still remove the blanket if the result is not good.
Posted 18 November 2013 - 12:25 PM
Posted 18 November 2013 - 01:08 PM
Posted 18 November 2013 - 02:01 PM
I tried to flock the spiders vanes using a very mat black paint. This was before I flocked the internal of my Skywatcher 200/1000 tube. I didn't noticed any improvement to the spikes, it was even likely to be worse. Then I rebuilt another tube for my telescope, initially to win weight. I flocked it with a very mat black paint that is very effective (I don't see any reflection even with the flash of my camera...). The spider vanes were still the same and though I noticed a huge improvement in the contrast, thanks to the internal flocking, the spikes were still the same. I am also using a dew shield that is 1.5 x the outer diameter of the tube.
One of the reasons, in addition to what Nils Olof talked about, is that the black paint is very emissive in infra red. It will pump out the heat of the secondary holder and drop the temperature a few degrees below ambiant air. This difference in temperature will generate laminar air veins around the vanes that will refract the light. As a consequence, the vanes will behave as they were thicker and the spikes look bigger and stronger.
To avoid this, one should avoid the vanes to see the dark sky (which has a very cold temperature), and the dew shield is a good companion. But it is not 100% effective. The alternative is to reduce the emissivity of the vanes surface. A shinny surface, like aluminum polished sheet or the silver or gold face of a safety blanket, will do the job. The emissivity of mat black paint is between 0.95 and 0.99, whilst the emissivity of these shinny surfaces ranges between 0.02 (gold blanket side) and 0.10 (aluminum paper).
However, the shinny surface will reflect the light coming from off axis bright stars. I can't check that because of the bad weather here in french Normandy since more than a month. This is why I was asking the question, to see if someone tested this solution and get his/her feedback.
Posted 18 November 2013 - 03:50 PM
I flocked it with a very mat black paint that is very effective (I don't see any reflection even with the flash of my camera...).
In my experience, for grazing reflections at low angle, even the most matte of paints on a smooth surface will be highly reflective - as is easily shown. To stop this, an irregular surface before painting helps (crushed walnut shells on glue has been suggested) or the fibres of flocking or velvet. What did you do? In a practical situation, light from an off-axis star may hit the primary, reflected towards the tube wall, and maybe the grazing reflection may hit the secondary and on to the EP.
One of the reasons, in addition to what Nils Olof talked about, is that the black paint is very emissive in infra red.
Seems most materials including white paints, anodized aluminium and the mylar side of a safety blanket are highly emissive/"black" in the wavelength range in question (around 10 micron).
This is why chromed details on a white car gets hotter in the sun than the paint - they are about as reflective of sunlight, but the paint is much more emissive than the chrome in IR and can radiate the excessive heat much better.
Posted 18 November 2013 - 04:08 PM
See http://www.insul.net/testdata.php for example.
I'm waiting for the clouds and wind to leave our sky to shoot some photos close to a bright star to test...
Concerning the paint I used. First, the material of the tube is aluminum, not finished (therefore already mat). I applied two layers of mat black paint (Rust Oleum mat black). It has a grainy surface but was not mat enough. I then applied two layers of Charcoal black paint (the one used to make a blackboard on a wall). The grain is now very visible and really, it is very black. Here is a picture shot with a flash. On the top, "my mate black", below the original Skywatcher tube :
Posted 19 November 2013 - 05:19 AM
On the top, "my mate black", below the original Skywatcher tube :
Certainly a big improvement over the original (typical for a commercial tube, I guess). But backscatter from light coming down the tube is not the critical thing - the light source ought to have been at the bottom of the tube, thereby simulating a reflection in the primary, scattered forward towards the secondary.
Here is a photo I took long ago, to study the effect of a baffle in a newtonian tube. Not the best quality, but I think it illustrates my point:
I use plywood, painted with moderately matt black. I hold the larger piece horizontally ("tube wall"), next to a mirror. Also, I hold a strip of the same material at right angles, simulating a baffle. The camera is held at a low angle to the tube wall. Most of it appears strongly illuminated by forward scatter from the mirror image of the camera flash, but you see a marked shadow of the baffle, where there is only backscatter (like in your photos!). Obviously, backscatter is negligible.
(The point I wanted to illustrate is that a baffle is brighter than backscatter from the tube wall - it means that you should only baffle the tube to cut forward scatter, if at all. Flocking is probably better!).
Posted 19 November 2013 - 09:01 AM
Posted 19 November 2013 - 09:56 AM
To avoid reflection, baffling and/or flocking the internals suffers no alternative. But the outer surface of the telescope can be designed to offer the lowest emissivity as possible, hence a very reflective coating (bare polished inox steel) would be the better choice.
Usually you have some clearance between the tube wall and the mirror diameter. So laminar air layers close to the inner wall of the tube are not a big issue (as long as the equilibrium in temperature has been reached). It may be a concern if a baffle is too close to the light path and the air layers are forced to travel inside it.
But the vanes lie inside the light path, you shall minimise the difference in temperature between the vane and the air as close to 0 as possible. Therefore flocking the vanes with a high emissivity material looks contra productive.
However, I tried to simulate the effect of a laminar layer around the vanes with Maskulator (adding 1 mm thick blurred and distorded grey around the vanes). The result is quite strange : the spikes are reduced ! In fact they look smaller and fainter but the energy of the spikes is dispersed all around, and the contrast drops a little bit.
Therefore, flocking the vanes adds some "blur" around the vanes that disperses the spikes. It has an equivalent effect - though lower - than with curved vanes, as long as you don't increase the thickness of the vanes.
Posted 19 November 2013 - 05:16 PM