Eyepieces, Barlows and diagonals - reducing internal reflections

[Ricochet]

On 05/05/2022 at 00:50, Mr Spock said:

I've noticed quite a few components, despite having black interiors, reflect quite a bit of light. So I've bought some of that (expensive) non reflective black paint to try and reduce reflections as much as possible. I'm doing the insides of the tubes I use for Barlows and eyepiece barrels to start. I'll then be taking the lenses out of my Barlow nose piece to do that; I'll also be dismantling my diagonal to give that the same treatment.

I've only done one coat so far, and missed a few bits, but the difference is still remarkable. I did the inside of one of my Svbony 70mm tubes I use with a Meade 140 apo nose piece to give x2.18; also the barrels of my orthos. I used the 9mm ortho with a painted and unpainted 70mm tube (I have two, plus a 56mm). The painted tube gave a much darker sky background making stars stand out more. I used Castor as the main test subject. It was much easier to pick out fainter stars with the painted tube, though the poor seeing conditions didn't help. The 9mm + 70mm Barlow is x173. 

I'm not going to do more tests in better conditions. I've seen enough already to show me it works and was worth the cash.

How is this paint holding up four months later? Have you had any issues with it flaking off? 

[Mr Spock]

10 minutes ago, Ricochet said:

How is this paint holding up four months later? Have you had any issues with it flaking off? 

None at all 👍

[Ricochet]

Excellent news, thanks. I've ordered myself a bottle. 

[Ricochet]

I used a combination of flocking and the musou black paint on the inside of my 72ED. Results as follows:

The dewshield has been flocked. You can see the original "Skywatcher black" in the foreground. The flocking is pretty black but there is a slight sheen at shallow angles.

The inside of the focuser has been painted. The finish isn't smooth and at shallow angles it is quite reflective, although better than the original Skywatcher paint. The manufacturer recommends several really light sprayed coats for proper blackness. Painting such a large area at a very shallow angle with a brush clearly doesn't work as well.

I also painted the inside of the nosepiece and extension tubes for the diagonal. They are ridged inside instead of smooth and the paintbrush can be used at a steeper angle. This has worked really well and is significantly darker than the original Baader paint.

The main body of the scope has a single baffle 2/5 of the way down the tube. The baffle itself was painted and again, painting a small area worked well and the baffle is nicely blacked. After the disaster with the focuser I flocked the inside walls of the tube either side of the baffle. The musou webpage says it can be used on fabrics so after a test on a scrap piece I painted the flocking inside the tube. If you compare the flocking at the far end of the tube to the first photo you can see that the performance of the flocking at shallow angles is greatly improved. Behind the baffle it is completely dark. There was no difficulty in painting the large area of flocking, but the drying time was a lot longer than for straight onto metal and I gave the piece a full day to dry before reassembly.

This is the view though the reassembled scope from the rear. The only thing not completely dark is the inside of the focuser. Based on a quick daylight test this might not be a problem but I need some clear moonlit skies to test it fully.

 

[Don Pensack]

No question that for moon viewing one needs to suppress any internal reflections a lot more than the manufacturers typically do.

[Ricochet]

I managed to get a quick test session in and the results were really good. With my delites and panoptic I couldn't see any reflections from bright objects off axis. There were some very slight issues with a couple of my SLVs but much better than before and only an issue because I was carefully looking to create them and unlikely to create a problem in practice. 

[Mr Spock]

The difference is noticeable on bright objects like Jupiter. Looking through the 'blackened' orthos is a revelation. No reflections or glare at all and a lovely clean disc and belts.

[great_bear]

On 05/05/2022 at 06:06, Alien 13 said:

Nice, have thought about getting some of that paint for the focus tube of my Mak. maybe it will then transform into a "Questar" 😀

I imagine that paint would be brilliant for camera lens hoods too.

 

Alan

I have recently painted the inside of the baffle tube for my Mak 180 Pro with the paint mentioned in the original post.

I was pleased at the very neat job that I did, but to be honest it has made absolutely no difference (thankfully it didn’t make it worse!). So it seems that at the very oblique angles that light strikes a Mak baffle tube, this paint offers no benefit. 

So don’t bother doing this on a Mak.

I may have an attempt at flocking instead at some point in the future - there is just about enough room in the Mak 180 if the flocking material isn’t too thick and will stay stuck down.

[great_bear]

On 05/05/2022 at 18:49, Mr Spock said:

My 12" needs flocking. It won't get to see this paint, not at those prices. The paint would cost as much as the scope!

You’ll only need to paint the area directly opposite the focuser and just the small length of the tube base that is visible from its reflection in the secondary mirror.

[Louis D]

3 hours ago, great_bear said:

I have recently painted the inside of the baffle tube for my Mak 180 Pro with the paint mentioned in the original post.

I was pleased at the very neat job that I did, but to be honest it has made absolutely no difference (thankfully it didn’t make it worse!). So it seems that at the very oblique angles that light strikes a Mak baffle tube, this paint offers no benefit. 

So don’t bother doing this on a Mak.

I may have an attempt at flocking instead at some point in the future - there is just about enough room in the Mak 180 if the flocking material isn’t too thick and will stay stuck down.

What is the rear port diameter?  In my 127 Mak, I get reflections off the rear baffle tube from bright stars when using 2" eyepieces because the rear port is only 27mm in diameter at best.  I keep thinking I'll install flocking inside the rear tube someday to tame those reflections.

[great_bear]

1 hour ago, Louis D said:

What is the rear port diameter?  In my 127 Mak, I get reflections off the rear baffle tube from bright stars when using 2" eyepieces because the rear port is only 27mm in diameter at best.  I keep thinking I'll install flocking inside the rear tube someday to tame those reflections.

I’ve got the older, gold-tube version which out-of-the-box only handles 1.25” eyepieces. I upgraded the diagonal to a Revelation 1/20th Wave 2” one but only for the rigidity and optical improvement (which was significant). So in my case, I’m not worried about 2” EPs because I never expect them to work anyway (although I do have a GSO Superview 30mm, which works well with no significant vignetting).

[Ricochet]

11 hours ago, great_bear said:

I may have an attempt at flocking instead at some point in the future - there is just about enough room in the Mak 180 if the flocking material isn’t too thick and will stay stuck down.

If the baffle tube is smooth internally there will be no issue with the flocking coming unstuck. If it has loads of microbaffles/is threaded then you might find it won't stick at all. If there is enough room inside the tube that there is no chance of diagonals, barlows etc coming into contact with the flocking then as in my photos above, I would recommend painting the flocking as well. 

[great_bear]

11 minutes ago, Ricochet said:

If the baffle tube is smooth internally there will be no issue with the flocking coming unstuck. If it has loads of microbaffles/is threaded then you might find it won't stick at all. If there is enough room inside the tube that there is no chance of diagonals, barlows etc coming into contact with the flocking then as in my photos above, I would recommend painting the flocking as well. 

Interesting.
 

I have next to me the chunk of sponge that I used to paint the Mak tube. Although the sponge is very stiff, I can squash it around and the dried paint shows no signs of shedding, which is good, and the paint hasn’t changed the visible appearance of the sponge (other than making it very black!). In a way, it’s behaving almost like ink, which is why I can see that painting the flocking paper might - under the right conditions - be beneficial. I’m just a bit worried about flattening the fibres in a way that spoils the flocking. 
 

I guess I could take two small test samples of flocking side-by-side, paint one of them and then test the result. 

[Don Pensack]

This isn't new, but illuminating (pun intended):

https://www.cloudynights.com/articles/cat/articles/observing-skills/flocking-materials-test-stray-light-reduction-r82