Improving Flextube contrast

[acey]

Since getting my Flextube 300P a year ago I’ve been aware that although it’s great for deep-sky observing, planetary views were iffy, lacking contrast, and really no better than what I got with my 8-inch. Guessing stray light to be the issue I bought an Astrozap shroud and made myself a dewshield. These help a bit, but not enough. Now I’ve taken further measures – with minimal effort or expense, and very pleasing results.

The problem is not the open-tube design, but a lack of baffling. I got advice from an excellent S&T article by Gary Seronik (April 2001) who makes the point that what matters is stray light reaching the focuser. In an ideal world you should see only the primary mirror when you look through the empty drawtube, anything else is a potential problem. In the Flextube I could see three sources of stray light reaching the eyepiece: the edge of the secondary (left of centre), the patch of tube wall (and rim) beyond the secondary, and the inside of the focuser.

(In the pic it looks like the scope is out of collimation: it's because I was holding the camera near the end of the focuser and not exactly square to the focal plane).

I blackened the visible upper part of the secondary edge using a Sharpie pen, simply reaching down (with a steady hand!) into the end of the tube. I didn’t go to the very edge of the mirror – whenever I take the secondary out for cleaning I’ll maybe do that. Next I applied Protostar flocking to the visible part of the tube wall. I cut it in manageable strips and was again able to do this job without removing the secondary. The only thing I removed was the shiny screw, visible from the focuser, that holds a knob used when extending the tube. I painted this screw matt black, flocked the shiny tube rim, then replaced the screw.

The focusser drawtube was the only other part I needed to remove. On the underside of the focuser there is a large silver set-screw (the focus lock), four small screws, and a central Allen nut (the tension adjuster). I loosened the tension nut a turn or two, then the four screws by several turns, until I was able to slide the drawtube right out. I unscrewed the plastic mount from the top end of the drawtube and flocked the smooth part of the interior using strips of Protostar.

The final step was to baffle the focuser. Given that you ideally want to see only the primary mirror when you look through the focuser, you could have a ring fixed to the end that blocks everything else. But this would cause vignetting (light loss towards the edge of field). To avoid vignetting, an anular baffle needs to allow you to still see a little bit around the secondary when looking through the drawtube. I simply guestimated a suitable size, made a ring of card, stuck Protostar on it and taped it to the end of the drawtube (flocked-side towards the eyepiece end). Wear marks on the drawtube show where two rollers press against it; I avoided taping over these so there would be no obstruction to smooth motion.

The last pic shows the effect of these changes (I held my camera just beyond the top end of the drawtube: the light on the focuser wall is coming from outside, not in). There is now far less visual noise reaching the eyepiece.

Does it make a difference? You bet! Previously I found planets to be pale, washed out, and with four enormous searchlight beams protruding from their blurry discs. After doing these mods I was treated to a fine view of Mars, with much more surface detail visible. The searchlight beams are much narrower and fainter, the sort of diffraction spikes one expects to see with a 4-vane spider. That’s good enough for me, but if I were to go further there are a few steps I might consider.

First would be to increase the size of the anular baffle. I measured and found that the the secondary is sized and positioned to give a fully illuminated field 6mm wide, good for those wanting low power wide-field views with no light loss at the edge – but actually I think I could manage with less than that. Maybe I’ll do some more measuring and use Newt to calculate an optimal baffle size. I could also install a second baffle further up the drawtube.

Another option would be to flock or paint other parts of the tube interior. The issue is how much light ends up reaching the focuser, and I’ve already treated the parts that have a direct light-path: any other light reaching the focuser is from reflections off those surfaces. I observe at a dark site where I doubt that those reflections are going to make any difference at all; at a heavily light-polluted site it might be different, though given the low reflectivity of Protostar, it still may not make that big a difference. The Protostar instructions talk about flocking the whole interior of the telescope tube, but Seronik’s article suggests this is overkill. From the single sheet I bought there’s plenty left if I ever decide to flock part of the lower tube, just above the primary, that is potentially visible at the focuser.

The Protostar instructions warn you should use a roller to remove loose fibres before applying: I was lazy and didn’t bother, but found lots of fibres coming loose when I was fiddling to make the anular baffle. I’m glad I did this job prior to mirror cleaning rather than after. But I suspect the fibres only come loose from rubbing – and given the small amount I’ve used, I don’t anticipate any problem. Another point is that some users have found that Protostar has a tendency to peel loose. I can see some bumps and loose edges that will maybe peel away. If they do then I’ll simply restick or apply new pieces.

If you’ve got a Flextube and are happy with the views then don’t let me spoil your enjoyment. But if you’re looking to improve contrast then these simple steps are well worth taking.

Andrew

[oceanheadted]

Thanks for the write up. I'll take a look down my 200p to see if there is any scattered light!

[brantuk]

I tried both holding the buttons down and also "stabbing" them. Stabbing doesn't seem to do a lot - but it depends what speed you're using and wether in tracking mode or not. I often overshoot the object. There's a fair bit of backlash on this dob so my intention is to balance up the alt with weights, and dampen the az with some thin carpet. I also ordered the alt/az goto synscan which I understand improves the movement quite considerably.

Here's a thread you may have missed - there's some great advice on it.

Cheers

http://stargazerslounge.com/equipment-discussion/85173-goto-skywatcher-flextube-auto-dobs-reality.html

[acey]

Mine isn't Auto - pushing by hand works fine for me.

[brantuk]

Woops - think I posted in the wrong thread - totally confused now - apologies

[xboxdevil]

Andrew, that is a really great bit of work, Thanks for sharing it, due to cloud I've not had a chance to use my 300p yet, but I'm looking forward to it !!

[mindburner]

this is a great idea. I have ordered the protostar flexiboard stuff from the US. It fits inside the tube and springs into position inside the tube. As my scope is new, I didn't want to mod it to much yet.

[WayBig]

I'll bookmark this and try it myself. I've coloured the edges of my secondary, sounds like I've got to get handy with the protostar to get the real benefits.

Do your changes also improve the views on faint fuzzier?

[acey]

Do your changes also improve the views on faint fuzzier?

Hard to say. In theory, improved contrast should help with DSOs, but you'd really need a side-by-side comparison to test it. In practice I'd say it's really on bright objects that I see a difference.

[WayBig]

I'd lend you mine, but it't only the 250.

I know what you mean about the searchlights though it's annoying, but useful to follow to help find the object!