Bright lines on the image

[almcl]

Hi, back with yet another question...

My last two imaging attempts haven't been crowned with success. 

Whilst trying to get the Flame and Horsehead nebulas in the same frame, an artefact that isn't on the darks or flats has shown up.  Seems to consist of two bright parallel lines.

Example below, where its in the upper right part of the image.

It almost looks as though it's some sort of reflection from Alnitak's diffraction spikes but on another evening, it showed up in a completely different place.

Any thoughts on how to go about finding the source for this would be appreciated.

The set up is a Canon 700D attached to a motorised but unguided SW 200P on an EQ5 mount, 20 sec subs, ISO 1600.

[ollypenrice]

I think it is probably a reflection from the vane.

You also need to give the vanes some attention, I think. I'm not well up on Newts but how good is your coillimation and is there a twist in the vane?

Olly

[laser_jock99]

Spurious reflections are always a possibility with bright stars like Alnitak in the field of view (or even just out of it). The line could be from one of the other Belt Stars?

Edited February 8, 2015 by laser_jock99

[RichM63]

Olly has a good point. When I first got my 200P one of the vanes was twisted and visible. Easily fixed though.

[Scott]

Olly has a good point. When I first got my 200P one of the vanes was twisted and visible. Easily fixed though.

How did you fix it Rich? For the benefit of those less knowledgeable (not me of course )

[almcl]

Thanks for the thoughts, Olly, Laser jock and Rich.  I have had a look at the collimation this morning and it was a little off, but on a previous occasion when the artefact appeared, it was spot on.

As for the vanes, I'm not sure.  Picture below seems to show the 12 o'clock one slightly bowed and the 9 o'clock one slightly twisted.  Would this be enough to cause the problem, I wonder?  Or is there something there I haven't noticed?

And Olly, you're far too modest, I am pretty certain that massive Dob you showed me the witch's broom and many other exciting things through last September is a Newt and that seemed to be in pretty good order!

[steppenwolf]

The spider vane does indeed have a twist that requires correction to stop that double diffraction. However, an analysis of your image (see below) shows that the diffraction on the right is most likely caused by the bright star Alnilam (the middle of the three Belt Stars of Orion).

[Leveye]

I can't help with the possible reflection there to the right but I did have the same offset diffraction spikes at one time while testing a three vane. That turned out to be a faulty secondary replacement mirror whose coating did not cover the entire surface on either side. Going back to the original mirror corrected the issue right away so in my case i know that was what was causing it. I'd inspect the secondary very carefully. Some of these Chinese imports are flawed and many companies are using them in their reflectors..

[ollypenrice]

I didn't recognize you by your forum name, almcl! Sorry about that.

Steve has it with Alnilam, I suspect. I gave the area a quick eyeball on a widefield and decided it didn't line up as a likely candidate but I hadn't allowed for the orientation of your camera. In doing it properly Steve has fingered the suspect's collar, I'd say.

Just in terms of good practice you have no flocking in the tube and you also have a couple of bolt intrusions into what might be the lightpath. SInce the tube also has a raised seam I might be inclined to flock it and trim the finder bolts. I've no idea whether this might be a player. I do have a big visual Newt, as you know, but imaging is far more exacting in terms of things having to be right.

Best,

Olly

Edit, I'd maybe blacken the vanes as well. Matt paints using pigments are best since dye based paints can reflect IR, it seems. Barbecue paints use pigments.

Edited February 8, 2015 by ollypenrice

[almcl]

Wow!  Olly and Steve in the same day.  I am definitely playing with the big boys now!

Thanks both (and Leveye) for the thoughts.  Flocking is another thing to add to the list and some bbq paint as well.  I'll also play around with the framing to see if that helps and try to untwist the vane.

[RichM63]

How did you fix it Rich? For the benefit of those less knowledgeable (not me of course )

Well in my case it was down to the PO and human nature, as the secondary fixing bolts could be adjusted he had done just that, causing the whole assembly to be moved off axis and badly twisting one of the vanes. (he sold me the telescope as 'broken').

All I did was to slaken off all the 4 bolts, straighten the bent vane and then sequentially retighten the bolts until the secondary mirror support screw was dead centre. Then fully recollimated the whole telescope, using images and Als Collimation Aid, followed by defocused star tests.

Full secondary/primary collimation guides can be found both here and on video. AstroBaby's guide is a good starter point.

The biggest cause for twisting is over tightening one side. I find that when I'm near, then I use both hands and tighten opposing bolts at the same time.

[Francesco Bisi]

Excuse me, could you tell me the distance of the spider's rays, are all four equally spaced?