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IC 410 - any thoughts please?


Steve 1962

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Hi

I spent Wednesday evening out under a rare clear sky trying to get 30 min subs from my FLT98 and NEQ6. All of my images showed egg shaped stars - even though the guiding was pretty well spot on.

This is a 300% crop of the centre of a frame.

post-6387-0-25950000-1386324097_thumb.jp

I got pretty frustrated because I've never had this problem before with my kit.

I was trying to image IC410 which started off fairly low in the ENE and as I toiled away trying to work out why the stars were eggy I noticed that they were rotating as the target moved across the sky. The collimation was checked - and the out of focus diffraction discs seemed very regular but I'm pretty sure that this shows some slop in the optical train. (i.e. gravity seemed to be pulling the camera out of true(?))

I have the scope, a Feathertouch focuser, a WO Flattener IV, a 15mm extension tube, a SX filter wheel and an Atik 460 in that order. The focuser seems very rigid with very little in the way of adjustment and everything was screwed up very tight with Delrin rings to prevent the threads seizing. 

Does anyone have any thoughts on this please?

Anyway, as IC410 went round to the south, I stepped back to 20 min subs and the eggs went away. So maybe it was guiding or field rotation (although the eggs were consistent across the whole frame)

Here's 5 x 20mins Ha.

post-6387-0-73214900-1386323856_thumb.jp

I don't know what you think, but it looks a bit "flat" to me - again, any comments would be appreciated please.

The full size is here: http://www.flickr.com/photos/perrybarn/11234170304/sizes/o/in/photostream/

Cheers

Steve

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Hi Steve, the cause of your earlier eggy stars is bit of headache, why they should be there in the 30 min subs, but not the 20 min subs is tricky to fathom out. Did you tighten anything up after seeing the initial downloads showing the problem? The reason I ask is that mid-week I also had some issues with eggy stars, and although I try to bolt everything down as firmly as possible, I noted that this time my scope clamps tightened another 1/8th inch when pressed so I suspect that with the repeated freezing nights followed by mild days things have worked loose, at least in my set-up. The tightening I did worked magic, since the eggy stars then disappeared, which was a relief - but I guess it was enough to give flexure.

Your actual image looks very good in my opinion, the stars look absolutely fine, and the details are there - perhaps with OIII added to give colour you may gain a bit more depth & the perspective that you're after.

HTH,

Martin

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Thanks Martin

I've had a good tighten up all round...and I've moved the spacer out to between the camera and the FW (moving the FW closer to the scope) , so we'll see how it goes next time out.

I'm pleased you think the image is OK - Oiii will also be next time out (subject to no eggy stars of course!)

Steve

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Unable to say much about eggy stars from one perfect night to another nightmare, except to check tightness and perhaps it is just a case that your mount didn't like where it was pointing. Sounds silly I know but with my HEQ5 there was parts of the sky I knew I couldn't guide well in and it was as annoying as that.

A little more contrast perhaps in the image, but the data looks good so far.

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Looks like a bit of flex there - especially being as its in the centre of the frame too.

One thing to look for - you might find that the hot pixels in your raw subs are travelling in the same direction. As already mentioned, tighten everything up and try again. Might be worth trying different parts of the sky, just to see if the result is consistent.

But whats more annoying is when you get it despite a flat graph!

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Yes maybe PA also in question as Ian & Peter suggest - easiest way to check PA is simply to run a 5 minute unguided sub and see how bad the star trails are. If PA is spot-on then your 5 min unguided sub should show nice round stars, and no trails. PEMPro is an excellent package to take the pain out of optimizing PA using software automated drift alignment routines - there's a free trial period as well.

Martin

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  • 2 weeks later...

Thanks again everyone for your comments.

I found a couple of issues:

The front tube ring clamp on my imaging scope was loose.  I have a 70mm guide scope bolted to the top of the tube rings, so this allowed some flex in the guide scope and slop in the main scope.

Also, my PA was out altitude wise. I think this explains why the problem went away as the target tracked southwards.

Anyway, using a combination of two drift methods - PHD and DARV, I think I've nailed the PA.

Here are links to the two PA methods which I used:

http://www.cloudynights.com/item.php?item_id=2838 I found this made it really quick and easy to identify the problem, then prove that I'd fixed it at the end.

http://stargazerslounge.com/topic/81943-phd-drift-polar-alignment/?hl=%20phd%20%20drift%20%20align and I used this method to make the corrections.

I was staggered as to how sensitive the NEQ6 PA adjustment is - in the end it came down to the tightness of the adjustment bolts - 1/16 turn made a lot of difference.

Now just waiting for another clear night to see if it worked!

Steve

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