NGC7635 Bubble Nebula

[MarkS]

Did a quick Bubble tonight before the clouds rolled in.  Sky wasn't yet completely dark but the result isn't too bad though a bit noisy.  That Bhatinov mask (http://forum.orpington-astronomy.org.uk/index.php?topic=3869)certainly made focusing really really easy!

23 x 5min using modified Canon EOS 350D on Celestron C11 with F6.3 focal reducer.  Ambient temp 10C.

[RobertM]

Very nice Mark, it's a difficult one up close.  The image looks a tad soft to me, I'm not sure why - perhaps it's collimation or just the conditions.

[Mike]

Looks good to me Mark. However, there are some strange shaped stars around the edges.

The centre section is really nice though, the mask seems to work well.

I guess the softness might be due to poor artmospheric conditions or maybe IR? I am not sure how good your IR blocking is with that kit.

[MarkS]

Robert, Mike,

The image has been scaled down by 2/3 so each pixel represents very roughly 1.2 arcsec.    I was using a IR filter plus a CLS filter.

The softness could be a mixture of things. 

1) Seeing was not that good
2) The wind was gusting (played havoc with my guiding)
3) I'm not sure the EQ6 will ever give the same results as your Takahashi!

It is also possible that collimation might be a problem.  What are the precise effects that bad collimation would have on the final image?

Mark

[Mike]

I don't think these are collimation issues. It is some kind of distortion due to perhaps misaligned kit (light path not straight), poor optics or bad polar alignment.

Collimation issues might cause weird shaped stars but the effect should be pretty uniform across the image. To me it looks more like your set-up alignment (light path) is not quite right.

[MarkS]

This image is a crop.  The distortion of stars at the bottom right is my usual problem which I think is field curvature - I put an example of it in this message:

http://forum.orpington-astronomy.org.uk/index.php?topic=3864.msg19108#msg19108

[RobertM]

Mark,

Inspect the flats you take and if the centre of the bright fully illuminated circle is not dead centre of the image then one of two things could be occurring.

a) The camera axis is off centre to the image axis.  This can be caused by  a weak or off axis focuser or the camera being off centre.  Also look at whether the camera is orthogonal to the image plane - my canon won't seat properly in the Baader visual back without a bit of fiddling.
b) Collimation is out (axis of the secondary is not pointing exactly down the centre of the focus assembly).

Of course it could always be a combination of both !

You don't need a Tak or AP mount for accurate guiding if you can tune up your EQ6 like Chris has but you will be pushing your luck if you want to put a C14 on it.  I've been trying to improve my guiding, with the ED80 and ATIK 16ic I generally get well below +/- 0.05 pixels under stable conditions though it does throw a wobbler occasionally and go up to +/-0.5 or even +/-1 rarely (could be backlash in the gears or other mechanical play) which isn't good enough to use the 9.25 at f10.  No one seems to publish their guiding errors so I've no idea whether this is good, bad or indifferent.

[MarkS]

Another attempt at the Bubble last night.  The skies were much clearer but even so, I still threw away 1/3 of the subs because of haze passing over.

33x5 min modified Canon EOS350D on Celestron C11 with F6.3 reducer.  IR filter + CLS filter. Ambient temp 8C.  Final image scaled down by 2/3 and cropped.

This was the image taken on the climb up to the Meridian.  I have yet to process the data taken later after the meridian flip (only 8 usable subs).  I also intend to add the data from the previous night's image.

Mac for your benefit, note that the stars on the LHS are less sharp and show slight coma.  I think this is because the CCD was not square.  After the meridan flip the stars on the RHS are less sharp so I need to judiciously merge the two!

[JohnP]

Mark - great result & the mask looks brilliant as well...

Cheers,  John

[Rocket Pooch]

Robert,

Olly had an 16ic and was getting .10 worse and .06 best at 380mm FL, my camera has smaller pixels just and I was getting .12 worse and .06 best 400mm FL, these we're both EQ6 Mounts.

This is in the realms of guiding with a 2 meter scope, I will be putting mine back onto the mount as soon as I can to get some closeup nebula imaging done.

Fay was getting 1 arc second guiding, and I think John was getting about .8 arc second guiding, K3CCD gives a different readout from AstroArt, which Olly and I was using.

Hope the numbers make sence.


Chris

[MarkS]

I've now added all my data into this image.  A total of 66 x 5minutes

Modified Canon EOS350D on Celestron C11 with F6.3 reducer.  IR filter + CLS filter. Ambient temp 7-10C.  Final image scaled down by 2/3 and cropped.

Mark

[RobertM]

Chris, that's very useful, I still have the bad figures to sort out but it sounds like I'm close to the right ballpark.  It pops over 1 pixel immediately after the dither between subs so I think I can ignore that but the other variance needs some attention.

Mark, that's much better and there's excellent detail showing - brilliant work.