The Cocoon Nebula and PHD2 issues

[joecoyle]

Hey

So, i attempted the Cocoon Nebula last night, imaging for 2 hours. Ended up having to reject 24 out of my 30 luminance images and rejecting 4 out of 10 RGB. 

30x2 min guided luminance images. 10x2 min RGB guided.

The work in progress is below... i will add more to it, but would like to understand why it went so so wrong!

I have attached a couple of the poor luminance and colour images for people's ideas... and the PHD2 log. 

Could someone with more idea about PHD2 please take a look at the log and tell me if that was causing the problem or something totally unrelated?

The colour images were a lot cleaner than the luminance ones.

Thanks

Joe

Work in Progress:

Blue:

Luminance:

PHD2 Log:

PHD2_GuideLog_2015-09-10_201639.txt

[Owmuchonomy]

There is something 'jerking' the mount in your poor luminance sub. Could be backlash, cable snag etc. what did your graph look like? Were there sudden corrections or movements?

[cfpendock]

Other more expert people will no doubt help here, but several things occur to me:

If you have a reasonable polar alignment with your Celestron mount, it shouldn't be necessary to guide for subs of only 2 minutes.

The first pic looks good - I like the colours.

In the second pic, it looks as though the rig moved.

In the third pic, something definitely moved the rig, either after about 30secs or about 90 secs during the exposure. (wind, cable snagging.....)

Don't know if this helps, others will no doubt be along!

Chris

[wuthton]

How is your guider mounted? If you are using the supplied Orion finder bracket then it's very likely your getting flexure.

[Welrod50]

First colour image looks promising.

The second sub looks to me like iffy guiding.

The third is most likely wind.

I sometimes get subs like these, usually if PHD/the mount decides to have a minute off and just stop tracking for some reason (rarely). The stars are all distorted in the same direction quite uniformly, suggesting to me tracking error.  In my own personal experience, I generally get subs like the second one if I get wind shear.  Usually if it's cable snag, tracking will discontinue quite noticeably, but that's just what I've found. To be fair, if you've lost quite a few subs I would suggest tracking/guiding error and stiff breeze.  Are the ones you discarded later on the imaging run or first off?  That will give you a better clue about what's what.

[joecoyle]

Hi,

My guider is mounted using the supplied bracket, so flexure is a possibility, although all my other pictures have not been subject to the same level of distortion.

I think it was possibly wind shear, there was a breeze but nothing too heavy. 

The discarded images came from all over the place, but the bulk were discarded from the middle (images 8-25ish).

With regards to my graph, i don't know what looks good and what doesn't, heres a picture...

It's all just numbers to me

Joe

[joecoyle]

With regards to 2 min subs. If I try any longer (3 or 4 mins) I get kidney bean / egg shaped stars.

So potentially my polar alignment isn't perfect. Although my mount reads 0*0"0' for alignment error

Sent from my iPhone using Tapatalk

[wuthton]

Hi Joe,

Firstly you need to adjust the scale on your graph so that you can see what is happening with the corrections. +/-25" should be better, then drop it more as your guiding improves.

Then you need to make sure that the guider is fixed, rock solid in position. The easiest and most effective way would be to glue it into the holder with epoxy. If you want to keep it removable you could drill three holes towards the front, tap them and use some nylon screws so that it's held firmly front and back. You could also start a topic and ask, I've never used a finder-guider but I sure there's some ingenious and cheap ways of mounting them that the SGL members have come up with.

Next look at your cable drag, cables from the back of the cameras to the floor is bad due to the leverage. Try to make it so that the drag point is as close to the mount as possible.

This is what mine used to be, I made a bracket for the mount to hold the cables.

I've now gone to even more extreme lengths and mounted the PC on the mount to reduce the cables to floor from 10 to 4. Even though my guidescope is held solid in tube rings the cable drag is still detectable.

[joecoyle]

Hi Matt, 

Thank you for your suggestions, i will look into mounting the guide scope more securely, whilst keeping it removable for now...

I've changed the scale of the graph, and this is what it looks like... Its clearer now that it was making larger corrections at the start of the imaging run and minor corrections at the end, when the overall image quality of my RGB images was much better. 

Cheers

Joe