orion nebula questions

[pkinchington]

I shot this when the seeing conditions were poor. There was high level cloud which obscured the running man nebula and I was unable to rectify this. Also the stars are egg shaped on the periphery I am not sure whether this is a coma corrector issue or the winds that were buffeting my 200mm f5 newtonian. What has caused the double spikes on the bright star to the left of m42? Camera used was modded sony A7r mirrorless using a clip in L3 astronomik uv/lr cut off filter. About 3h of 120s subs. (Added tweaked image with running man a bit more visible).

[ChrisV]

Poor seeing and wind aren't going to do you want favours. The newt will act like a sail in the breeze - I don't even try with mine under these conditions.

The eggy stars are uneven across the image. Top left, bottom right/left are all splaying outwards diagonally, but the top right is fine. You probably have the spacing between the coma corrector and camera about right, or a least close.
- It could be a collimation issue. This could account for the double spikes on your stars.
- There could be some tilt to the camera. That would produce the uneven stars as astrocameras can have issues with this. But I would be surprised it that was the case with you sony.

So maybe (1) check scope collimation first without the coma corrector. There's many collimation pros around IIS
Then (2) camera tilt without the coma corrector, then (3) have a look again with the coma corrector.

[RyanJones]

I think you e got a couple of things going on here not just one. The left of the image looks like classic coma but quite extreme. The center of the image seems to be just left of M43 but then the egginess of the stars then changes to a single direction to the top and bottom of frame but with round stars in the center. I think it’s likely to be a combination of tilt and collimation or maybe collimation that far out it’s stopping the coma corrector from being able to correct the field ? It is a strange one to diagnose especially down to one thing.

Cheers

Ryan

[Startrek]

I can only recommend to go back to basics and start from the ground up due to this image having multiple possible issues as mentioned by Chris and Ryan

1/ Level tripod and set up with front leg facing True South then install mount and scope

2/Check payload on mount is balanced in Ra and Dec ( adjust slightly east heavy , move counterweights no more than 10 to 15mm )

3/ Set mount to Home Position

4/ Check Secondary and Primary mirror is clean

5/ Collimate Newt using both a Cheshire and Laser ( ensure primary mirror is not pinched, ensure secondary mirror is not slight tilted with respect to the focus tube )

6/ Ensure camera and coma corrector are fitted with correct manufacturers distance to camera sensor

7/ Ensure camera and coma corrector sit level in focuser without tilt

8/ Polar align to under an arc minute

9/ Perform a Star test ( slew to a bright star say magnitude 1.0 to 2.0 and take 10 sec, 30 sec and 60 sec subs unguided ) Check images and ensure star is round and has 4 diffraction spikes not 6 or 8
If more than 4 , secondary mirror may have tilt or spider vanes could have an offset )

10/ Run your guiding to ensure guiding error is under 1 arc sec

11/ If all of the above is ok , then image say M42 again , this time take 60 sec subs , review images and then if ok image with longer subs 2 or 3 min

Caveat
Avoid imaging during cloudy weather even high cloud
Avoid imaging in windy weather with newts ( they act like a sail )

Hope the above offers some practical advice moving forward

Cheers
Martin

[Startrek]

NB: A couple of important items I forgot to mention , make sure your mount is not loaded beyond its Astrophotography capacity ( general rule of thumb is around 65% of the mounts stated maximum capacity)
What mount are you using for an 8” f5 newt ??
Also check mount for excess backlash in Dec and Ra main motor drives and worm drives
What guiding software are you using when using ?

[pkinchington]

Hi Martin, Ryan and Chris,

Thankyou for your input. I think tonight should be clear and still (fingers crossed). I have not checked the collimation on my newt other than seeing my eye is centred when looking at the secondary mirror through the focussing tube. I am using an Heq5 pro with the original worm gears (no belt modification) and phd2 for guiding. Usually my RA and Dec are around 0.25 (first figure shown on phd2) although this was exceeded on the night in question and was variable - probably due to the wind. I certainly exceed 0.65 of the suggested payload for the heq5. I have attached an image where I managed to get a recognisable running man out of my lousy subs.
Cheers Peter (Kanga)

[RyanJones]

To me that looks like bad coma with the optical axis being between M43 and the Running man and up from the horizontal center. I’d be initially willing to guess the coma corrector spacing isn’t correct which should be a fairly easy fix but first things first I’d be making sure the collimation is right. Good luck tonight and I hope our comments help you work though the issues.

Cheers

Ryan