Owl Nebula, M108 and oops.

[JohnH]

I stupidly changed the distance between the sensor and the reducer after taking the L and R images - and it shows.



A bit of a shame because I have managed to get a little bit of detail although I now wish that I had taken some Ha.

Taken in Bromley (Bortle 7 if I am lucky) with Sharpstar 15028 HNT (1.87 arcsec/pixel) at the end of March.

L: 60 secs x 45
RGB: 180 secs x 20

Camera: ZWO ASI 1600 Pro MM at -15C attached to ASIAir Pro. The ASIAir has now added the ability to plate solve and go to a previous image which makes it a little easier to return to a target.

Imaging is now more difficult because I have to set up each time and the weather has been untrustworthy. I am waiting for Bromley Council to consider my planning application for a small roll-off roof observatory (unfortunately, we do not have the normal planning consents).

JohnH

[Carole]

The Owl Nebula has come out well and generally it looks much better than previous images (the spacing in the corners apart which you have explained).  But there is something very odd going on with the brighter star diffraction spikes.

I wonder if that too is related to the change of spacing between the L and the R images, I think it is likely.

Carole

[RobertM]

That's a great image from Bromley John, you have a few faint fuzzies in there to.  A bit annoying about the spacing but we all make mistakes from time to time (I've made more than enough !).

I know what you mean Carole about the spikes, red towards the top and blue the bottom, they do looks a bit unusual.

Robert

[JohnH]

Thanks Carole and Robert,

I am finding the spacing difficult. The reducer screws into the focussed but here is no stop to position it, it is very much trial and error. I have discovered that the plate solving script in Pixinsight produces a graphical chart of distortion which suggests that I still need to increase spacing.

I have a suspect list for the spikes:

• Changed spacing.
• A heating band on the secondary mirror.
• The cable from the heating band running along the top of one of the vanes.

I have done what I can to minimise 2 and 3 for the future, only time will tell.

As for 1, obviously I shouldn't change the image train like that and try to use the images together. If the spacing is changed like that there must be distortion.

In the meantime, it was good practice with PI. I have discovered the joys of WBPP and Photometric Colour Calibration. The last has massively improved the colour balance.

Regards,

John

[Roberto]

John

This is an excellent outcome for a difficult pair. The galaxy and nebula require different filters and processing and you have avoided any reflections from Merak.

Roberto

[The Thing]

Nice composition John. Something to have another go at as it looks nice and its unsual.

The spacing lark is difficult. The reducer position in the focuser shouldn't matter on a reflector, it only changes the relative position of the focus tube when your focused. Its the distance from the back of the reducer that's the pain. Luckily it looks from the spec sheet that there is an easy to identify point on that reducer to measure the required 55mm from. My ASI294 sensor is exactly 6.5mm from the front face of the camera. Unfortunately in my imaging train with my TS1506UNC the filter drawer and OAG are not exact mm widths and I need 50.5mm. I need 0.21mm of Baader and ASI (very thin) shims to get it spot on. Then when I change everything to my Sharpstar refractor its 55mm and start measuring again!

[MarkS]

It's well framed and despite the problems you had, the end result is pretty good.

Mark