Alnitak interference pattern - 8" f6 Newt.

[alpal]

Hi guys,
I was re-processing my Flame & Horsehead Mosaic pic last night
when I came across some shorter sub frames I'd taken.
I also re-processed the RGB just for Alnitak with
a linear stretch out of the stacks to work on.

I got a wonderful interference pattern that looks like
twisted rope on the mono frames & comes out very nicely in colour.

I used a 25 second exposure frame of Ha to get the smallest
star size - which was not reaching the well depth - & used it for the
center of the boosted colour shot.

I believe that the star near Alnitak it's one of it's companions?

I include a tiny re-process mosaic of the Flame & Horsehead where I tried to use
the Alnitak rainbow spikes - the re-process was much better
but the spikes were hard to put in with all the colour here.
larger version is on flickr:

http://www.flickr.com/photos/24719437@N03/12483840583/in/photostream



All comments welcome.

cheers
Allan

[Joshua Bunn]

Hi Allan,

Interesting, I don't mean to offend, however it almost looks as if the scope isn't focused for these stars??

love your horsehead :)

[alpal]

Hi Joshua,
It was focused with a Bahtinov mask just before taking these shots.
I think the horsehead is as good as it can be with a small amount of data.

cheers
Allan

[Joshua Bunn]

Thanks Allan.

[alpal]

Actually - you might be right that the pics were slightly out of focus.
When I look at this webpage:
http://www.bisque.com/help/theskyxsaeandpro/focusing_principles.htm

it shows this pic:
http://www.bisque.com/help/theskyxsaeandpro/theskyx_pro_user_guide_files/image618.jpg

It shows 2 spikes are formed when slightly out of focus.
Is this what I'm seeing in my picture or is it correct
& just a normal interference pattern?
I know rainbow colours are correct but should they look like my pics?

Maybe someone can chime in?
Maybe that is something to look for when checking focus?
I'll take some more pics next time & deliberately put it before & after focus
to see what happens.

cheers
Allan

[strongmanmike]

Those diffraction spikes do look ever so slightly double and thus out of focus to me :question:

I have always just focused by eye looking at an enlarged star on the screen in real time (usually 1 or 2 sec exposures) and without any sort of mask. I just press the in and out buttons until the focus star is as small as possible :shrug: in reasonable or better seeing it is quite a quick and easy process however in periods of bad seeing it does waste some extra minutes as it is harder to tell if I truly have the very best focus due to the star going in and out of focus due to the seeing...especially with the tiny CFZ at F3.8.

Mike

[alpal]

Thanks Mike,
Do you have any shots of a bright star such as Alnitak that you could post - just for comparison?

I have re-collimated the telescope since those Alnitak images.
I have had a look at many other images before & since the Alnitak
image & I can't find any evidence of a twisted rope like
formation - rainbow colours - from any star's diffraction spikes.
Then again - none were so bright as Alnitak.
I will revisit this during my next imaging session.
It could be normal - but it could also be that it was slightly out of focus.

cheers
Allan

[strongmanmike]

Not as bright as Alnitak but the rainbow effect is pretty obvious, how are the bright stars in this (http://www.pbase.com/strongmanmike2002/image/153955170/original)? Looking again at your Alnitak I think it is very slightly out of focus.

[alpal]

That's perfect Mike -
the rainbow colours but not split into 2 lines or looking like a piece of twisted rope.
OK - you've solved it -
I must have been slightly out of focus.
I will be able to check this next time after using a Bahtinov mask.
Me or anyone else watching this thread should now have a way of double checking the focus
before making a run of subframes.
It showed up on any of the RGB frames.
I hope to get even sharper pictures now. :)

cheers
Allan

[alpal]

Here is a shot taken the same night but in Ha.
Exposure 10 minutes binned 2x2.
All other 11 Ha frames looked about the same.

Notice that there are not 2 lines but there is still a twisted rope effect? -
the extended spikes get dimmer then brighter.
This view is at 200% where as the first was at 100%.
The other stars do look more in focus.
Notice that the camera must have rotated as the companion
star of Alnitak is now inside a diffraction spike.
The focus was changed for Ha as it's different to the LRGB filters
which are supposed to be all parfocal.
The seeing was about 3 arc seconds & the guiding inside 0.33 pixels rms.
Ignore those hot pixels.

Question - was this one correctly focused?

[Joshua Bunn]

That looks much better Allan, but it is hard to tell if this was the best you could have gotten that night. Also, if collimation was off a little when you originally took the first image in the OP, and you used a Bat mask to focus on a star on the opposite side of the frame from where you cropped (if so) that star from, you could get mixed results. Maybe you used the Bat mask on this star though?

Do you use focus software?

Josh

[alpal]

Hi Josh,
The Bahtinov mask was used for Alnitak which was at the edge of the frame.
The collimation was a bit out & adjusted it since then.
I should have used 1x1 binning not 2x2 for focus.
I am unable to use the EZCap QHY9 focus software.
The focus part of the program just doesn't work.
I use it for all other capturing.
We'll see what I get next time.
I will really try to nail the focus.

cheers
Allan