M83 and Imaging Blog

[Robby]

Hi All,
Well after two nights of stuff-ups I finally managed to image M83 last night. I only got in 18mins worth of exposure before begin clouded out, and I'm not 100% happy with the image but thought I'd post it anyway.
I reckon I break all the rules of imaging, but rules are made to be broken ..

Rule #1: Guide at at least 1x, preferably 1.5x the focal ratio of the imaging scope.
Broken! I guide at f/5 & image at f/10. 0.5x!

Rule #2: Shoot in Raw mode
Broken! I shoot in jpeg and have yet to find a reason to go to Raw. I started with Raw, but it didn't make any difference.

Rule #3: Image in a dark sky.
Broken: Most of my images are taken in my light polluted back yard.

Rule #4: Use hartman mask or other focus aid.
Broken! I focus with the eye & trial and error with the camera screen until I get it close enough.

Rule #5: Take dark frames every time you image.
Broken! I took some dark frames ages ago at different ISO & time settings. These are the masters I use every time now. Seem to work ok.

Rule #6: Do a flat field..
How can I break a rule if I don't even know what in means!...

Anyway that's enough rambling. I guess my point is that you don't need to follow every rule in the book to produce images and experimentation is the key!

[Vermin (Tom)]

I think a flat field is a uniformly illuminated frame (shows up vignetting).

Nice shot, plenty of detail.

[iceman (Mike)]

Great Robby, I like it! There's so much to know about DSO imaging to do it properly.. takes a lot of time and patience, two things that I don't have a lot of

[ving (David)]

great shot
love galaxy shots. and there is a lil' double forming just above it too

[[1ponders] (Paul)]

woowoo Robby, I hope my galaxy shots turn out that well.

[Comet Hunter]

Great shot Robby

[gbeal]

Well done, full credit, and apart from the blobby look it is as good as you could ask for. Better than what I ended up with, clouds and all.
I know what you mean though, I tried guiding at f5 (400mm), and imaging at f12 (2160mm), and it wasn't pretty.
Oh boy.

[Robby]

Gary,
Blooby blob blob.. Your sounding like your brother
Focus wasn't quite spot on, and the 0.5x guide ratio means that I do get a little bit of trailing which makes the blobs slightly bigger after stacking.
Full credit indeed......
I got lucky with the clouds. Had a break for about 40mins.
Cheers

[rumples riot]

Lovely shooting Robby, flat field shots are taken with an opaque white light source, through your scope, (basically a light box) they are easy to make from what I have seen, but like you I break this rule all the time.

Still for all the rules that you broke, I think that this is a tremendous shot. Spiral arms are evident, and so too are the dust lanes.

Keep up the good work.

[gbeal]

Brother indeed!!
What was the one from last year/season like? Better/worse?

[Robby]

Gary,
I had star-trailing ptoblems with last years image. No star-mate back then! It was manually guided. Full credit.
Here it is anyway. Not sure why the colour is so different?
Cheers

[gbeal]

Yeah, thats it, I remember it. Nice, and not as bad as you suggest.
Colour, or lack of it, is probably a result of you forgetting how to use the RGB channels in P/Shop.

[MiG]

"Rule #1: Guide at at least 1x, preferably 1.5x the focal ratio of the imaging scope.
Broken! I guide at f/5 & image at f/10. 0.5x!"

Isn't that meant to be magnification? It doesn't make sense to me otherwise.

[Robby]

What I mean is that if you image at f/10 you should guide at f/10 (1x) or f/15 (1.5x).. Strickly speaking it's not a direct mathematical relationship, but you get the idea.
Problem is if you guiding at f/5 and imaging and f/10 (as I do somethimes), every 1 pixel movement in the guide scope = 2 pixels in the imaging scope (assumming similar pixel size as I have). ideally you'd want it the other way round.
Cheers

[MiG]

But I don't see how F ratio relates to pixel size. I understand how 0.5 magnification ratio will result in your example, but not F ratio.
F ratio is just the focal length divided by the aperture.
I am missing a piece of the puzzle aren't I? What is your guiding method?

[Striker (Tony)]

Nice work Robby........btw that reply regarding F10(1x) 1pixel guide 2pixel cheers.....means as much to me as it sounds.....lol

Time for me to head back to the beginners forums.....hehe

[Robby]

Mig see my website for my guide setup.
I'm not a mathematician, so I'm not going to try and explain the maths. I wouldn't even know where to start.
But it's all to do with image scale.

Take an 80mm scope at f/5 into a 100x100 pixel 10um chip...
Image FOV will be 8.6' x 8.6'
That same chip at f/10 will be 4.3' x 4.3' It's all to do with image scale. Now it's a little more complicated than that as often imaging scopes are different aperture from guiding scopes. The 1.5x f/ratio is simply a guideline..

But let's look at my setup with our fictitious 100x100pixel 10um chip..
Guidescope 80mm @ f/5 : Image size = 8.6' x 8.6'
Imagescope 200mm @ f/10 : Image size = 1.7' x 1.7'
So actually to get pixel for pixel correction I really need my guiding scope to be f/25 or 2.5x that of the imaging scope.

Right that's probably lost everyone... or put them to sleepIncluding me! I was probably a little optimistic in my previous post. My setup is worse than I thought!! I'd just never worked it out before!.. But as I siad, I don't care much for rules..

Cheers

[Starkler (Geoff)]

Originally posted by MiG
But I don't see how F ratio relates to pixel size. I understand how 0.5 magnification ratio will result in your example, but not F ratio.
F ratio is just the focal length divided by the aperture.
I am missing a piece of the puzzle aren't I? What is your guiding method?

The F ratio is important as the imaging chip sits at the focal plane, and not via an eyepiece which determines magnification.
More accurately its the focal length which will determine pixel size.