Blobs blobs and more blobs

[IvanT]

Hi,

Just a quick one (hopefully), in the image below the stars seem to me to be quite blobby. I normally use a Hartmann mask when prime focused so I don't think it's a focus problem, from this clipped upper left segment of my 1st effort at M42, does the collimation look off or do I simply need more data to bring out the brilliance of the surrounding stars? I don't want to go messing with the collimation anymore than I already have unless I really need to.

Thanks

[SteveL]

If you are using the 1000D and the 200P, are you also using some kind of coma corrector as well? If not, this issue at the edges of the image is coma, where the image is only in focus in the middle and not at the edges. Its inherent with newtonians, so not much you can do without a coma corrector to make that "in focus" area larger, and enough to cover all of your DSLR chip.

[IvanT]

It is the 1000D and the 200P. I don't have a comma corrector fitted but the blobs are also apparent at the center of the image in the M42 picture I linked.

[FraserClarke]

200P is f/5 is it?? So that gives you a 1000mm focal length. A 1000D has pixels of about 5 microns, I guess, so that gives you a pixel scale of about 1-arcsecond/pixel. I think your 'blobs' are just what the stars naturally look like under imperfect seeing (usually 2--3 arcseconds in the UK), and a little bit of guide errors/wind shake. There will also be an effect from the processing that makes the stars look 'bigger' when you try to pull out the faint detail in the nebula.

I wouldn't worry too much about it. Steve's point about the coma corrector is a good one -- Newt's don't (by design) deliver good image quality away from the centre without one.

Don't be fooled by huge pixel scales of these short focus refractors most people use these days -- they are just cheating by putting all the light into one huge (3--4 arcsecond) pixel You're sampling the image properly

[IvanT]

Thanks guys, looks like there is a coma corrector going onto Santa's list.

Sent from my iPhone using Tapatalk

[Tim]

Focus looks ok by the spikes on the stars, but to remove ANY doubt about that, a Bahtinov mask makes it easy to achieve perfect focus every time, a lot easier than with a hartmann mask in my experience, especially with a dslr.

[IvanT]

I had looked into one of those TJ, but after using Nebulosity for the first time recently (its framing & focusing features are fantastic), I don't think focus will be an issue going forward (hopefully). This was focused using the notebook out beside the scope, I didn't need to look at the camera at all and being able to frame the shot before committing, well, I can't begin to say how much time that's going to save.

http://i214.photobucket.com/albums/cc318/rimwald/Astrophotography/M34.jpg

One thing, what do you guys do with regard to notebook dewing up when outside?

[Tim]

cover it up! the heat from the fans on mine under a cover kept it frost free.

i use nebulosity with a batinov mask, easier than just relying on the fwhm figure

[ollypenrice]

Cardboard box works well, upside down. Keeps the light down as well.

One thought about coma correctors; some are on the long side and push your camera a long way outboard, raising worries about flexure and orthogonality. At least I found this when trying to hang CCD gear off a Newt.

Olly

[IvanT]

Thanks for that Olly. The coma corrector & LP filter are now in place (for the love of God someone tell me to STOP SPENDING MONEY), just waiting for a decent night to try things out. Will hit this same target again to get a feel for any improvement.