Deepsky not going really deep

[coenie777]

I have been imaging for just on a year now and still need to learn a lot.

I recently started to image exclusively with my AT65EDQ apo. I shot some nice unguided images before my guider arrived about a month ago.

I thought that once I would do 300", the results I have seen on 45" and 90" so far would far be exceeded. This is however not happening. I would appreciate some advice on what could possibly be the result of these mediocre results, guided.

I image without any filters and with an un-modded Canon 40D. My image site is at best mag 4 skies with plenty of light pollution right round me.

My image used in the compilation of these images were all tight stars with slight elongation on the unguided shots on around every fourth one. Again, to my eye I got a better image from that than the round star guided images.

The first shot is badly colored but I am looking more at detail now than getting the color right. It was a combination of two session's data. I shot 30 x darks, 30 x flats and 30 x bias frames.

The second shot was a combination of 46 x 300" and 39 x 180" guided with FWHM reported by DSS to be below 4.1. Sensor temp were a bit high with EXIF data showing 27 - 29 degrees. I shot darks x 20, flats x 30, flat darks x 30 and 30 bias. Histogram were pushed very far right with three shots actually not making it due to the drop off in FWHM due to excessive light.

To my eye I got more detail from the 45" shots than the guided shots. I used Startools for processing and tweaked in CS6.

Any views/advice would be appreciated.

Image 1 : http://www.astrobin.com/195204/B/

Image 2: http://www.astrobin.com/212273/

[wxsatuser]

Most likely 300sec subs at that ISO? over exposed for your sky brightness.

You will get to a stage, as you over expose, where the faint stuff merges with the sky brightness.

You can lower the exposure to bring the hump in the histogram back under 40%.

or lower the ISO to bring the hump back down

[coenie777]

Thanks Mike

What would work best, reducing ISO or exposure time?

You are right, looking at the histogram in DPP (Canon software) the histogram is over to the extreme right.

I will aim for the 40% mark you mention next time.

[wxsatuser]

Personally I would want subs as long as possible.

The best ISO for read noise for the 40D is 800 or 1600.

I would use 800 and expose for the hump to be no more than 40%, that will be your best sub length.

If you not getting near the 300sec sub then just get more of the shorter subs.

Your original image is not to bad considering, where was the histogram hump for those subs?

[coenie777]

Thanks Mike, 800 or 1600 iso was what I was aiming for. I used to image at 400 after misreading a Craig Stark review of a 350d and advising 400 as optimal. Somehow I thought this applied to 40d too and just kept shooting at 400 until fairly recently.

For the 45sec shots the histogram looked like this in Canon's DPP:

The 180sec looked like this:

I will make sure to read this histogram to know where 40% is as I would prefer to not have to check the back of the camera after the test shots.

[coenie777]

I see there is a n RGB histogram option as well.

Here it is clear that on the 180" shots I pushed the red channel off the histogram probably clipping it before I even started processing. Hopefully this post could serve as advise for someone in a similar situation as me.

The 45" histogram sits well within the 40% band (I think):

But the 180" show clearly why I could not process these images to something respectable:

[Rico]

What about a light pollution filter? It can make a huge difference to your allowable sub durations.

[jtlandreneau]

I shoot at ISO800 or 1600, I prefer 800 (1600 just has too much noise) with my 600D. I have a ton of light pollution that was limiting my exposure times, buying an AP light pollution filter made a huge jump in the image quality - it let me do longer subs to pull out the faint detail I was trying to catch