Lagoon Nebula M8 / 4 x 30s / First Guiding & Stacked, Help Appreciated

[dylan_odonnell (Dylan)]

Hi all,

Although terribly daunted by the high quality of the "beginner" section of this forum, I was still excited enough at having achieved a result however shaky and would love some advice going forward.

Details :
9.25" Celestron SCT + CG-5 eq Mount
80mm Guidescope + Starshoot 3 (?) Pro Mono CCD (in reality it's CMOS)
Nikon D5100 attached to Star Diagonal / 1.25" adapter / IR remote
4 x 30s subs @ 1600
Stacked in Keith's Image Stacker (OS X)
Star Bloat removed in post using PS Colour Range selection and -> "minimum" filter. Inverted / Levels.

The process :
I got a rough polar alignment, found the target, used live view on the DSLR to zoom right in and focus on a star, started guiding with PHD and took a bunch of exposures at various speeds and ISOs so I could compare later. Only 4 were good enough to stack and a tree overhead ended my night early.

Some questions for the more experienced if anyone can help!

- Stars were a bit blobby.. I assume this was the seeing and their brightness overall and maybe some softness overall. Is there any way in my process I can reduce this? Is there a better way to get focus?

- The red line (DEC) was slowly drifting up. In PHD the star was slowly drifting down. The target was straight up (zenith?) Does this mean I need to adjust my latitude on the mount or it's direction south to the SCP?

- If I do get alignment better and somehow manage to get minutes long exposures, should I bump the ISO down and do those.. or is is better to stick to 45s - 1m exposures and simply do as many of them as possible. If it's the same thing either way I assume it would be better to do shorter ones to isolate any shakes, bad moments when it comes to stacking right? Or is it better to stop down the ISO for less noise and do longer subs?

Thanks for bearing with me, and I do enjoy all your posts even if I don't post to say so!

Dylan.

[cometcatcher (Kevin)]

It's a good start Dylan.

About the star aberrations, you have an unusual way of connecting the camera to scope. It looks like it's a little tilted as the stars on the lower left corner are sharp. If you can get an adapter to go straight through (no diagonal) that would be better.

Expose for as long as you can in order to keep the majority of subs and collect as many as possible.

[Eden (Brett)]

Hi Dylan,

You're certainly headed in the right direction. Try not to be daunted by what you see other folks posting in the beginners astrophotography section -- what matters is the fact that you've gone out there, given it a go and brought back some images.

I don't use a DSLR so I can't be of too much help in that department but I can offer you a few general pointers:

It looks like you might be experiencing some vignetting around the edges. This is probably due to the 1.25" adaptor shadowing your DSLR sensor. If you haven't done so already, consider getting a 2" adaptor for the SCT and connect the DSLR straight into the back of the telescope without using a diagonal. The diagonal might be the culprit which is causing the slanted appearance in your stars.

With respect to Dec guiding errors, I had exactly the same problem with the CG5 and other people have reported likewise. There's no silver bullet for this one unfortunately. Doing a google search to read what others have had to say on the matter is a good start, but briefly:

- The working load capacity of the CG5 is less than what Celestron advertise and when it's overloaded, Dec is where it shows. Try guiding without your DSLR or diagonal connected and see if the problem persists. That will help you determine if it's overloaded or not.

- Double-check to make sure that the weight of the scope is properly balanced on Dec.

I hope this helps a bit!

[dylan_odonnell (Dylan)]

Thanks Kevin and Eden for the kind and encouraging words and assistance. It's much appreciated! I do indeed have the 2" adapter to hang the DSLR from the rear of the tube without the star diagonal and will use that next time.

I'm amazed about the CG-5 DEC / weight error! This is good news in a sense.. in that my process is ok... but I risk divorce if I spend any more money on this hobby Hopefully without the star diagonal and a little more time spent balancing I can get rid of the DEC drift otherwise I'm stuck! Unbelievable that the scope can be overloaded using the very same 3 weights provided with the mount! It seems balanced to me, it doesn't move no matter where I position the OTA before locking it down and I give it a slight touch more weight on the opposing side for the motor to work against (I've read this is the right thing to do?).

Super grateful for the feedback. You've both probably saved me hours of digging around!

Dylan.

[LightningNZ (Cam)]

Hi Dylan,

Nice capture! I really love that you've managed to keep the colours of the stars looking great - lots of people screw up the stars when trying to get as much out of the nebula as possible.

As the others have already said, having as little in the way between the camera and scope is Good Thing(TM), especially the little 1 1/4" star diagonal which is about the same size as the sensor in your camera (good choice of camera BTW, I have the same one).

Cheers,
Cam

[raymo]

Hi Dylan, You can remove any uncertainty with your focusing by buying
or making a Bahtinov mask, and using it in conjunction with "Live View".
Buy on ebay, or from Bintel[about $40-45,] or make one by downloading
a template from the internet.
raymo

[Eden (Brett)]

Hi Dylan,

Before worrying too much about the mount itself, take some time to refine your polar alignment. The more accurate your PA is, the less work PHD will have to do. You might also need to wait a while after you begin guiding so that PHD has time to "settle". If you find that PHD is showing Dec errors even after refining your PA then it's possible that the mount is overloaded.

When you say the mount was supplied with 3 weights, how much do they weigh?