Suggestions to an imaging issue please?

[Chris81]

Hi everyone

I am relatively new to astrophotography and have had some successes but twice as many disappointments in all honesty...

I am currently using a Canon DSLR via my 8" Reflector with GoTo/Tracking. I a stacking images in DSS and then processing them in Photoshop to get a final image. Given that I do not currently have a guide-scope with a star tracker (such as Synguider or similar), when shooting for 45 second exposures or more I am getting star trails, so the final image is poor and star detection in DSS is low too (maybe that's another issue altogether..?!).

What would you recommend for now? Is it better in this instance to take shorter exposures (Say 20 Seconds) and stack more frames (Say 30+)? I am aiming mostly at Clusters, Galaxies and some nebulae.

Any advice/tips is greatly appreciated.

Thanks!

[oldfruit]

I have had some reasonable results with a very similar set up to yours. Yes you would need to reduce the exposure times. Firstly though make sure your polar alignment is as accurate as possible, as this will make a big difference and could possibly be the source of some of the trailing. The work through different exposure times until you have reduced the trailing to your satisfaction. As an example, try 40 seconds, then 35 seconds, then 30 seconds and so on until you get the desired results.

[johnrt]

You have a big heavy scope on a mount that is under the general accepted minimum for imaging work, so you are going to need to go back to the basics - make sure your mount is polar aligned as well as you can and balanced in each axis perfectly. Even then, you will be doing well to get 45 sec subs without any guiding on that mount & OTA

[Chris81]

Duly noted, thanks. Yes, Polar allignment could probably be tweaked,it's accurate enough to find objects easily enough, but for photography? probably not. Yes the OTA is pushing the capabilities of the EQ5 Mount, that is pretty clear to me these days, a mount upgrade is definitely on the horizon! but for now, I will experiment with exposure times and better allignment and see what happens.

Thanks for the advice.

Chris

[astrosathya]

Hi Chris,

Try to drift align. Even a basic drift alignment should give you about 90 seconds of exposure which is good, but will need a lot of frames to collect data. Sky-Watcher has released the latest firmware upgrade wherein there is a polar alignment routine that can get you up to 5 arc seconds close to the North Celestial Pole, which is very good. It will depend on how level your mount is, because if the mount is not level, an adjustment in one axis will put the other off slightly. I have tried this routine myself because from my rooftop where I image I cannot see the pole star and this routine does not require the pole star. It simply picks two bright stars one North (ish) nd the other West (ish).

Before you use this routine however, it would help to align the polar scope reticule to the RA axis. A very easy procedure is explained in the video tutorial bu Dion of Astronomy shed. Remember that there are little grub screws in the polar scope that are screwed in and out to adjust the reticule. However, they are too small and often fall through into the polar scope. Good news is, they are M3 screws, and can be found at any online hobby store. Just get yourself half a dozen of them, about 19mm long with an allen head.

Hope this helped.

Regards,

[Chris81]

Thanks Astrosathya i will give that a go!

[pete_l]

I am currently using a Canon DSLR via my 8" Reflector with GoTo/Tracking. ... when shooting for 45 second exposures or more I am getting star trails, ... I am aiming mostly at Clusters, Galaxies and some nebulae.

If you have enough back-focus, could you buy a focal reducer to help ?

The F-R would do two things. First, it would make the field of view that your CCD "sees" larger, so the star trails would be smaller (they'd still be the same angular size, in arc-minutes but by getting more arc-minutes of field in the image, they would trail across fewer pixels).

Second, the F-R would allow you to take brighter images in the same time, or to take shorter images with the same results (but smaller targets in the final picture). The shorter sub-exposures would also help reduce the trailing.

Be aware that you'd need to make sure you could fit the F-R, both to the camera and the 'scope AND that you could still get it all in focus, given the longer optical path. Though once you do get the trailing fixed, either with a bigger mount or by guiding) a F-R is a handy piece of equipment to have.

[rfdesigner]

The Focal reducer is a reasonable idea.. but not easy for a Newt and a DSLR. (cost)

Your best bet would be to use the skywatcher coma corrector which is important for newt imaging and gives you a slight reduction too. FLO seem to be saying 0.9x, and in your position, every little helps.

How long to take?.

If you image at say 5s and have essentially no drift, but a 10s you have just noticable drift, what's happening is that the background (skyglow) is now twice what it was in your 5s image, but your star is no brighter because it's illuminated different pixels in the last 5s compared to the first 5s. So a 10s image will have worse signal to noise than a 5s image!

So optimal performance will be achieved at the point where your stars aren't quite drifting.

Next:

You will have dim subs with lots of readout noise and other camera artifacts.. getting darks to work on a DSLR is not easy as the noise is dependant on sensor temperature which is changing.

I use IRIS for processing and I did recently find that it has a dark removal tool that can automatically scale the dark frame so as to get best correlation to the light frame.. i.e. it can automatically compensate for your DSLR temperature drift. It's still important to get dark frames that are taken at roughly the same temperature as your lights.. but with this they only need to be roughly the same.

The command (on the command line only) is OPT

clear skys

Derek

[dph1nm]

I have an 8.5" f6 on an HEQ5. I never use more than 30s exposures - periodic error on the mount is the killer, not drift alignment. But it works fine. You need to stack maybe 100+ subs though. I never use darks on my Canon 1000D - total waste of time.

All these shots were taken with this setup - you can see the total exposure times listed. Just divide by 30 to get the number of subs!

NigelM

[Chris81]

I have an 8.5" f6 on an HEQ5. I never use more than 30s exposures - periodic error on the mount is the killer, not drift alignment. But it works fine. You need to stack maybe 100+ subs though. I never use darks on my Canon 1000D - total waste of time.

All these shots were taken with this setup - you can see the total exposure times listed. Just divide by 30 to get the number of subs!

NigelM

Thanks for this, it's useful advice. I took a look at your images on the website too. most impressive I have to say. thanks