My second DSO - m31

chuang · Post by **chuang** » Fri Oct 31, 2008 12:05 am

Hey guys,

Took 12 x 10sec 30 min ago. Stacked using DSS with 6 darks. Focused using liveview. Canon 450 through vixen 80ed on sphinx, unguided, no drift alignment done (too lazy).

chuang · Post by **chuang** » Fri Oct 31, 2008 12:08 am

another -

Tachyon · Post by **Tachyon** » Fri Oct 31, 2008 8:32 am

I think 10 secs is too short to capture DSOs. The minimum would be 30 secs. Although you tried to stack 12 pics of 10 secs to give 120 secs of exposure, unfortunately it doesn't work this way (else we could get away with 120 pics of 1 sec). 10 secs may not be sufficient to capture enough signal to stack.

Just my views... the pros can comment further...

Meng Lee · Post by **Meng Lee** » Fri Oct 31, 2008 11:50 am

Yah correct. It does not work that way.

When we say 12 x 10 sec = 120 sec image is true when the photon shot noise is the most dominant. But in a 10sec image, readout noise is still more dominant, especially at the dim parts of the object. So in such cases, further stacking will not result in much improvement.

So try to go for longer subs

chuang · Post by **chuang** » Fri Oct 31, 2008 12:31 pm

Ya i agree. Anyway I was not intending to do further stacking to achieve anything. I stacked 12 cos I have 12 haha. just fooling around.

First time I am using the mount and the camera. first time focusing also. first time unguided also. also first time using DSS so didnt expect too much to come out. Anyway did these in 30 min while going through mount orientation. Longer subs definitely requires some guiding quite possibly through a guidescope with guider. I will try these but slowly ok. and that will be after I do some drift alignment, PEC etc. steep curve.

Post by **perseid** » Sat Nov 01, 2008 1:27 am

A very good try. The focus is right on. Next step would be fine tuning your drift alignment. With Sphinx, I am sure you can get about 1 minute unguided shot.

If you don't have illuminated recticle to do the drift alignment, you can try doing it the digital way. This is easy to do since your camera has live view feature. First align your camera orientation so that north is up and east is right. See where the star drift to and make the necessary adjustment. If you don't see any drift after one minute duration, then you can start imaging.

Start with 30 frames at least. 60 would be good. Between exposure allow 30 seconds for the camera to cool down.

If you don't get a good result on the first few tries, don't get discourage. M31 is quite a challenging object to image especially from light polluted location. Most probably, you'll get a big blob in the center and quite a featureless disk.

Good luck.

Post by **weixing** » Sat Nov 01, 2008 2:17 am

Hi,
Yes... Try to get at least 1 minute for sub frame. 1 minutes unguided exposure should not be difficult for your scope and mount... especially a GOTO mount with a light load.

If you don't have illuminated recticle to do the drift alignment, you can try doing it the digital way. This is easy to do since your camera has live view feature. First align your camera orientation so that north is up and east is right. See where the star drift to and make the necessary adjustment. If you don't see any drift after one minute duration, then you can start imaging.

Although the digital way is slower, but can be quite useful if seeing is bad (the star "jump" all over the field in reticle eyepiece at high magnification). Just expose the star for the require exposure time with tracking on, then switch off the tracking for a few seconds which will cause the star to leave behind a star trail from east to west at the end of the drifting star. The direction of the brighter star trail before the tracking is off will indicate whether the star drift to north or drift to south and you can make the approximate correction.

Anyway, base on my experience, you should be able to get at least 2 to 3 minutes of unguided exposure if do a good drift star alignment with your scope.

Start with 30 frames at least. 60 would be good.

Yap... more sub frames give better signal-to-noise ratio, but eat up your hard disk fast... a single colour image will take up 70MB for Canon 450D during processing!

Between exposure allow 30 seconds for the camera to cool down.

Does 30 seconds gap produce significantly less noise?? I usually don't wait for so long... may be only 10 second for the DSLR to save the image because we usually don't have long period of clear sky... especially this year... still didn't manage to image any DSO this year.

Good luck and happy imaging.

Have a nice day.

PS: Hope weather can clear up soon.

chuang · Post by **chuang** » Sat Nov 01, 2008 9:59 am

Thanks for the advice! I got a few questions -

1) Drift alignment using live view - I suppose I can switch on the grid function to do this. Do I rotate from the eyepiece holder part of the flip mirror assembly to achieve this? Not from the T-ring right?

2) Long exposure in light polluted areas - If I do a longer exposure, like 30 secs the whole sky brightens up. Do I do a few shots to determine a suitable exposure time to use? How to choose?

3) Imaging small DSOs - I have 2 refractors of 700mm and 800mm focal length. What do I do if I want to zoom in on small DSOs like m15 or m57 or clown nebula?

Thanks

Post by **weixing** » Sat Nov 01, 2008 10:55 am

Hi,

1) Drift alignment using live view - I suppose I can switch on the grid function to do this. Do I rotate from the eyepiece holder part of the flip mirror assembly to achieve this? Not from the T-ring right?

You connect to the flip mirror using the T-thread on the flip mirror, right?? If so, you need to rotate the whole flip mirror assembly. Also, IMHO, the live view isn't that effective for drift alignment because the live view zoom is not 100% which mean small drift error might still show up on the image even you didn't see it on live view.

2) Long exposure in light polluted areas - If I do a longer exposure, like 30 secs the whole sky brightens up. Do I do a few shots to determine a suitable exposure time to use? How to choose?

Use the DSLR histogram to determine how long can you expose. As long as there there is some gap at the right edge of the histogram, it should be ok. All my past DSO RAW image (1 to 2 minutes exposure) from my past F5 scope look over expose by light pollution, but it's still very decent after the processing. You can also use light pollution filter to reduce the light pollution, but you might need to over expose longer due to some light lost cause by the filter.

Imaging small DSOs - I have 2 refractors of 700mm and 800mm focal length. What do I do if I want to zoom in on small DSOs like m15 or m57 or clown nebula?

Use an extender (expensive method) or a barlow (low cost method) to increase the focal length, but the drift alignment become more critical at longer focal length.

Have a nice day.

swat_pup6433 · Post by **swat_pup6433** » Sat Nov 01, 2008 1:06 pm

Use an extender (expensive method) or a barlow (low cost method) to increase the focal length, but the drift alignment become more critical at longer focal length.

note that if you were to increase your FL your scope, the Focal ratio of ur scope increases too. you might end up with a F ratio too slow for any use other den visual. and as for very small and dim dso in singapore.. i would suggest. you stay away from them first. Bigger stuff would be more rewarding like the M8 and M42 for a start. and at that kind of FL, guiding the mount is a must. else u'll just end up with interesting looking lines for any exposure past 1 min.