My second DSO - m31

[chuang]

Did another tonight. I think the processing is a bottleneck. Anyone has any tips for DSS?

[Meng Lee]

Hi,

I checked your image in Photoshop, black point is just a bit high.

I don't think image processing is the bottle neck here. The intensity of the spiral parts are about the same as background noise, so the signal to noise ratio is very low. One way could be to lower the ISO and take longer exposures. Photographic LPR filters will get the contrast a bit higher also.

Also, M31 is an object with a large dynamic range, it is not really an easy object to take as it seems.

Just my opinions.

[chuang]

Thanks Meng Lee.

My current iso is 1600 do you think I should try 800? As for exposure, I am doing 2 minutes now. It seems that the background of the images are a bit bright. Is this normal?

If I use an LPR, what kind of exposure time should I try?

I did not use bias images perhaps I should also try that?

Do you think the light polution has a large part to play here? Thanks

[Meng Lee]

Hi,

Normally I don't recommend high ISO, especially in light polluted areas. Lower ISO also gives u more dynamic range to take in objects whose brightness varies over a large dynamic range.

When you subtract dark frame the bias is also removed as the dark frame contains the bias in it. People sometimes take bias frames seperately to create scalable dark frames.

Light Pollution is the major problem really. Imagine the spiral arms are as bright as the sky background (especially m33 and m101) then expose longer is useless as both signals from spiral arms and sky background builds up. So the aim is to create a difference of intensity between these 2 signals. I know of 2 ways:

a) Imaging LPR filters like those of Hutech or Orion
b) Gradient removal from image

Of course the best way is to go to a dark sky location! You will find that data is much better, processing is also easier and results are much better. Hehe.

[perseid]

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.

I read somewhere that the time is 40 seconds. This is the time given to the camera to cool down to its normal standby temperature. Of course, our weather is different.

I chose 30 seconds. I too can't wait idling for so long. 30 seconds is long enough for me to check my image to see if any unexpected crops up, such as dew on the lens, guiding ran off or to see if the transparency get worse.

[perseid]

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

Sorry for the late response.

1) I haven't personally use the live view function before. But I believe you can magnify on certain star. And if you have grid, that would be great. For the camera orientation, you have to align the camera so that the axis of your mount is the same as the CCD chip. You can check this by putting a bright star at the edge and press your controller to slew the star along the edge. The distance between the star and the edge should remain the same throughout the whole length. IF this is not the case, simple rotate the camera around the focuser, then do the above process again. This will make your life easier.

2) You can start by using LPR filter such as the IDAS one. With or without the filter, you can determine the maximum exposure by looking at the histogram. The majority of the signal should fall around 3/4 to the right. If there are signal that clip to the right, you have to reduce your exposure time or ISO. I suggest reducing the ISO to increase the dynamic range.
From my semi polluted area, I can do 180 seconds @ 800ASA.

3) For small and usually dim objects, you should use a reflector such as 8" F4 or F5. This way, you can have around 1000mm FL and still have very fast optic. You can still opt to use LPR filter if your sky is too polluted.

[chuang]

Thanks for the tips guys. I will try again when the sky is good and when I catch back all my lost sleep.

BTW Perseid, thanks to your liveview drift alignment tip! I figured it out before your explanation and it really works. However Grid view does not work cos the grid is black...doh.

[weixing]

Hi,

Did another tonight. I think the processing is a bottleneck. Anyone has any tips for DSS?

The image is quite good... the dark lane can be seen with some additional processing. How many did you stack?? To get better image, stack more.

My current iso is 1600 do you think I should try 800?

If your sub frame exposure is longer than 1 minute, lower ISO will give better result. ISO 800 is a good choice.

If I use an LPR, what kind of exposure time should I try?

As long as your mount can give you and without saturation.

I did not use bias images perhaps I should also try that?

You do not need the bias frame if all your sub frame had the same exposure time... the bias frame is include in the dark frame.

Do you think the light polution has a large part to play here?

Yes... more work required to get a decent image.

Have a nice day.

[cataclysm]

Hi, you did very well chuang! M31 is a very challenging target to image under our light polluted sky. I had attempted several time too, non successful. Keep on the good work, the more you do it, the better you will get.

[chuang]

Hi Wei Xing,

Thanks. I stacked 9 2min and 1min exposures. Anyway I stacked the final upon itself and got some other detail appearing, but thats cheating right. Just hope the noise does not amplify instead.