Star trailing and stacking

[MND]

Hi all,

Im planning to do some widefield tonight (hopefully) and have a question.

Im going to take 20 or so 20 second exposures on a tripod with 14mm lens. Will DSS still be able to stack these or will the 1st and last frame be too far apart to align?

Is there a limit to this kind of stacking on a tripod?

[jambouk]

It will be able to stack them. You will lose data at the edges, but at a focal length of14mm over 7 minutes it won't be much. Problems will arise if you static things in the image (trees, houses etc).

You could stack twice, one using the software to align the images so the stars are points of light, the other turning alignment off so you get a star trail and any meteors etc will appear as straight streaks.

You might need to experiment with the lens, as i have found if you have the iris fully open (lowest f/ value) you get coma aberration at the edges of the image; stopping down a few stops reduces this, but obviously reduces the amount of light hitting the sensor.

Depending on your camera, it will probably also be wise to do some dark frames at the end of your imaging run, just to reduce the noise and amp glow, or whatever it is called.

Good luck and look forward to seeing your images.

James

[kalasinman]

It depends on what you are trying for. I stay around 15s near the CE with a 12mm. Nearer 45 degrees DEC you might get by with 20+. I find that much past 20 subs, field rotation starts to ruin the edges. For crop sensor cameras a rough guide is 400/FL=secs.--Jack

[MND]

Excuse the terminology but hopefully you will understand what I mean. I don't know if this will work but I had planned to use a layer mask with the landscape and just have 1 frame of that in with the sky.

I'm hoping to do some experimenting at different ISO and exp length. Both with Samyang 14mm and nifty fifty. I will take darks and bias also. I am heavily light polluted here and am hoping the calibration frames along with gradient exterminator will help with this.

Of course even though forecast is for clear it is thick grey stuff at present!

[jambouk]

Sounds good.

I don't think you "need" to do bias frames as the camera does them automatically - but this is beyond my level of understanding, just what i have read and probably mis-understood

Look forward to seeing the results. Still pretty cloudy in nottingham too but it's all heading south east.

James

[dph1nm]

This must depend how much distortion your 14mm lens causes. If you try to stack distorted frames which are rotated relative to one another then the edges will never align (well, DSS won't align them - if you have some software to take out the distortion first then it might work).

NigelM

[davefrance]

 Might be talking b.........ks here but I have taken 50 frames with an 18/55mm lens @ 18mm on D600 and tried stacking in DSS and got total garbage. Repeated several times with different settings but unable to resolve. In desperation I tried Microsoft ICE which is a free panorama stitching program and it seems to stitch the frames together but unlike DSS trees etc remain clear. Not sure how or whether it is actually doing it but aircraft, satellites that appear at various times all appear on the finished "stack"  so it leads me to believe that it is combining some data.

Skies cleared here around 10.00 so "here is one I prepared earlier", Not the best Milky Way I have produced but it will, hopefully, show you what I mean.

Not recommending it, just passing on my experience. 

[jimmyjamjoejoe]

The samyang 14mm has pretty harsh moustache distortion so you'l need to correct that first. The good news however is that it performs VERY well with regards to coma so dont be afraid to shoot wide open.

[MND]

Pretty rubbish results from last night.

I took 10 x 25 seconds @ ISO 3200 and though I can see the milky way faintly on individual subs there is nothing after stacking with darks and bias. Could it maybe be the moustache distortion causing this. Should i correct each sub before stacking?

[jimmyjamjoejoe]

I would, yep. Try to keep pre-processing to a minimum though. Individual subs and the stacked result will never look like the final image though. Stacking squishes more data into a file, it's the post processing which will reveal it.