30 2 Minute Expoures on Alt AZ (Stacked) for DSO versus 1HR GEM Exposure.

[sabana]

Excuse my complete and utter ignorance on this subject. But I've ascertained that you can do up to 120 second exposures on an Alt AZ if properly aligned to North and tracked as well as you can on this mount.

And then stack the Alt Az images in DeepSky software.

At the same time if you do a long exposure on a GEM with tracking of the same object (say 1 long one).

Will the results when compared between the two be different and if so in what way?

If there is little difference between a stacked 2 min image to a long exposure why do people prefer the inconvenience of a GEM ?

[Shibby]

Sabana, here's the way I see it...

First off, you'd be doing very well to get 2min subs on an Alt-Az mount without tracking errors and field rotation (depending on the FOV).

But assuming you could reliably get good 2min subs, there is only so much signal you can pick up in 2 minutes. Longer subs, while having more noise, actually have a higher signal to noise ratio - you need to expose for a long time to gain a decent signal on the fainter nebulae simply because there are so few photons hitting your sensor.

By stacking lots and lots of short subs, you can eliminate most of the noise, allowing you to aggressively stretch the histogram, pulling data out of the "dark end". However, there is only so much bit depth in the image and therefore only so much information that can be stored in such a tiny fragment of the histogram.

On another note, I wouldn't necessarily call a GEM inconvenient - quite the opposite in many cases!

[hemihaggis]

Mmm it would be doubtful to get more than 20 sec subs with an alt az , even an unguided eq mount would have to be perfectly aligned , drift aligned , have all the errors ironed out , and be in the hands of a very experianced person to archive 2 min subs with no rotation ,

Hope this helps

[Macavity]

It seems an unavoidable and accepted that e.g. (randomly chosen/modified quote):

"Stacking improves Signal to Noise Ratio, over a single short exposure, but the result is STILL worse than a single exposure of the same total length, due to extra noise..."

Note: I haven't defined "noise", but I'm now reasonably convinced of the above, by the popular opinion, and the more detailed mathematical formulae sometimes shown.

P.S. Nowt WRONG re. *experimentation* with Alt-Az (Video!) DSO astronomy, stacking though!

[dph1nm]

"Stacking improves Signal to Noise Ratio, over a single short exposure, but the result is STILL worse than a single exposure of the same total length, due to extra noise..."

Popular opinion is often wrong!

You can image with 30s-1min exposures on an alt-az mount and do as well as an equatorial in many cases (you might well need to autoguide an equatorial to do much longer than this anyway). However, the GOTO alt-az mounts seems to have rather rubbish tracking, which doesn't help, so you need to be thinking about quite large pixel scales (but that is OK for wide-field shots).

The important thing is for your short subs to be long enough so the sky noise overcomes the read noise. This, of course, depends on many factors.

NigelM

[jgs001]

2 minutes is possible with an Alt Az, low in the east or west without field rotation (I've achieved it). However, the exposure time decreases the further N/S or overhead you get. All I've read and my findings seem to bear this out, the focal length is not relevant to this. However, actually getting usable 2 minute subs (without issues from the drive train) is another matter entirely. I was losing 50% of all my subs to errors. I was able to get the bubble nebula (IR) to show up with an unmodded camera using 168 x 45 second subs.

As for which is better... that I can't answer, I've never tried 1 hour exposures, given the choice... I'd take a load of 5 or 10 minute exposures, over the equivalent number of 1 minute exposures though.

[dph1nm]

However, actually getting usable 2 minute subs (without issues from the drive train) is another matter entirely

Quite! I have no doubt that it is possible to build an alt-az mount which tracks as well as a high end equatorial, but would you want to spend the sort of money it would probably cost, given you are still limited to ~2mins max? I suppose the solution to that is even more expense for a field rotator. However, most (all?) modern professional scopes are now alt-az (with rotators), so maybe this is the way the amateur scene will eventually go.

NigelM

[klaatu]

Naive point - I kind of know I am wrong but have to say it.

A 16" tracking dobsonian would cost about £2k all in.

TELESCOPE SUPPLIERS - SKY-WATCHER TELESCOPE

You could spend a similar amount on a EQ5 or 6 + 4" refractor + second refractor to guide + guiding camera + reticle EP for calibration + old laptop + loads of wires + loads of faffing about.

The dob would have rubbish tracking and maybe only make 20 seconds without trails but it is 4 stops faster than the 4" refractor so you would have to do 320 seconds on the refractor to catch as many photons. With breaks in the cloud only lasting 4 minutes the dobsonian approach seems appealing. You could image several deep sky objects before putting the umbrella back up.

I know - it is image quality that counts .... this is just fantasy kit building.

[dph1nm]

Gary Honis gets fantastic images with a tracking alt-az 20" Dob and short exposures, so it can be done if you have enough aperture.

NigelM

[sabana]

This is really fascinating. I agree for really DSO then every photon does count (Re Steppenwolf's excellent book).

Steve, if you read this maybe could respond and advise. Most grateful all to have replied so far.

[jgs001]

I was using a £40 from eBay NexStar SLT with an ST80 clone, so really cheap. Keep the subs under 60 seconds and the keeper rate goes up to probably something like 95%. I bought an eq mount as I got fed up with fighting a losing battle against sub length.

Basically, Sabana, go for an eq mount, the biggest you can afford and be able to cart about for imaging, anything less leads down the path of frustration. There's more than enough of that anyway, what with the weather and all.

Steve's book is the best bet.

[steppenwolf]

This thread raises some very interesting points but the way I see it is this.

Longer, exposures yield greater detail and colour in deep sky images but you still need lots of them to stack and thus increase the signal to noise ratio. Many more but shorter exposures stacked together will produce reasonable images but will be deficient in detail except on the very brightest of deep sky objects.

The alt.az. v EQ mount argument is, IMHO, easy to resolve. Celestial objects show an apparent movement across the sky in the form of an arc so for the best results over the whole field of view, it is necessary to follow those objects in a matching arc. A correctly polar aligned EQ mount will move in pretty much a matching arc, allowing for periodic errors. An alt.az. mount will follow the object and keep it 'centred' but only in horizontal and vertical steps which cannot match the arc movement of the sky and it too will suffer from periodic errors but this time in two planes.

Best quality deep sky images are thus obtained by stacking numerous long exposures taken with a camera and telescope mounted on an accurately tracking equatorial platform - either a German Equatorial Mount or an alt.az. mount on a heavy duty wedge.

I am now ensconced firmly behind my parapet!

[sabana]

Thanks Steve. There is a place for both I guess. But as advised there is more room for manouevre with the EQ mount.

[Macavity]

I have "imaged" on everything from a (fixed!) Giro III, through a (single arm) Synscan, to an Ioptron Minitower - I now have an HEQ5. There are clearly limitations! But many of us are content with a personal record of activities. But many of the above produced some sort of (software stacked even) result - Often better than expected! Give "imaging" a go, Eh?