Unguided Imaging

[gfletcher]

Hi Guys

I was imaging the Leo Triplet last night thro my C8N on a standard HEQ5.

28 subs each 60 secs.

I had to ditch 11 of the subs due to star trailing.

The others seemed pin sharp.

Is this normal ( 60% ) without star trails when you image unguided?

I can see that trailing may be more noticable the further you are from Polaris.

The Leo Triplet i think is about +12 degrees DEC.

thanks

Graham

[Ben Ritchie]

They can trail from polar misalignment and periodic error - the fact that some are ok suggests that PE is the issue. That sort of percentage seems broadly correct to me, the worm drive has a 10m 38s period and some parts of it will be smoother than others so some subs will be better than others.

Of course, transient vibrations like wind, touching the mount etc. can also cause some subs to go bad.

[gfletcher]

Thanks Ben

[Ben Ritchie]

Here's a plot of the (out of the box) PE on mine

taking this as an example, any time you took a 60s unguided exposure on a 'flat' bit of the graph you would get good stars (assuming good alignment, no wind etc.) while one of the steeply-sloping bits would show trailing from PE. So i'd guess that about 50% good, 50% trail would be about right.

[Dangerous-Dave]

What Ben said. Also factor sampling rate into the equation - the shorter the focal length, the less likely any small deviation from sidereal is to show up on your images.

[gfletcher]

Ben

What site do i look at to get a plot graph like that for my standard HEQ5 ?

Looks very useful info

thanks

Graham

[roundycat]

How severe are you being when you trash frames for trailing? What sort of combination are you doing? If you use a standard deviation based method of combining frames you may well not need to drop any of them, or certainly nowhere near as many as 50%.

Dennis

[gfletcher]

Dennis

I purely look at the RAW images in DPP , delete any that show any star trailing and put the remaining goods ones through DSS and process in Photoshop CS2

Graham