Imaging Guiding Tip

[TrevorW]

When you set up PHD guiding pick a fairly bright star to guide on.

I set up an imaging session last night with the idea of taking 20 x 3 minute subs with my 120 f/5 achromat.

Spent the first 5-10 minutes after setting everything up (no permanent site) getting the focus as near as perfect with DSLR Focus actually I was happy with this works a treat.

Set up PHD on a smallish dull star at 3.5s exposures PHD calibrated and said guiding OK.

1st image taken came out fine, so I went inside for an hour, then when I came out again I found that PHD had lost the target star sometime during the second expsoure and all I had for the nights effort apart from the 1 good sub was a series of star trails.

I can only assume that the guding failure may have been caused through some low cloud coming out of nowhere blocking the light from the dim guide star I had picked.

So I suggest if you do set up like I did

a) pick a bright as possible guide star
b) wait until at least a quarter of the subs have been taken before leaving the mount too go it alone.

[leon]

Trevor I think you nailed it when you mentioned some possible haze or a cloud passing through, that will stuff it, it has happened to me.

Can i ask why you set the exposure at 3.5, I never go past 1.5 and even lower, and it will guide on the faintest star, if it stays clear of coarse.

No mate never leave your rig, just in case something happens, even like cloud, I sit next to mine the entire imaging run, and it has payed off at times, as some things do go wrong occasionally.

Trev, pick a medium star, not a very bright one, it will work much better.

Leon

[scopemankit (Chris)]

Leon,
You have hit the nail on the head! Although you may be auto imaging never leave the site. I have 30 hours of jazz set up in a computer just dedicated to music to ensure i don't get bored and a small hi fi playing it all the time I'm working.
Chris
Cape Town

[jase (Jason)]

Indeed, I wouldn't be picking a very bright guide star. The brighter it is, the harder the algorithm needs to work to compute the centroid. As Leon points out, a star of medium brightness is best - certainly from an accuracy perspective.

[multiweb (Marc)]

If your mount tracks reasonably well PHD should pick the guide star again when the sky clears provided it is still wihin the square search area. You can increase it from the default 15 to 25 so you provide for some kind of buffer in case of drift. I always leave my mount alone and go watch telly unless I'm close to do a meridian flip. I even fall asleep most of the time

[celstark (Craig)]

Bright is good -- to a point. Just don't let that guide star saturate the CCD. If you do, you're loosing accuracy in determining where the star really is. If you're at 3.5s of exposure and it's just picking it out -- yea, probably a bit dim and dim enough that when some light clouds come by, all is lost. But Sirius -- not so good to guide on! (assuming you blokes can even see it)

Craig

[iceman (Mike)]

I'm too lazy to sit next to my scope while doing an imaging run. I often leave it running for hours at a time while I go back to bed

Occasionally i've lost some subs but being near the scope wouldn't have helped because it's been when blanket cloud has come over anyway.

And most times it's worked perfectly and I've caught up on some much needed sleep