Unguided Limit

[martin_h]

I have just spent a very cold few hours drift aligning my mount - HEQ5 - I used the program EQAlign and have got the drift down to almost nothing in all directions(to small to correct) but when I tryed an unguided test I can only get about 1.5 mins before the stars are oval - So what is going on?

Any ideas?

[Jamie]

Periodic error?

[Tim]

Martin, even the best of amateur mounts will only stretch out to a couple of minutes before the stars go off-round. Once you start autoguiding you will notice that the mount has to be slightly corrected every few seconds to keep the stars round, even with great alignment.

As Jamie mentioned, Periodic error will also rear its ugly head every 4 or 5 mins and give a glitch in the stars too.

Other things that can cause oval stars;

Bad collimation

Wind

Cables tugging/blowing/wobbling

Longer focal lengths show up the error much more quickly than shorter focal lengths. A mount with a short fast refractor with a DSLR/wide field of view will keep better looking stars for longer than an SCT with a small chip camera for instance.

Well done on getting great alignment btw, but take care not to waste clear nights just setting up and missing out on the clear spell!

Cheers

Tim

[ollypenrice]

Ditto the others. You don't mention your focal length but Tim's point is key. You would probably be able to image for ten minutes with a 50mm camera lens but as the f length goes up so does the egginess of your stars!

On my EQ mounts I set the guider to correct every 0.5 to 1.0 seconds.

Top mounts (Astro Physics, 10 micron, Paramount, etc etc) have a periodic error of plus or minus 2 arcseconds. The EQs might be 20 or 30 seconds. But the top mounts still need autoguiding. It is a fact of imaging life but is now cheaper and easier than ever before.

Another refinement is balance, which is very critical. Some like to weight the east side of the mount a little heavy to keep the gears pushing and in mesh on the same side. Having the camera end heavy may also help. Mounts seem to vary with regard to how they respond to this.

Olly

[martin_h]

The focal length is around 760mm and I'm using the QHY9 which is a fairly large chip. BTW I'm not wasting imaging time at the moment - cant do much with that huge floodlight in the sky.

[ollypenrice]

760mm is getting serious. Even on a 5 arcsec PE Tak mount and half that focal length I need decent autoguide values to image. I would put the comfort zone of an EQ mount, autoguided, as not much more than a metre though with effort at tuning folks do get results at 2 metres.

The thing is, you pay for good resolution in your optics so there is not much point in smearing it all away in tracking errors.

There are inexpensive solutions if you are prepared to fiddle around. EG webcam operating through ST80 or even finder-guider. There will be lots of help available on all the possibilities. I use Atik 16ic cameras here and ST80 guide scopes.

The only unguided I have done was with an 85mm camera lens on the Tak mount. Ten minutes was fine, no trailing at all was visible. But that is 85mm for you.

Olly

[martin_h]

Ah! I do autoguide - ST80/QHY5 - I just decided to "test" my drift alignment unguided and was surprised to only get 1.5 mins, I know I'm not going to get a huge amount of time, but thought it might be a bit more than that.

The only reason I had a go in the first place - and this will make you laugh(or cry) is that my 'horizon' in my obs is about 30 degrees so I can't see orion neb but I can see the horses head for about an hour when its due south.......but....... I use a side by side arangement and when slewed it becomes and over and under....the guide cam cant see the HH but the image cam can when in the east side just before the meridian... so I thought I'd try some unguided shots...Ahh well.

[Earl]

I have managed in the past 3 mins unguided, over 20 images 9 or so were spot on others showed trails.

[ollypenrice]

Ah, I see where the post was going now!

Olly

[martin_h]

HeHe...It might be easyer just to cut a foot off the obs wall...How many subs at f7 at 1.5 mins would i need to make an image? LOL

A job for the summer me thinks.

[dph1nm]

The periodic error on my HEQ5 averages about 3 to 4 arcsec per minute. Doesn't seem to matter what load you put on it.

NigelM