My pics always trail in dec

[riogrande100]

I set my mount up with a fairly good polar alignment. however i notice that all my pics seem to always trail in dec axis, whilst RA tracks well. i will strat auto guiding soon but trailing with a 30 second sub seems strange

[astroman001]

A polar align scope alone will not give much better than 30-60 secs before trailing. You would need to use the drift align method to achieve better.

Polar Alignment Procedure - Drift Method

regards

Peter

[johnrt]

I can get a 2 minute exposure on my eq5 by aligning using the built in polar scope. But I have found for every good sub I get, I have to chuck 2 away due to trailing.

If I walk next to the mount that ruins the shot. I also have a train line at the back of the garden - they don't help either.

How are you leveling the mount? I found the spirit bubble to be way off on mine and now use a proper spirit level.

You should be able to go more than 30 secs imo. As stated above drift aligning is the next step up and will be much more accurate. I haven't tried this yet.

[steppenwolf]

An equatorial mount does not have to be level to work although it helps for repeatability in setting up from session to session. DEC trailing is a sign of poor polar alignment - could it be that your polarscope is not correctly calibrated so that your polar alignment is not as good as you think it is?

[riogrande100]

My polarscope well never seems to get the star in focus, maybe it's too weak. I got my guide kit so will start autoguiding soon which should hopefully get me past the issues of star trails.

[johnrt]

Do you still need good polar alignment when guiding?

[riklaunim]

Yes, you need good polar aligment so the field won't rotate. And on bad polar aligment guiding won't manage to correct the big shift.

[MartinB]

Do you still need good polar alignment when guiding?

You need it to be reasonable but some people deliberately offset the alignment when autoguiding so that the dec corrections are all in the same direction thus avoiding problems with backlash. As has been mentioned, with very long exposures polar misalignment will cause problems with field rotation.

[ollypenrice]

Just to agree with Steve's point, so unimrtant is the levelling of a German mount that the Tak EM200 with wooden tripod has no levelling facilities on it at all. It simply isn't necessary.

And to add a smidgen to Martin's point, when doing long stacks from a very long run on a misaligned mount all you have to do is make a set of sub stacks - say 4 of an hour each if you have a 4 hour run - then co register them (de rotate the field) before combining them.

The dec misalignment trick sounds crafty. I need to read up on this.

Olly

[riogrande100]

Thanks guys this forum is a fountain of knowledge for the noob

[davew]

Getting the tripod as level as you can helps in so far as when you make a change in one direction it doesn't impact on the other.

Think of it this way.... If the tripod was at 45* E / W ( you would never do that, the thing would fall over ! ) if you adjust the dec the ra would change by the same amount.

If you can't level, no worries, it just takes slightly longer to fine tune.

Dave.

[Astro_Baby]

JUst saw the post on the polarscope not focusing - there are two adjustments on a Synta polarscope. The firts is the knurled ring at the end of the polarscope (the bit closest to you eyeball) this is often VERY tight from the factory but it does turn. Turn it counterclockwise - this is the reticule focuser to get the star maps in focus.

A little known (or at least its seldom mentioned) focsuer for the main polarscope optics also exists. You have to remove the polarscope from the mount to get at it.

Heres how

1. Grip the polarscope barrel and turn it to the left to unscrew it from the mount. They can be VERY stiff from the factory and if you cant hand turn it wrap a cloth around it and apply some mole grips. Do this VERY carefully.

2. With the polarscope removed you will see behind the date circle of the polarscope there are three grub screws. Release these a little bit and you wil find the main polarscope tube unscrews from the eyepiece end. THIS is the helical focsuer for the main polarscope (ie the bit that brings stars toi focus).

Now - seeting the focsu on this can be a bit of a curse so heres how you do it.

3. Tape the polarscope to a camera tripod or use elastic bands to secure the main polarscope tube to the tripod. Make a simple lightshield for the polarscope out of some card and use an elasctic band to secure the lightshield to the polarscope.

You have to do this because the scope has no baffling or lens shield of its own and will be affected badly by any surrounding light sources.

4. Point the polarscope at a distant object - a star is good but I use a light on a radio mast thats about 5 miles away.

5. Turn the polarscope end (the eyepiece but but not the reticule adjuster) until the scope is in focus.

6. Retighten the small grub screws

7. Reinstall polarscope to the mount.

Voila - you should now have a perfectly well focused plarscope that will be fixed focus on infinity for stars while the knurled end will focus the reticule for you.

Hope thats of some help.

Mel