EQ6 bubble level problem

[mark7331]

I need a sanity check from you guys please. Sorry if this is a bit long but I wanted to give you as much detail as possible.

I've just put an ambubble onto my NEQ6 to level it and the ambubble shows a different level than the built in bubble. I've checked that the ambubble is sitting correctly and also put a good quality spirit level on the arm of the ambubble and its perfect.

I decided to try the ambubble on the EQ as my unguided tracking is awful and I've gone through everything I can think of to get the setup perfect. I use a compass to point the mount at polaris then level with the built in bubble, rotate the RA axis so polaris is at the bottom of the scope, set the RA to zero, read the SynScan controller top line and set the RA (bottom line) to match. Then I centre polaris which sometimes is way way out in azimuth but usually dead on altitude, then the rotate RA axis back to home and start aligning.

I can align without problems (3 star), though the stars, not just the first of 3, are sometimes quite a way off.

Now if I pick, say Albireo, I slew to it and sometimes it's dead centre, most of the time it's off slightly. If I take short exposures it starts to drift off.

So is my built-in bubble level wrong, am I expecting too much or being incredibly thick?

Thanks in advance all.

[Ianstraton]

The buble levels in the eq mounts are generally not good, I have an eq5 with a built in bubble mount that is about 15 degrees off! However that said getting level is not hugely important, it does complicate getting polar aligned a little but once aligned it makes no difference to the tracking or slewing of the scope.

[steppenwolf]

I'm with Ian on this, getting an EQ mount level with a bubble level (any bubble level) is not a pre-requisite for accurate tracking - all it does is to potentially speed up the set-up time if you always set up at the exact same location. Even though the built in EQ6 bubble is often inaccurate (but REPEATABLY so), this is not a problem.

The drift that you are experiencing could be many things including an inaccurate polar alignment. Have you checked that your Polarscope is itself calibrated correctly - if not then your apparently good polar alignment will also be inaccurate. It is possible (although unlikely) that your mount is not tracking at sidereal rate but I'd double check that all is well with your polar alignment first.

Unfortunately, the EQ6 is not known for its good tracking accuracy so this 'feature' is also a likely cause of the apparent drift that you are seeing - this is why for long exposure (in excess of 1 or 2 minutes) you should be considering some kind of autoguiding.

[mark7331]

Ian, Steve thanks for the info. I've just qone a quick check on the polar scope alignment and it's out quite a way. The cross-hairs move left to right as I rotate the RA 180 degrees to the right.

And I won't worry about the level - as you point out it's good enough and not related to the tracking problem. The mount is set to track at sidereal rate and I usually check this before starting to align.

I have an LVI SmartGuider which works perfectly but I wanted to check that my mount was set up properly.

Many thanks again.

Mark

[steppenwolf]

I've just qone a quick check on the polar scope alignment and it's out quite a way. The cross-hairs move left to right as I rotate the RA 180 degrees to the right.

Resolving that will make a worthwhile difference

[TopHouse]

Resolving that will make a worthwhile difference

Sadly it's one of those jobs that either snaps straight in and is a pushover or involves lots of twiddling and cursing. Mine was the latter

[mark7331]

Just had a go and found it difficult to tell what was going on. I found that it was quite difficult to look through the polar scope at a consistent angle so I popped a collimation cap on the end of the polar scope and hey presto - same angle each time.

I've got it as near as I can using a distant object - I think I will check it, clouds permitting, against polaris as soon as possible.

Thanks all.

[TopHouse]

The secret is to undo your alt bolts so you can tip it up as far as possible, saves on the neck a bit, but the colli cap sounds like a brilliant idea.

[mark7331]

The secret is to undo your alt bolts so you can tip it up as far as possible, saves on the neck a bit

Another good idea. Cheers Dion

[gfletcher]

Brilliant idea that collimation cap over the polarscope eyepiece

Graham

[barkis]

Can I clear something up regarding accurate polar alignment.

If the polar axis is tilted to one side of the true Celestial Pole, and a long exposure of an object was initiated, and for arguments sake, we will make the exposure in the order of one hour.

A suitable guide star has been selected, and the tracking is underway. The scopes tracking software is keeping the g/star spot on the centroid, by making adjustments to any drift that occurs.

So, since the polar alignment is not accurate, why would there be no rotational arcs at the edge of the field. The stars near the guidestar will be fine to a point, but those at the edge must suffer, since there is no field derotator being used.

Does the software somehow compensate for this rotation around the guidestar, which must surely happen if the polar alignment is not perfect.

I'm happy to be educated in this matter .

Ron.

[steppenwolf]

Ron, you are absolutely right that drift will occur if polar alignment is not accurate regardless of whether you guide or not. However, the 'levelness' of the mount is not an issue here for the following reason:-

Consider a long pole made of gold-pressed latinum at -272 degrees Celsius (i.e. a pole that is very very solid and does not bend - quite expensive too ). Imagine that pole reaching to the NCP just to the side of Polaris.

Now, remove your polarscope from your mount and thread your pole (which by coincidence is a perfect interference fit in the channel that normally holds the polarscope) though the mount. This is, of course, the axis on which the mount will rotate in RA at sidereal rate following the movement of the skies.

Finally, rotate the mount to any angle you fancy and using quick drying cement, built a wall to support it - the mount will continue to revolve perfectly aligned with the NCP with that rod going through it despite the fact that you have mounted it an angle and, therefore, not level!

The mount does not need to be level to track accurately.

[TopHouse]

Consider a long pole made of gold-pressed latinum at -272 degrees Celsius (i.e. a pole that is very very solid and does not bend - quite expensive too

And Quark would be your very best friend!

[steppenwolf]

Ah Deep Space (nine) just gotta love it .....

[Astro_Baby]

Setting the polarscope reticule can be a total bitch of a job. My advice is do as tophouse says and use the latitude bolts (better stioll just remove them) and have the mount set up so that the polarscope is horizontal - now aim it at a distant object. I use a radio mast with a red light on the top thats about 5 miles away.

Work from a lit room cos its much easier than trying to work in the garden.

[mark7331]

Astro Baby, I meant to say thanks earlier - I downloaded your guide before I had a go. Brilliant - thanks!

[mark7331]

Just an update. I've had my gear setup tonight and for the first time I am able to take unguided subs of about 1 min without star trails.

Thank you all so much for your help.

Mark