Whats happening here then ??!!!

[Skipper Billy]

Second outing last night with my new scope - ES 208mm f3.9 on an AZ-EQ6-GT guided by ST80 and Lodestar - camera Atik 460EX mono.

The seeing was truly appalling but my guide graph was spot on.

My 900 second subs all had double stars - see attached images. (Single sub stretched and converted to Jpeg.)

I dropped to 450 seconds and they were all good.

How can that be if the guiding is bang on the money ??? Can really bad seeing cause that ??

The main scope and guide scope are mounted side by side on Losmandy plates on an ADM dual bar and as far as I can tell there is no differential flexure.

[ollypenrice]

It's always best to identify the axes of RA and Dec as seen on the image so that we know which axis is trailing (if it is just one.) Do you have this information?

Double stars, barbells or overlapping stars a bit like a figure 8 are consistent with backlash because the mount is spending most of its time in one position or the other and moving between these positions, so creating a stellar image from both but moving too quickly between the two positions to leave much of a connecting trail. However, the guide trace shows no evidence of this.

So rather than mount backlash, how about mirror flop? The guider would not be able to 'see' this. It is a kind of diff flexure but not in the mounting hardware. It fits the evidence so far. The mirror flops from one position to the other quickly, creating two stars, but the guider is only 'aware' of keeping the OTA pointing in the right direction. It knows nothing of the mirror within it.

Edit: if the trailing is aligned along one or other axis this hypothesis is unaffected but if the trailing is diagonal then the hypothesis becomes far more likely.

Olly

[LightBucket]

Do you balance in all three axis with your side by side rig, rather than just the two needed for piggybacking..?

 

[Skipper Billy]

12 minutes ago, ollypenrice said:

It's always best to identify the axes of RA and Dec as seen on the image so that we know which axis is trailing (if it is just one.) Do you have this information?

Double stars, barbells or overlapping stars a bit like a figure 8 are consistent with backlash because the mount is spending most of its time in one position or the other and moving between these positions, so creating a stellar image from both but moving too quickly between the two positions to leave much of a connecting trail. However, the guide trace shows no evidence of this.

So rather than mount backlash, how about mirror flop? The guider would not be able to 'see' this. It is a kind of diff flexure but not in the mounting hardware. It fits the evidence so far.

Thanks Olly

Much to think about there - Off hand I cant identify which axis it is.

More investigation required !

[Skipper Billy]

13 minutes ago, LightBucket said:

Do you balance in all three axis with your side by side rig, rather than just the two needed for piggybacking..?

 

Yes - but I will recheck everything!

Its the first time I have used this mount at Zenith so it may be a balance issue - maybe its too well balanced ! (I usually try to keep it east heavy).

[ollypenrice]

1 minute ago, Skipper Billy said:

Yes - but I will recheck everything!

Its the first time I have used this mount at Zenith so it may be a balance issue - maybe its too well balanced ! (I usually try to keep it easy heavy).

Zenith position will have no effect on RA balance but at the zenith an imbalance in Dec reduces to zero because the heavy end is directly above or below the light end. Maybe you have a backlash-reducing imbalance in Dec which you lost at the zenith.

Olly

[Skipper Billy]

1 minute ago, ollypenrice said:

Zenith position will have no effect on RA balance but at the zenith an imbalance in Dec reduces to zero because the heavy end is directly above or below the light end. Maybe you have a backlash-reducing imbalance in Dec which you lost at the zenith.

That might well be it - I removed the mirror clips as they were causing weird shadows but the mirror was really well glued in and there was no discernable play in the mirror cell fixings.

Will investigate in the daylight!

Many thanks.

[Dave In Vermont]

As you mentioned 'seeing' being, perhaps, a culprit, this oddity crossed my way:

http://agenaastro.com/unihedron-sky-quality-meter-lens-sqm-l.html

I've never seen one of these. It looks interesting. Might be worthwhile to look into over your way.

 

 

Hmmm...

Dave

[alacant]

Hi. There are various bits I had to do to mine. All cheap and easily doable. HTH.

[ian_bird]

1 hour ago, ollypenrice said:

Zenith position will have no effect on RA balance but at the zenith an imbalance in Dec reduces to zero because the heavy end is directly above or below the light end. Maybe you have a backlash-reducing imbalance in Dec which you lost at the zenith.

Olly

Hi Olly

Is there anyway of dealing with this (Dec at Zenith backlash) issue? I remembered your bungee cord dodge - but surely there is a better way? Well - short of buying a better mount! 

Cheers

Ian

[LightBucket]

1 hour ago, ollypenrice said:

Zenith position will have no effect on RA balance but at the zenith an imbalance in Dec reduces to zero because the heavy end is directly above or below the light end. Maybe you have a backlash-reducing imbalance in Dec which you lost at the zenith.

Olly

But at zenith on a side by side rig there could be balance...!!

[ollypenrice]

10 minutes ago, ian_bird said:

Hi Olly

Is there anyway of dealing with this (Dec at Zenith backlash) issue? I remembered your bungee cord dodge - but surely there is a better way? Well - short of buying a better mount! 

Cheers

Ian

The bungee cord is the last resort of the desperate. Actually it's hosiery elastic! I never found it very effective and now I don't use gear driven mounts anyway so problem solved. (I've heard of others using elastic in extremis, too.)

There is another way and this worked well enough on the Tak mount in question. You deliberately go for a small polar misalignment and then guide only in the Dec direction needed to correct the resultant drift. (Experiment to find which direction to disable and remember to reverse after the flip.) This stops the guider from tossing the mount back and forth across the backlash endlessly.

I still don't know why this wouldn't show on Skipper's guide trace, which is why I thought of the mirror.

Olly

[ollypenrice]

2 minutes ago, LightBucket said:

But at zenith on a side by side rig there could be balance...!!

There could, and balance might be the problem. There could also be imbalance, which might cure it! Another kown solution to this problem is the magnetic weight which can be put off to one side near the zenith to introduce some imbalance.

Olly

[ian_bird]

4 minutes ago, ollypenrice said:

The bungee cord is the last resort of the desperate. Actually it's hosiery elastic! I never found it very effective and now I don't use gear driven mounts anyway so problem solved. (I've heard of others using elastic in extremis, too.)

There is another way and this worked well enough on the Tak mount in question. You deliberately go for a small polar misalignment and then guide only in the Dec direction needed to correct the resultant drift. (Experiment to find which direction to disable and remember to reverse after the flip.) This stops the guider from tossing the mount back and forth across the backlash endlessly.

I still don't know why this wouldn't show on Skipper's guide trace, which is why I thought of the mirror.

Olly

It's odd. You spend ages getting perfect polar alignment, and then need to deliberately set it off to account for a deficiency in your mount! 

I might look at the offset weight on the DEC axis to, as you say, introduce some imbalance.

So in summary - perfectly balance your rig on your mount - and then set it East Heavy with a DEC imbalance. Or Polar Align, and then deliberately mess it up!

I love this hobby!!! 

Cheers Olly

Ian

[ollypenrice]

I never try for perfect PA. If I can do 30 minute subs I'm happy. We can't dither the dual rig so a touch of misalignment isn't a bad thing.

Olly

[ian_bird]

31 minutes ago, ollypenrice said:

I never try for perfect PA. If I can do 30 minute subs I'm happy. We can't dither the dual rig so a touch of misalignment isn't a bad thing.

Olly

Out of interest - how much do you let it be off by? And in any particular direction?

Ian

 

[ollypenrice]

 

10 hours ago, ian_bird said:

Out of interest - how much do you let it be off by? And in any particular direction?

Ian

 

No particular amount. I just stop tweaking the drift alignment when I'm confident that it will be fine for 30 minute subs. When we had the Tak EM200 with backlash in Dec I found that Tak's outstanding polar scope system (quite unlike most others) got me close enough/far enough to let the one-way Dec guiding trick work OK. I never tried to quantify the offset I'm afraid.

Olly