Field Rotation....

[RichieJarvis]

Hmmmm - well, looks like I've picked up some nice field rotation. This is 5 x 20 minute exposures. Unfortunately, I can see the rotation in a single 20 minute exposure as slightly enlongated stars (but only for dim stars for some weird reason), which is really annoying....

Whats the secret to drift aligning?

[GazOC]

For me it was blindly following a set of "if...then...else" instructions and not worrying "why".

[RichieJarvis]

I suppose the other question is:

Is this field rotation?

The reason I ask - that image is 5 x 20 minute (1 hour 40 minutes) stacked with NO alignment. The guidestar has constantly been less than 0.5 pixels from position throughout the run.

So if the guidestar hasn't moved, why would I get trails?

Thanks,

Richie

[SteveL]

Looks like differential flexure to me, rather than field rotation. I had the same thing with the MakNewt until I swapped the dovetail... the guidestar in the ed80 would be rock steady, but the 5 mins subs started to show some trailing, and it was even more obvious in 10 min subs.

I believe field rotation would show as the star field curving/rotating, rather than all the stars trailing in one direction. I`d check for flex in the system somewhere... easier said than done. Could be anything in the system between the guide camera and the imaging camera.... dovetail flex, guide or imaging camera wobble, rings, side-by-side plate, etc.

[SteveL]

Oh, and as for drift align, do you have Maxim? If so, I`d definitely look at PoleAlignMax for accurate polar alignment.

[RichieJarvis]

Thanks Steve....

My first port of call is going to be the guidescope rings then I guess....

I'll have to take it off and hope I can find a guide-star in the FoV

[George]

Doesn't look like field rotation to me Ritchie, more like a guiding issue or the mount was knocked...was it windy?

[steppenwolf]

Differential flexure I'd say - not easy to resolve but it is achievable. Ironically, a side by side arrangement made matters worse for me so I have stuck to a piggy-backed arrangement and with everything tightened down firmly, the problem is minimal.

[ngc2403]

Flexture, A slow slipping of one of the telescopes relative to the other resulting in this sort of trailling. This is not a problem with the mount, it's everything above the mount head.

as steveL said you should try to test all the possible part of the setup, not easy when clear nights are hard to come by, over i would try going through the setup with a fine tooth comb to see that everything it tight and nothing has a wobble.

Good Luck, it's a nasty thing to have.

[RichieJarvis]

Thanks guys - i think! Another problem for me to solve.

It only seems to be an issue when I shoot 20 minutes or above - you can see it on individual subs at 20 minutes, but at 10 minutes I can't see it...

The setup is an Astro Engineering AC694 side-by-side bar, with A ST75 5" scope rings on a Skywatcher 21cm dovetail for the AC 102ED, and the WO Zenithstar 66 is on its 'shoe'. This particular exposure, I was guiding with the 102 and imaging through the 66 with a 2x barlow.

I am wondering whether the weight of the SXV-H9 could be causing the problem as well - my accessories only have single screw to lock them down - the SXV-H9 had a 1.25" nosecone, plugged into the 2x barlow, and then into a 1.25" -> 2" adaptor and then into the ZS66 2" to SCT fitting.

The guidecamera was a SX Guidehead, into another 1.25" -> 2" adaptor, and then into the 102ED.

And as for wind - last night it wasn't wind - the setup is permanently setup in my roll-off roof observatory.

Anything anyone can suggest would be good - I think I've had this problem for a while, as I've noticed my subs don't line up despite accurate guiding for a while now - its just not been an issue whilst I've been taking 10 minute subs!

Cheers,

Richie

[steppenwolf]

my accessories only have single screw to lock them down - the SXV-H9 had a 1.25" nosecone, plugged into the 2x barlow,

There is plenty of chance for DF in that set-up, especially over a 20 minute exposure. Wherever possible I try to use 'T' thread adapters throughout but it can't always be done - my WO focal reducer for example comes with a 2" nosepiece built in which is removable but the exposed thread is not a 'T' unfortunately so no choice there!

[Starflyer]

My first port of call is going to be the guidescope rings then I guess....

I'll have to take it off and hope I can find a guide-star in the FoV

I saw this gizmo in the Jan edition of Astronomy Now. Looks like it'll make finding a guidestar easy and removes the chance of chewing up the finish on your guidescope. I guess if there's any slop in the threads / slide then it'll introduce more problems than it solves though :?

It's available from SCS and the Widescreen Centre.

Ian

[roundycat]

I hate to spoil the party but why are you persevering with 20m subs? In parts of the sky that I use that would give me up to 12 aeroplane trails in each one.

Dennis

[CELESCOPE]

Yes i was thinking that as well Dennis , i started out doing 20 and 15 mins subs when i started guiding, but after a while found that 8 to 10 mins or sometimes less gave an equally good result, and like to say i was loosing a load due to planes etc , but anyway thats going off the subject i guess.

[RichieJarvis]

Thanks for the advice - having a check through the imaging train today, I think its the camera attachment to the barlow that caused the problems last night - there is about a 1mm movement on it - everything else is solid.

As for why 20 minute subs, well, because I can

I usually shoot Ha and OIII, but what with the wheel out at the shop for the moment I am reduced to monochrome. 20 minute subs and above are where I am headed for narrowband.

I might be near to Gatwick, but the flight path misses my southern vista most of the time, so I am quite lucky with planes as well.

Cheers,

Richie

[Merlin66]

Richie,

I ended up building a support bracket which bolts to the support plate and to the camera/ spectroscope to "tighten" everything up. Takes the slop out of all the adaptors etc and holds the instrument firmly.

My 2c