Accurate SCP alignment - but RA drift of 1 arc minute per 10 minutes?

[g__day (Matthew)]

I have Vixen Atlux permanently set up - and after drift aligning it with a CCD and running a 40 star MaxPoint model to confirm - it appears my alignment to the South Celestial Pole is accurate to within 30 arc-seconds.

So on a 600 second shot I see effectively no DEC drift. However I do see about 5 mm star trail in RA. Now given a Canon DSLR normaly prints to a 6" * 4" shot this equates and that 6 inches can just capture the Moon's diammeter being 30 arc-minutes. So a 5mm drift in 600 seconds indicates about 30 * 5 / 150 = 1 arc minute of drift in RA per 10 minutes.

It doesn't matter if PEC is on or off - it seems the RA is just slightly slow.

Is this to be expected and more importantly is there anyway to command the SkySensor2000-PC V2.05 to speed-up the Sideral rate to be 6 arc minutes an hour faster?

Am I likely to see better RA tracking if I change it to unaligned Polar mode and ignore PEC?

Many thanks,

Matthew

[citivolus (Ric)]

You say the moon, are you tracking lunar or sidreal?

[Dennis]

Hi Matthew

AFAIK, the SS2K will track at Sidereal Rate but when you select e.g. a Planet, the Moon or a user-input Comet, it will perform real-time calculations and track at the computed rate of those respective objects?

Cheers

Dennis

[joshman (Josh)]

my first thought is your mount is slightly unbalanced on the RA.

dunno if that helps, i'm still to find the time (and clear sky) for me to have a proper go at polar alignment

[Kal (Andrew)]

You don't guide? I've read that if you guide it is quite often beneficial to have the RA drive slightly slower so that all your corrections are in the one direction.

Also, how many gear cycles did you train the PEC over?

[h0ughy (David)]

gee I would be getting a guide camera and GPUSB and PHD and going for it. if your that close you can balance the rest with some guiding, with a DLSR you would get fogged out with background glow anyway so 5-10 minute shots would be the go stacked.

[Terry B]

I remember reading an item on the SS2000 forum that described the same thing. The person there blamed his mains power connection. If he isolated the computer and mount from the mains the tracking rate was more accurate.
I use to find that I had better tracking with the PEC off realizing I had one spot on the worm that was dodgy and had to discard images that had that bit of poor tracking.

[g__day (Matthew)]

Guys thanks - in order of questions asked.

I say the Moon - because I used it to determine the size of my frame.

The I see drift normally after alinging through Refer stars on three stars - so the rate I presume should be should be sidereal - not lunar!

The Sidereal rate as I say seems slightly slow.

Balance on RA - drift is evident at all points in the sky equally - no matter East or West of the Meridan, nor high or low in the sky. The mount can image with 22kgs - so 14 isn't going to be stressing it - pretty sure its not a balance thing.

I guide all the time - but I wanted to see the raw tracking errors! The trick is to have as good tracking as possible - to avoid corrections whenever possible = sawtooth. I have to be careful to set pulses low in PHD and set aggressive low as well else it overshoots and see-saws - even if adjustments are only every 3 seconds (to minimise seeing errors).

Houghy - I do Meade DSI -> 80mm Megrez or 130mm MAK, via GPINT and PHD. Yes DSLR can only go about 10 minutes and yes I stack on long shots,

Terry - yes - I've asked there too - rings a bell - I am supplying plenty of current via a regulated power suplly - via a UPS (which line filters) - from mains.

PS

Now I've gone Losmandy bars and loaded the mount right up - still purrs! With 3 scopes the set-up looks like:

[g__day (Matthew)]

Wierd - put it in polar unaligned mode - tracking seems perfect! Put it in aligned mode RA is too fast actually!

[g__day (Matthew)]

A bit more detail - its becoming a battle of pointing versus tracking.

If I switch on the mount and only align it on one star the RA tracking seems fine (but gotos aren't as accurate as I'd like). However if I add one or two more stars well when I slew to these targets a small adjustment in pointing is required (say 4 degrees in RA for star 2 and 1 degree for star 3). After adding just the second star I notice drift in RA appears. Perhaps this small delta changes the tracking rate by adjusting how the hand controller allocates angular distance to the gears?

So I am wondering if this calculation of sky pointing model changes the accuracy of the RA tracking - even in polar aligned mode - any thoughts on what is happening and how to fix it guys?

Thanks!

[Karls48 (Karl)]

Mat, I got similar problem with my mount. It is Autostar controlled modified EQ4 mount on the pier. It is reasonably good polar aligned. GoTo’s are about 8 Deg off the target. If I accept it, then tracking is very good. If I make corrections then Autostar do its calculations and GoTo improves, but RA tracking became slow. I can’t figure it out.

[g__day (Matthew)]

I just wonder if I did something really dumb (very possible). I did a state change from polar unaligned to aligned - but kept telling it to align on 3 stars.

I tried last night re-building my Maxpoint sky model and by the fourth star everything was really wierd - so I re-started it all and aligned on one star - great - then I went to star two - almost centre of the CCD chip - but when I aligned on this or the next star - it kept on saying just one star align (its supposed to reject a new alignment star if DEC hasn't changed or if the target is > 10 degrees out of position) I say my targeting was well within 5 arc minutes or better...

I have to read the manual on alignment when you're in polar aligned mount mode. Tracking didn't seem to bad and pointing was pretty darn good too. Maybe in polar aligned mode you can only align on one star - you just have to get date, time and location very accurate - which is no problem for a fixed pier.

Of course by then it was 2am - I did a few 240 second shots of Orion and there seems to be minimal if any star trails - with no guiding - then of course clouds rolled in!

So next good viewing - I think I'll stick with a one star alignment - build a 40 star sky model, then test pointing and tracking with and without guiding thoroughly!

Matt

[g__day (Matthew)]

The manual really doesn't reveal too much more - so I stuck with 1 star aligns.

To this I added a 30 star sky model from MaxPoint - brought the pointing error from 5 arc minutes down to 1.7. MaxPoint estimates my SCP alignment to be within 30 arc seconds on Alt and Az - and flexure has about halved moving from Celestron to Losmandy saddles - so I'm definately getting better.

Tracking has improved about 4 fourfold - I see about a 12 arc second RA drift in a 10 minute shot now. I've tuned the DEC backlash better am are stilling learning optimal guiding settings

I then tried a PC based goto NGC 2070 - it landed spot on - see attached shots:

360 seconds unguided, 600 seconds unguided and 600 seconds guided.

I still have to tune PHD's settings - but I'm slowly getting there.

PS

All these shots - well I haven't adjusted the focus once in 4 months - carbon fibre seems to keep its expansion well under control!

Matthew

[g__day (Matthew)]

Well after adjusting weights a tad - about 10 kgs 1 cm upwards - tracking (unguided) is twice as good as tracking guided was!

Thanks to folks discussing this is in the thread on PHD. Basically I realised the mount was slow when facing East and slightly fast facing West - indicating its was the weight too far down!

I'm grinning now - I just have to tune PHD better