Drift alignment - what am I really seeing question?

[g__day (Matthew)]

Just thought - if I check elevation on every 20 minutes and re-centre target - and if the the SCP alignment was out by the average elevation of the star Sirius but West and below the SCP by say 37 degrees - I could hit a Larange point where at 20 minute intervals there would be no DEC drift in elevation between 30 - 50 degrees - but the longer angle could cause a speed up or RA - relative to the angular rotation from the SCP alignment position. If the angle was just right and the delta was the right size I would see an apparent 1 part in 300 speed up in RA!

More research still!

[edwardsdj (Doug)]

Maybe you would but that's hardly your problem

I'd be tempted to try without that SkySensor2000-PC altitude based atmospheric refraction model and see if the RA drift gets any better or worse.

[g__day (Matthew)]

Doug,

Its hard wire in and you can't turn it off. From memory its only 1 arc minute at Sydney latitude at elevation 20 degrees and it decreases as you elevate the scope (helps having a dad who was a navigator and surveyor - amazing the old school has all this knowledge at their finger tips).

So refraction alone isn't enough to account for this degree of tracking error - unless they modelled it really badly.

I think I will experiement with maxpoint to build a 50 star sky model and see what it thinks my alignment errors are. My gut tells me I somehow just slightly blew polar alignment in an esoteric way that is producing what I am seeing naturally!

[edwardsdj (Doug)]

Hi Matt,

Sorry to misunderstand. I just thought you had eliminated dec drift in all sky positions. I agree that a polar alignment error will manifest as combined RA and Dec drift.

This is why ideally, drift alignment would be done on a star on the horizon due East or West (where altitude error is maximally manifest as drift in Dec) and at the intersection of the equator and meridian (where azimuth error is maximally manifest as drift in Dec).

If there is no Dec drift for all sky positions (East, West, North, South, Zenith) then the polar alignment must be correct. I'm sure the effect you are referering to would only exist in two particular positions in the sky. Maybe I misunderstand?

Good luck with it!

Have fun,
Doug

[g__day (Matthew)]

Doug,

What I refer to should only occur at one point in the sky - so my thinking or observations must be a tad wrong!

I did a 50 star model with Maxpoint - raw pointing error was 158 arc seconds that Maxpoint brought down to just 34 arc seconds since I've tuned DEC. So for a 12mm eye piece nearly every target was close to dead centre.

MaxPoint diagnosed my alignment to be about 50 arc seconds West and 30 arc seconds lower than the SCP. So being within one arc minute is so far not bad.

If I could just cure this rotten tracking rate - arrrgggh!

I did take the chance when I removed my Canon 400D to visually observe NGC 2070, Dor and Saturn with Vixen LVW eye pieces 22mm, 17mm, 13mm, 8mm and 5mm - about the first visaul astronomy I've done in 6 months! They looked good but I think my collimination is just slightly out! In the next two months I will probably ask Bintel to service - clean front corrector plate - check focuser backlash and re-colliminate my main OTA.

I finally got the chance to mount the Meade DSI on the main OTA whilst the scope was targeted high at the Meridan (where drift appears minimal) I trained PEC using PHD to auto-guide on the main OTA. The worm was pretty smooth bar once spot - corrections were at peak under 10 bar one spot that reached about 20, finally summation of the 300 step 8 minute process was a net delta of about 2 - so that's looking good too.

So next clear night I plan to drift align with my piggy backed 80mm Megrez refractor and/or with my 127mm MAK and see if I can see RA tracking error there. If I don't that kinda says its the mirror or imaging train of the main OTA that is giving my observed shift.

Finally I may track Sirius on my main OTA from elevation 30 - 90 or 100. If in 4 hours I see a drift of say 4 - 12 arc minutes which increases then decreases I will have more data to deduct with. I can't see how pointing can be superb when tracking appears to have problems - something doesn't add up there.

On night two I might centre on Sirius on my main OTA and every hour into the tracking issue a goto Sirius and see if it moves at all.

Thirdly I may try a 3 star alignment and see if tracking improves or worsens when its running in polar aligned mode.

I do feel all the diagnostics are helping to hone in on the cause of the error!

thanks all,

Matthew

[Merlin66 (Ken)]

Going back over this thread, and remembering it's an SCT; I'm beginning to think it may just be mirror flop movement?????

[EzyStyles (Eric)]

i just want to say ..... AUTOGUIDING

I can't even do 30 sec exposure shots without trailing. No perm observatory, a 'really' rough polar alignment and plonk the EQ6 into the marked concret ground thats it. PEC what? never heard of it

I remember i spent heaps of time adjusting drift alignment, getting polar alignment to be exactly in the end, problem all solved with Autoguiding.

Anyway g'luck with it.

[g__day (Matthew)]

Well I put the illuminated reticle on my refractor and and tracked Canopus for an hour - it barely moved off the cross hairs - and at high elevation it was excellent.

So after all this hassle - it starting to appear that the mount is doing all its supposed - gotos from the hand controller are landing very close to dead centre - and from MaxPoint - well its five times better!

Auto-guiding therefore isn't really going to help me if its mirror flop in the primary - unless you do it at prime forcus with a S-BIG!

So I will experiment more to confirm these findings - and I'm happy that my most expensive piece of gear is performing right to spec - but now I have to seriously think how do I solve mirror flop.

I guess on the cheap I could try some sort of off-axis guider - but my shopping list at the moment is looking like:

1. Off axis guider
2. 4" - 5" APO
3. S-BIG
4. Electronic, computerised focuser for the APO

Oh dear! There goes any sanity or fiscal responsibility!

[tempestwizz (Brian)]

Personally, I agree with EziStyles. Autoguiding fixws most , if not all problems. I was a little dissapointed when I took delivery of my Tak EM200 mount. There is no PEC feature. PHD guiding for my system was/is plug and play. I have not (yet) found the need to make any adjustments. I am up to 5 minute exposures with no noticable (to me at least) problems.
Mind you, I am only using a 500mm focal length scope, with a DSLR imager.
Higher fl's and smaller imaging areas will magnify any tracking/guiding errors.

HTH
BC

[EzyStyles (Eric)]

If your getting mirror flop then polar alignment, autoguiding, pec, drift etc etc will make no difference. Guess fix that problem first.

ps.. mmmm. sbig...mm..drool.

[g__day (Matthew)]

Yep - mirror flop can't be fixed other than by adjust how the mirror moves on the tube - or on axis guiding.

[Karls48 (Karl)]

Just for interest – you can do it with GStar. This camera got two outputs (Composite Video and S-Video) that can be used simultaneously. So you can guide with AVI and capture with S-Video. You would need fast laptop with two frame grabbers. One PCMCIA card and other USB.

[g__day (Matthew)]

Yes - given you use an on axis shot!

I have done great 10 minute shots unguided at the full 2.3 metre focal length - so in some parts of the sky the mirror isn't moving too badly on the baffel tube - in others it is.

What I have to work out is why and if its addressable!

[g__day (Matthew)]

Hmmm,

Spoke to BinTel today about Lumincon on axis guiders and what I thought was mirror flop - they say that's unusual because usually mirror flop is:

1. a sudden shift - not smooth throughout the movement hour by hour and
2. most often seen during a meridan flip

More research required still - will move the weights a tad and see if this worsens or improves matters!

[edwardsdj (Doug)]

Originally Posted by g__day

Hmmm,

Spoke to BinTel today about Lumincon on axis guiders and what I thought was mirror flop - they say that's unusual because usually mirror flop is:

1. a sudden shift - not smooth throughout the movement hour by hour and
2. most often seen during a meridan flip

I agree. That is when you see mirror flop. You also see it when changing the direction of the focus knob. If you always find focus by making the final adjustments with the mirror moving against gravity, the mirror position should hold unless you move east/west to the other side of the meridan or north/south to the other end of the sky.

[g__day (Matthew)]

Moving the weights down has improved things a fair bit - so maybe things where out of balance! Which would account for poor RA rate at some elevations, not all.

Plus drift guiding with a MAK took some learning - it requires moving the scope in the opposite directions to my SCT.

[g__day (Matthew)]

How switching from polar unaligned mode to polar aligned mode affects tracking


I initiated my mount in polar aligned mode and synchronised on one star. I thought my polar alignment was good because of several factors: pointing was superb, MaxPoint modelled alignment on fifty stars and said I was within 30 arc seconds of the Celestial Pole and I could see no minimal DEC drift on 1 hour shots.

But the mount appeared fast in RA to the tune of 60 – 120 arc seconds an hour – more at low elevations and less at higher ones.

After seeing most people’s comments I ran through several configurations of isolated DC power, filtered and conditioned mains power – but nothing helped.

Next I tried more carefully balancing the counterweights – this slightly improved things. Then I adjusted the DEC gears to reduce slop – this brought pointing to within 34 arc seconds. But tracking persisted to be too fast – all over the sky, not just East or West.

So I reverted to polar unaligned mode and added 1 star at a time up to 3 stars and checked the tracking at each point – still no improvement.

Then I switched from unaligned mode to polar aligned mode and executed a goto on one of the sync stars I was currently pointing at – and it moved over 30 arc minutes!

Suspicious of this I ran long sessions with PEMPro V2.0.39 which finally has Meade DSI support directly and it informed me I had to move my mount Eastwards about 40 arc minutes. This puzzled me given the other checks and balances said I was fine, but I experimented and suddenly tracking was a whole lot better.

Now I noticed pointing was a bit off, and I went to inspect my settings, three things surprised me. The first it said my polar elevation was 33.9, which about 0.1 degree too high for my precise latitude. Second Az said it was 180.1, but I expected for Sydney SCP is 11.4 degrees East of magnetic South – so I expected to see this. But the big one was my polar alignment coordinates. I expected them to read something like:

0 +123 + 123 +0 +0

But they didn’t, they read:

0 +120 +75 +54 + 48

Now the first two three digit terms are simply adjustments required from the scopes initiation position in case it’s not perfectly pointed East and level at power on. But the last two coordinates are more cryptic, poorly documented axes adjustment parameters. In polar aligned mode I expected these to me zero.

It appears switching from unaligned to aligned mode left these parameters non-zeroed. The result – well tracking Sirius (due East) elevation 30 degrees – 45 degrees for one hour show no RA drift and maybe 10 seconds of DEC drift – sensational!

I ran several more runs of PEMPro – which generally makes clouds appear, until its corrections was coin toss +/- 0.2 arc minute adjustments over a 20 minute run, which I imply means get a better CCD!

I also synced several times on each of three stars – until goto’s where spot on for them, and noticed they weren’t the first few iterations (about 2 arc minutes off in DEC). Also after each alignment on Sirius, Canopus and Acrux I check the alignment parameters – and noted the last two digits of the polar alignment coordinates were varying wildly – from -300 to +300, generally with only a few points separating the two values.

So let me sum up where things are at. I was surprised too see Maxpoint and PEMPro disagree significantly – but I guess PEMPro was correct. It was too late to confirm tracking accuracy – but if its under 20 arc seconds an hour, and mostly in DEC – then I’m smiling. I am still suspicious pointing has detoriated (but MaxPoint should resolve this) but will live with it if tracking is back to superb!

Lastly I really would like to know what those two parameters of the alignment model are. I ponder if it was that – rather than the rotating the mount East 30 – 40 arc minutes that significantly improved tracking!