Source of drift only in piggybacked refractor

[rogerg]

I'm struggling to understand why I have so much drift when photographing using my piggybacked Megrez 90.

Mounted on a paramount ME the primary OTA (12" SCT) has no drift in 5 minute exposures and slight in 10 minute exposures. However the piggybacked Megrez has noticeable drift in 5 minutes and significant drift in 10 minutes.

The drift is mostly dec but not pure 100%, some RA.
The drift is ongoing not cyclical (the FOV shifts increasingly one direction)
The drift is there even when the primary OTA is autoguiding with good accuracy.
The drift appears to INCREASE over time from one exposure to the next, the stars become more and more illongated (see example attached).

Surely it has to be flexure if the primary OTA is known to be tracking OK? But how could it be so consistent and ongoing?

Confused.:help::shrug::screwy:

Regards,
Roger.

[ZeroID]

Check your polar alignment first obviously, I thought I was pretty good till I did my little exercise in rotating long exposures. No almost no guiding required and almost zero drift. Thread here if you're interested. My symptoms were identical to yours.

http://www.iceinspace.com.au/forum/showthread.php?t=151186

The other factor could be balance of the mount/scope/counterweight. Do you rebalance when you change scopes over ?

[rogerg]

Polar alignment is good, 1.1 arc minutes off in Alt and Az. Also remember that the same exposure time in the primary scope has no drift when autoguiding, almost none when unguided, yet it's 2180mm focal length vs the piggyback refractor at 600mm. If it were polar alignment I would expect to see more exaggerated drift in the 2180mm not the other way around.


I never change the scopes over, as in, 6mo+ between any physical changes to the system at all, 4 years since I changed equipment/balance. I have checked the balance and it's OK. But,whether somethings is pulling on the refractor in some direction ..... maybe but can't see it .. must be, but still can't see it :)

Hmm

[raymo]

It looks very solidly piggybacked, so flexure of that magnitude seems
unlikely, so it would seem that something is moving. Could the weight of the camera and/or snagged/heavy cables be causing the focuser to sag a tiny bit. This movement would cease when the focuser tube came hard up against the casing or the locking screw[s], so it would be easy to check by doing an extremely long exposure, and seeing if the star elongation continued ad infinitum, or ceases after a given amount of time.
raymo

[Atmos]

I'd say what you're experiencing is mirror flop. Now this is fine as you're off-axis guiding is correcting for it on your main OTA but as your secondary refractor doesn't suffer from this, it will show up as drift.

[rogerg]

Ohhhhh yes! You're probably right. The dodgy old 12" SCT bites my ass again. Grrr.

It certainly does have significant mirror flop, as you say not normally noticed.

Hmmmm, I guess if that is it, I"m either best not guiding, or ideally need to buy an OAG for the refractor.

[Atmos]

You'd probably be better getting a HEQ5 Pro and using it just on that :) The problem with mirror flop is that you can only image with one at a time (although you may be).

[PRejto]

Roger,

Are you permanently set up? I would guess you are with an ME! So why not just use a good model + Protrack and forget about guiding all together? With a short FL scope going 5 min ought to be very easy. I can go 5 min without any problem with my TEC180/MEII.

Peter

PS. Are you trying to image with both scopes at the same time? There is a way but it's complicated and would necessitate using AO on one scope. I can post details if you mmight be interested.

[rogerg]

I sometimes setup my GM8 in the observstory to allow dual use but it gets a bit squeezy :) Much easier to just keep it on the PME :)



Yes well normally I image through the 12" for 5min with no guiding, 90% of frames are good, and 600s with no guiding but becomes less reliable. Strangely I don't get any better with the 90mm refractor at 600mm FL. It's probably about time I did another model and improved the PEC, havne't touched it for a year plus, but I get lazy.



I'm just doing one at a time. Tried two a while back but haven't since.

[Tony_]

Regarding mirror flop - I always focus my SCT "uphill" (counterclockwise with Celestron scopes). That way it is already in the "flopped" position - never had an issue with mirror flop since then.

Tony.

[rogerg]

Mine is always focused using a motorised Crawford and the primary is in theory tensioned/locked, but I have measured the drift due to mirror flop in the past and while I can't remember exactly it was something like 5 arc minutes of drift.

[rogerg]

Well, thanks for the inspiration to re-calibrate my setup Peter. I have done PEC and a new mapping run now. It hasn't changed much but has slightly improved my tracking accuracy. So .....

Unguided 600s exposures:
1 : Meade 12" prime focus
2 : Megrez 90 piggybacked

being unguided, there must be a whole bunch of movement somewhere in the megrez!!! it just keeps on drifting. Amazing. Going to have to hunt it down.