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Radar

The Impossible Image

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This is worth writing about. I've got this one object in my southern sky that just eludes perfect tracking. It is the Tarantula Nebula. Knowing how illusive this object is for my mount (for whatever reason) I polar align earlier tonight on the Northern Horizon, then the East. Using the tracking graph and 20 years of experience I can tell you that my PA is incredibly good. But I also know that my pier has some slight flexure (probably because I use a lifting column). Anyway, as predicted my perfect PA did not translate to the Tarantula (this specific region of sky). My mount, telescope and counterweights hovers about 180kgs, so flexure is certainly possible. 

So tonight, I decide to do PA on the Tarantula itself, until there is no drift. I perform this with the tracking graph as well to ensure I've got no drift. Once I had no drift via the graph or tracking image, I refocus the equipment, guide, and start imaging. Guess what? First sub shows drift. 

I still have some theories on what could be causing this but its pretty annoying. Any other area of the sky has pristine PA and never drifts. This one object is cursed. I'm actually enjoying the challenge. Suggestions and conspiracy theories are of course welcome. 

Ray

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Hmmm, strange one Ray. If it was PA then you would see field rotation that close to SCP as opposed to drift. My guess would be flexure of some sort, probably due to your lifting column pier. Is replacing the pier with a solid one an option? Even short term to test if that is the cause.

 

From what I understand you are better off, for guiding purposes, to have a "small" (read tiny, something like 0.5 arcsec - 1 arcsec) amount of polar misalignment.

 

Shane

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Yes, I might install a solid pier about 1/2 the length of the lifting column and put the lifting column on that. This way I can still use the column but only at half mast. I believe it is only when I come close to fully extending the column that it shifts slightly (maybe 1mm). But at 4000mm times 7000 light years, that's a massive error. 

Ray

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OK - so I am guessing you have a separate guide scope that you are doing the drift measurements with?  In which case you are probably not seeing drift in your image but differential flexure.  If you DON'T have a separate guide scope but are using an OAG on the main imaging scope - then this makes no sense at all.

Greg

 

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6 hours ago, wingeing Pom said:

differential flexure

The guidescope guides beautifully 99% of the time. When we are guiding on the Tarantula, the Tarantula is sitting right on top of the SCP, meaning the main scope and guide scope are directly above and below each other with the counterweight bar pointing straight down, which makes me think it wouldn't need to flex at all, however Greg, stranger things have happened so I'll double check this and see if something is loose. Could be the camera or dovetail plate, who knows. Thanks for the tip, certainly worth looking into. 

Ray

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Ahh - I didn't know the Tarantula was right on top of the SCP - that's completely different.

You cannot guide at all when at the poles as you don't get any appreciable movement in the axes up to the 2 minute limit of Maxim DL (or possibly whatever guide package you use).  So basically when at the pole you have no guidance.  Now I even have this problem when imaging Polaris, so I turn off the guiding altogether and I am limited to 5 minute subs at 450mm FL with my polar alignment.  So if you are at a much longer FL then it is no surprise that you have virtually no time before you see polar drift.  Not sure there is anything you can do about that :(

 

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In fact I will go so far as to say there is nothing you can do about that.  All you can do is get the best PA possible, and then take 1, 2, 3, 4, 5, 6, 7 ... minute subs and see where things become unacceptable.

Greg

 

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Radical thought perhaps Ray. Are you still using that adjustable guidescope holder? If so have you tried, once aligned on target, to adjust the guide scope to an area slightly away from target and then guide, if you follow what I mean? This may introduce field rotation so exposure time would need adjusting also.

I have imaged the Tarantula with my RC on SW NEQ6 PRO which is way below the AP mount in every aspect and had no problem guiding. Daniel also has imaged it and he is not far from me in location.

 

Shane

 

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9 hours ago, wingeing Pom said:

All you can do is get the best PA possible, and then take 1, 2, 3, 4, 5, 6, 7 ... minute subs and see where things become unacceptable.

I'll try this Greg and let you know how I go. Always something to learn in this craft.  

5 hours ago, poppasmurf said:

dical thought perhaps Ray. Are you still using that adjustable guidescope holder? If so have you tried, once aligned on target, to adjust the guide scope to an area slightly away from target and then guide, if you follow what I mean? This may introduce field rotation so exposure time would need adjusting also.

Excellent idea as well. I'll certainly try this. 

Ray

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Ahh - just thought of another issue :) I worked all this stuff out about 10 years ago which is why I have forgotten it.  You might actually be guiding on a star, but if you are close to the pole only RA adjustments are necessary, so if your guider is also doing DEC adjustments, it'll all go haywire.  This is why I don't guide AT ALL when ever I am anywhere near the Pole - and keep the subs to 5-minutes or less.  You could find (as I did) that the star drift is WORSE with guiding on than with it off completely.

Greg

 

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Cheers Prof. I don't guide in dec at all usually. But I'm also thinking that at the counterweight down position, that the mount has no pressure either side of the worm and possibly is free wheeling a little. Soon as this cirrus clears I'll try the above. 

Ray

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1 hour ago, Radar said:

Cheers Prof. I don't guide in dec at all usually. But I'm also thinking that at the counterweight down position, that the mount has no pressure either side of the worm and possibly is free wheeling a little. Soon as this cirrus clears I'll try the above. 

Ray

Yes - that too is a possible.  I know one guy used a bit of bungee chord attached to the counterweights to make sure there was always a bit of resistance (in one direction) for the gears to work on.

Greg

 

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