Polar Alignment & Permanent setup

[Rodd]

Not sure this is the right forum--but here goes:

I have 1 mount and 2 scopes--a 4" frac and an 11" Cass.  I am planning on nailing my polar alignment via the drift method (like 1 hour with no discernible drift).  Lately I have had my frac set up for 3 weeks--through several rainy days and nights under a complex multi-gortex jacket, reflecting emergency blanket,  and waterproof tarp cover I devised.  Works great, though I understand they make a 24/7/365 cover designed for this very thing.  I am measuring to see if 1 cover will fit both scope setups.  I will graduate to this if it does.  I have always been hesitant to spend the time learning and performing a drift alignment as it would take away from my imaging time.  However, since I am operating in a permanent/semi-permanent fashion, I have decided it would be worth doing so I can maximize my goto and guiding accuracy, and eliminate rotation issues on long subs.  I have 2 main questions:

1)  Which scope should I use to nail my polar alignment through the drift method? The TVnp101is or the C11Edge HD.  I will be using a 9mm illuminated reticle eyepiece.  The C11 will provide greater magnification, but the TV would provide a sharper image. Which would prove more accurate.  I will be imaging with both scopes.

2) After I achieve spot on PA (perhaps "If" is a better word choice), will I be able to switch scopes and maintain the PA alignment?  If this proves possible, it also means that I do not have to leave my scope set up all the time--just my tripod and mount, which will be easier to cover and 1 less thing to worry about.

 

Thanks--Rodd

[ChrisLX200]

I don't think it would make any difference on the whole, you can put a high power eyepiece in the refractor and use whatever magnification needed for this job. My mount stays outside but I won't leave the OTA attached - apart from security concerns I prefer to store it in a climate-controlled area (low humidity and out from under the hot sun). I can replace the OTA without doing anything more to the pointing model, I cannot do that with my 12" ODK because the mount needs to be re-balanced and it's more sensitive to small tracking errors anyway, so a new model run is needed. If you are going to do this I would suggest installing a permanent pier if possible.

ChrisH

[michael8554]

Hi Rodd

Switching ota's may well change the PA, so first try a 10 minute PA, swap ota, see if it's changed.

if it's good, go for the long PA, though an hour?  How long do you need to get into the range that PHD will easily handle?  I don't know.

Do it on the 11", focus doesn't matter, in fact it will be easier to see with the reticle if the star is slightly defocussed into a small disc.

Michael

[Rodd]

11 minutes ago, michael8554 said:

Hi Rodd

Switching ota's may well change the PA, so first try a 10 minute PA, swap ota, see if it's changed.

if it's good, go for the long PA, though an hour?  How long do you need to get into the range that PHD will easily handle?  I don't know.

Do it on the 11", focus doesn't matter, in fact it will be easier to see with the reticle if the star is slightly defocussed into a small disc.

Michael

Thanks Michael--The reason I say an hour is to get as accurate as possible.  Presently, with my right angle PA scope--I get pretty close, but on 1 hour subs taken near the pole (Pacman Nebula in Cassiopeia), there is noticeable rotation elongation on small stars.  Even 30 min subs have eccentricity values of .6--which can't be seen unless you zoom way in, so I can live with it.  Even teh best guiding can't guide rotation out.  I use Maxim DL to guide--which I am happy with. I would like to remove all rotation from my subs.  I am trying to remove all variables from the equation in efforts to pinpoint causes of image deficiencies  Also, I am hoping to get real accurate goto so I can pretty much dead center any target--thinking of a mosaic and it seems goto accuracy would be helpful.

 

Rodd

[ollypenrice]

I don't think it will matter which you use and PA should not alter with a change of scope if the mount is all tight. I'm not sure why Michael feels it would be he may have experienced this? I don't fnd any problem.. However, I do find that the occasional re-do of PA is necessary. I've no idea why. It could be ground swell or a number of other things.

What will change with the scopes is cone error, the angle between the optical axis and the mount's axes. It isn't normally important though it will affect any permanent GoTo alignment and sky model for unguided imaging.

Olly

[Rodd]

22 hours ago, ChrisLX200 said:

I don't think it would make any difference on the whole, you can put a high power eyepiece in the refractor and use whatever magnification needed for this job. My mount stays outside but I won't leave the OTA attached - apart from security concerns I prefer to store it in a climate-controlled area (low humidity and out from under the hot sun). I can replace the OTA without doing anything more to the pointing model, I cannot do that with my 12" ODK because the mount needs to be re-balanced and it's more sensitive to small tracking errors anyway, so a new model run is needed. If you are going to do this I would suggest installing a permanent pier if possible.

ChrisH

Thanks Chris--I forgot about re-balancing, which  will definitely need to be done, which will mean I will lose my "last parked" position.  Shouldn't be that much of a big deal though--I find that when my PA is accurate, my pointing is good enough for my purposes.  For me, that means the first focus star anywhere on the sensor so I don't have to hunt for it.  That is a pain, especially with the C11Edge with its narrow FOV.  Since my finderscope has to be aligned each setup, it doesn't really help with the first star--except in the Winter when I can see the tower 2 miles away and I use that to align the finder-not perfect, but good enough.  Once I synch to the star, the target is usually easy to get on the chip.  

[Rodd]

1 hour ago, ollypenrice said:

I don't think it will matter which you use and PA should not alter with a change of scope if the mount is all tight. I'm not sure why Michael feels it would be he may have experienced this? I don't fnd any problem.. However, I do find that the occasional re-do of PA is necessary. I've no idea why. It could be ground swell or a number of other things.

What will change with the scopes is cone error, the angle between the optical axis and the mount's axes. It isn't normally important though it will affect any permanent GoTo alignment and sky model for unguided imaging.

Olly

Thanks Olly--Once I have done the drift method I will be more able to tweek PA every now and then.  It will be interesting to see teh correlation between the AP RAPA and a drift method alignment.  Hopefully I will be able to track changes in teh RAPA scope and will see when it is time to re-establish PA.