themos

Much better going tonight! Used a tiled roof and managed to get the guide camera (ASI 120MM) in (rough) focus at the same time as the (slow, tiny) Atik16ic with the ZWO OAG. I hope it is good enough for guide stars. ZWO manual filter wheel in place, too, and the Baader MPCC coma corrector. My first RGB galaxy beckons!

I tried an OAG setup last evening and I was facing the western horizon from the first floor balcony. I chose an electricity pylon as the main camera target. Bad idea. The pylon did not extend as far as the prism and I was getting all kinds of random foliage in focus in the guide camera. I need to find a brick wall at a good distance.

Hello, The TS 10"f/4 reflector was pointed at the Moon last night. A TeleVue Barlow helped up the focal length to the ASI130MM mono camera. SharpCap and AutoStakkert did the rest. Enjoy!

Hello, I did something utterly stupid today. I have a Xagyl filter wheel, very early model (the 5x125), probably a v1. I haven't used it in years and I thought to have a play with it ( I am considering buying a mono cooled CMOS). I accidentally "updated" the firmware with incompatible software and now the wheel will not connect properly. I meant to hit the Cancel button but the mouse slipped and I clicked OK. Has anyone had similar problem and has a solution? I've contacted the company and they seem doubtful whether there's anything that can be done. Themos.

Surprised that I noticed the Great Conjuction 11 years ago!

Just be careful with the worm holder, I've had mine break in bits.

I should add that the method I used in PPA also appears in SharpCap http://www.sharpcap.co.uk/sharpcap/features/polar-alignment so if you like the extra bells and whistles you might like to give that a try.

I think it would be great for someone to add this, but I wouldn't be using it myself. Given that hardly anybody has a mount with motorised elevation/azimuth controls of the polar axis, the PPA process will always involve a human being iterating moves and closing in on the error. People with observatories can take their time and people that set up every night can use a piggy-back camera (even a compact camera will produce good enough stars in 10 seconds or so).

I am sorry, that's not it! The first couple of images will fix the pixel position of the polar axis no matter what orientations you used. The requirement for the second image to be horizontal is to translate the x/y correction that is calculated (which is merely along the long side and short side of the sensor) to the left/right and up/down instructions needed for the person doing the mount correction.

Hello Steve, I've avoided using the reflector at prime focus because 1) the image scale is a bit excessive for the purpose ,2) reflector images have the wrong parity which messes up the move instructions and 3) it's more tricky to know what horizontal is. Best way of making the mapping from arcminutes to screw turns is to solve images (with PPA!). I guess I would do the first couple to get the polar axis, then give a full turn to raise the polar axis and do an improvement solve, then note the new error (the Up/Down move instruction). Repeat for Left/Right. For this to work, you must make sure your horizontal image is really that. I used to do it with a spreadsheet and just the positions of Polaris and Lambda UMi (like David Rowe had suggested) doing the geometry you described. Then I realized that the NCP is quite near the line connecting the two and that magnifies errors. The next idea was to use more stars and take an average of the geometric solutions. I finally settled on a numerical method to find the fixed point of the transformation implied by the two solved images. The mapping is (pixel x, pixel y) -> (ra,dec from 1st solve) -> (pixel x, pixel y from 2nd solve). The axis of rotation is the pixel that gets mapped to itself. When I do the Improvement step, I already know the pixel position of the axis of rotation. That will not change, no more RA/DEC moves allowed. I also know that my camera is horizontal. Any moves with the screws of the mount would just shift the sky in the field of view, so I just move to bring the NCP onto the known pixel position. Themos

It should be ok, I guess you will need to experiment. But you don't have to slew, you can just undo the RA clutch and swing the axis manually. Atsrotracs can't do that but your CG-5 should be able to do it.

My Stellarium (0.17.0) says, when I click on Polaris, RA/Dec (on date): 2h55m07.03s/+89d20m23.7s. So that is nearly 40mins off the current NCP.

Hello, I've added your ideas in the GitHub Issues. I am not sure when I will find time to add these but they are there in case anyone is tempted to implement them and do a pull request. Polaris was at ~44 minutes in J2000 but not now! PPA does adjust for the wandering of the Earth's axis (precession).

Hello, whenever I have verified the PHD GuideLogs after a session, I find an estimated PA error that's quite similar to what PhotoPolarAlign (same method that Robin has put in SharpCap) measured. I use PHDLab for analysing PHD GuideLog files. A couple of things to check: It could be that PHD's Drift Align procedure is not as robust as PHDLab's analysis (remember that PHDLab can see the entire session, not just 10 minutes, and tell how much Dec correction was applied). The other thing is refraction. If you are imaging fairly low in the sky, there will be significant and varying refraction. The path of the star will not be a perfect circle and you will get apparent drift no matter how perfectly polar aligned you are. I image at up to a metre focal length and up to 10 minute subs and consider any polar error under 5 arcmin as good enough.

Hello Franco, I am not sure which pdf file you mean. Themos