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!

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.

5 hours ago, SteveBz said:

So this is my proposal for the last few steps, to avoid the need for a horizontal angle.  Please tell me what you think:

Here is a webpage I found, laying out the calcs:

http://www.stargazing.net/kepler/altaz.html

Which translates to:

I'd love to hear your thoughts.

Steve.

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).

Quote

So as far as I understand, horizontal is only important to avoid starting all over again.  Ie to remove one degree of freedom from the problem.

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

2 hours ago, SteveBz said:

I would also like it to operate more remotely.  I don't have a goto scope, so I can't slew at speed, but I can probably slew 30 degs in RA in under ten minutes, which is acceptable.  Would a 30 degree separation in planes be enough to polar align, do you think?  Obviously I still need to go outside to twist the alignment screws.

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.

3 hours ago, SteveBz said:

I can see that you are correcting using AstroPy, so it should be correct.  However, I looked at Stellarium and it says it's 'Current date'.  RA & Dec (Current date) are (43:78, 89:34).  89:34 is 26' off NCP, so it would seem to be the just past the 20' circle not on the 40' circle.  Have I got that right?

Steve

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).

13 hours ago, davidgr1976 said:

Hello,

[...]

I've been testing it. I get 20" error but when i check it with PHD2 the PA error after 10min is 4 arcmin... What could be the cause of this big difference with sharpcap?

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. 

On 22/08/2016 at 11:27, Fraleo62 said:

I just diacovered this topic and read the original pdf....with the math

I dont understand the need of the initial rotation to the zenith..

Can someone clarify ??

Thx

Franco 

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

Themos

Revelation 15x70 with anti-dew heated mirror mount and heated laser finder (keeps the laser diode warm, not shown).

I take both a vertical and horizontal at the end, mainly because I like to rotate the mount through large angles to make sure it's settled. I usually do this with light portable mounts, not so much with my EQ6.

Yes, I take just one vertical image and then, after the first horizontal image, I find the polar axis (left button). All images after that are horizontal and each can be used to show the improvement (right button). At the end, I confirm the alignment with another vertical/horizontal pair.

Hello calberts,

I think you would like the new numbers to appear on the GUI window, but below earlier values, is that right? By the way, the numbers (and directions) do not refer to Polaris as such, but to how you should move the mount's polar axis (imagined as a point on the celestial sphere).  

6 hours ago, calberts said:

Hoi  themos

I like to have option that the popup with the picture is disabled after clicking "Find Polar Axis" or "show improvement" but the calculation is still done i think the change of values are the most inportant of you program. Although the pictures makes the program complete and visable what is going on.

Any posibility in the settings screen for this?  and if possible  the before and after values,  i lke to have  in the screen  ;-) Sorry 

Iam a newbie in python i am not yet able to do it my self.

Chris

 

 

 

 

Open PPA.py in Notepad and find the lines like 

self.create_imgwin

and turn them into comments, like this.

#self.create_imgwin

Then , find the function update_display and add the last line "print inst" so that it lines up with the previous line

    self.wvar9.configure(text=inst)
    print inst

The picture serves as a visual check, by the way. You are supposed to check that there's stars (and the right stars!) in the centre of the circles.

I apply the precession correction using the astropy package in python, it's in line 969 in

https://github.com/ThemosTsikas/PhotoPolarAlign/blob/master/PPA.py

 cpj2000 = cp.transform_to(FK5(equinox='J2000')) 

The bottom line is "don't sweat the small stuff"

Polar axis alignment requirements for astronomical photography

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1989JBAA...99...19H&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES

The granddaddy of PhotoPolarAlign

http://faculty.mu.edu.sa/public/uploads/1333168346.5416Final_ICAM.pdf

15 hours ago, Vox45 said:

I still see on a regular basis people starting threads on PA and confusing 3 stars alignment with polar alignment.

one-star, two-star, 3-star procedures should always be called Syncs, not alignments.

On 13/05/2016 at 15:03, Vox45 said:

(1) How does PPA compares with Kevin Sipprell's PoleFocus ?

PPA measures the Polar Alignment error in angular displacement in Azimuth and Altitude. It does not require a Goto mount or a LiveView capable camera (because I wanted it to be useful even with basic quipment, like barn tracker mounts and compact cameras).

These error numbers are exactly what  PoleFocus requires to help you correct the error (by adjusting the direction your Polar Axis is pointing). It requires Goto capability and LiveView.

So each bit of software does a different job. AstroTortilla also calculates the Polar Alignment Error but people have had mixed results with it (AT uses GoTo) and development of AT seems to have stopped.

On 13/05/2016 at 15:03, Vox45 said:

(2) I see people using PoleMaster software, which looks similar to your software, and pay up to 300€ for the camera that is placed on the Polar scope. From what I understand, your software uses a camera mounted on the main scope, so are these people throwing away 300€ ? I mean, is there a reason to get a dedicated cam on the polar scope, what is the logic behind that if we can achieve the same result using a DSLR on the main OTA ?

I can't say that they are throwing away 300Euros. I can say that PPA, for me (and a few others) works just fine. The extra hassle is that I have to mount my DSLR with a 135mm lens, piggy-back on the scope, for polar alignment and then take it back to prime focus of the telescope, but it's not a deal-breaker! And I need to carry enough plate-solving software on the laptop, but again it's not too bad. 

On 10/05/2016 at 09:55, AndyKeogh said:

I do not use live view on the camera but have an image on my Ipad/laptop that I take to the scope when making adjustments.

Is anyone aware of a tool similar to AstroGeeks CrossHare that could overlay the live view image with a grid in arc minutes or arc seconds?

Or is there a simpler way to do this?

Andy

Hello, 

I was going to try Kevin Sipprell's http://www.scopefocus.info/polar-alignment to see if it works nicely with PPA.