×
INDI Library v1.8.5 Released (19 Apr 2020)

April 2020 release of INDI Library v1.8.5 introduces new drivers while providing fixes and improvements to existing devices and core framework.

Strange result during Polar Alignment

1 month 3 weeks ago
PK
Gold Boarder
Gold Boarder
Posts: 292
Karma: 9
More
Topic Author
Strange result during Polar Alignment #54077
I have been using the Polar Alignment tool for months without any problem.
But tonight, something strange happened. I am using the same version of KSTARS over the last month, and had no problems until tonight - even though I have not updated KSTARS recently.
I started the Polar Alignment routine, and it came back and told me I was off by a couple of minutes. Nothing unusual there.
But the SCREEN was showing me being WAY off - almost out of range of the tool. I selected a star as usual, and the amount I needed to adjust was appropriate.
I did this three times, getting closer to perfectly aligned each time, but the screen was not zeroing me in. Very odd.



iOptron CEM25P
William Optics Star71
ASI 1600Mm Pro
Moonlight motorfocus
LodeStar X2 and ZWO OAG
DewBuster
NANO PC-T4
Attachments:

Please Log in or Create an account to join the conversation.

1 month 3 weeks ago
Ihoujin
Platinum Boarder
Platinum Boarder
Posts: 901
Karma: 3
More
Strange result during Polar Alignment #54078
The screen is just showing you where the camera is pointed, and the error correction vector is the magnitude of your alignment error, not a defined position such as the celestial pole.

It appears to me your telescope was a little bit off the home position of 90°dec. Not enough to worry about.

INDI/KStars on Raspberry Pi 4, 4gb
Raspbian Buster with AstroPi3 script configuration
Skywatcher HEQ5 Pro Mount
Canon 600D Camera
Orion SSAG/ASI120mm @280mm Guide Scope
PHD2
Adafruit Motor Hat shield
Adafruit GPS Module
Generic Bluetooth Joystick.
Startech 7 port powered USB Hub.

Please Log in or Create an account to join the conversation.

1 month 3 weeks ago
maxthebuilder
Platinum Boarder
Platinum Boarder
Posts: 343
More
Strange result during Polar Alignment #54413
Andrew,
Are you saying that even if the scope is slightly off axis, the polar alignment will still be good?
The procedure takes care of the misalignment?
It would be great if that were true!

I've been using a mount without a polar scope for quite some time and I am sure my scope has been always a bit off.
I would run the PA align procedure in Ekos and the tracking would be good (with auto-guiding though).

Now. I recently got an HEQ5 with a polar scope. I don't know how well the polar scope is aligned but I hope it's good.
If I polar-align with the polar scope and then run the PA procedure in Ekos (with the main scope), I get 30'-50' difference.

The question is - which alignment to prefer?

Thanks!

Ihoujin wrote: The screen is just showing you where the camera is pointed, and the error correction vector is the magnitude of your alignment error, not a defined position such as the celestial pole.

It appears to me your telescope was a little bit off the home position of 90°dec. Not enough to worry about.


Max S.
RPI 3 B+ with ABerry and RPI 4 B (4Gb) with SMate.
Nikon D5500 H-a modified. 180 mm f/2.8 antique Nikon. Tamron AF 28-75mm f/2.8. Rokinon 135mm f/2.
AT72EDII.
ZWO ASI294MC Pro
ZWO ASI120MC-S on ZWO 30F4 MiniScope.
SW AZ-GTi.
SW HEQ5.
iOptron SkyTracker Pro.

Please Log in or Create an account to join the conversation.

1 month 3 weeks ago 1 month 3 weeks ago by dmsummers.
dmsummers
Gold Boarder
Gold Boarder
Posts: 194
Karma: 2
More
Strange result during Polar Alignment #54424
@Max: "The question is - which alignment to prefer?"

Without trying to be a smart..., you want a true PA. If PA tools/methods conflict, the way to know definitively is to drift align. PHD2's drift align tool is painless to use and simple; I ALWAYS use it. My advice would be to run both procedures, then compare answers (run PHD2 drift align last). You can trust what PHD is telling you about PA assuming you setup correctly (2 stars are required, one near DEC 0 south meridian, another near DEC 0 east or west horizon). If the methods agree, great. If they don't, you'll at least know. You can calibrate your polar scope position from the answer. Cheers, Doug

RASA11, Celestron CGX-L, ASI183mc Pro, 60mm guider + ASI290mm mini, ASI EAF focuser, PPB, Rpi4-4Gb+SSD, Powered USB3 hub, hardwire Ethernet.

Please Log in or Create an account to join the conversation.

1 month 3 weeks ago
Ihoujin
Platinum Boarder
Platinum Boarder
Posts: 901
Karma: 3
More
Strange result during Polar Alignment #54429
Yes It's true. If the FOV is slightly off it can still work. But you should try to at least have the celestial pole in frame. Also a camera assisted polar alignment does not require being on the axis of rotation, only roughly parallel to it. That is why it is possible to use a guide scope, or mount a QHY polemaster on the dovetail bar of a telescope.

INDI/KStars on Raspberry Pi 4, 4gb
Raspbian Buster with AstroPi3 script configuration
Skywatcher HEQ5 Pro Mount
Canon 600D Camera
Orion SSAG/ASI120mm @280mm Guide Scope
PHD2
Adafruit Motor Hat shield
Adafruit GPS Module
Generic Bluetooth Joystick.
Startech 7 port powered USB Hub.
The following user(s) said Thank You maxthebuilder

Please Log in or Create an account to join the conversation.

1 month 2 weeks ago
maxthebuilder
Platinum Boarder
Platinum Boarder
Posts: 343
More
Strange result during Polar Alignment #54455
Oh... This is great!
Still a bit skeptical since as far as I remember when I PA aligned with the main scope and then repeated with the guider - the results were different.
It was a while ago.. Will double check...

Ihoujin wrote: Yes It's true. If the FOV is slightly off it can still work. But you should try to at least have the celestial pole in frame. Also a camera assisted polar alignment does not require being on the axis of rotation, only roughly parallel to it. That is why it is possible to use a guide scope, or mount a QHY polemaster on the dovetail bar of a telescope.


Max S.
RPI 3 B+ with ABerry and RPI 4 B (4Gb) with SMate.
Nikon D5500 H-a modified. 180 mm f/2.8 antique Nikon. Tamron AF 28-75mm f/2.8. Rokinon 135mm f/2.
AT72EDII.
ZWO ASI294MC Pro
ZWO ASI120MC-S on ZWO 30F4 MiniScope.
SW AZ-GTi.
SW HEQ5.
iOptron SkyTracker Pro.

Please Log in or Create an account to join the conversation.

1 month 2 weeks ago
maxthebuilder
Platinum Boarder
Platinum Boarder
Posts: 343
More
Strange result during Polar Alignment #54456
I never used PHP2 and didn't realize it had a PA procedure.
Will give it a try.
Thanks!

dmsummers wrote: @Max: "The question is - which alignment to prefer?"

Without trying to be a smart..., you want a true PA. If PA tools/methods conflict, the way to know definitively is to drift align. PHD2's drift align tool is painless to use and simple; I ALWAYS use it. My advice would be to run both procedures, then compare answers (run PHD2 drift align last). You can trust what PHD is telling you about PA assuming you setup correctly (2 stars are required, one near DEC 0 south meridian, another near DEC 0 east or west horizon). If the methods agree, great. If they don't, you'll at least know. You can calibrate your polar scope position from the answer. Cheers, Doug


Max S.
RPI 3 B+ with ABerry and RPI 4 B (4Gb) with SMate.
Nikon D5500 H-a modified. 180 mm f/2.8 antique Nikon. Tamron AF 28-75mm f/2.8. Rokinon 135mm f/2.
AT72EDII.
ZWO ASI294MC Pro
ZWO ASI120MC-S on ZWO 30F4 MiniScope.
SW AZ-GTi.
SW HEQ5.
iOptron SkyTracker Pro.

Please Log in or Create an account to join the conversation.

1 month 2 weeks ago
ChrisRowland
Platinum Boarder
Platinum Boarder
Posts: 496
Karma: 9
More
Strange result during Polar Alignment #54469
There's no fundamental reason why polar alignment needs to be done with the scope looking at the pole. The celestial sphere is rigid so the polar alignment error affects the whole sky by the same amount.

Celestron do this with their All Sky Polar Alignment.

Once you have the align error - which can also be done using data from the whole sky - the tricky bit is working out the error so advice can be given on how t move the mount relative t the sky it's looking at. The mount is using sky coordinates but the mount adjustments are in Azimuth and Altitude

I've been dabbling with this for a while and the trick seems to be to transpose the polar align error to AltAz coordinates. The transpose the position of the mount to alt az and add the polar align error in altaz to the mount altaz position. Then transpose back to Radec. This gives the position the mount needs to be moved to in order to get the mount polar aligned.

This is all somethng for the developer to worry about, all the user has to do is follow the instructions.

While it's true to say that the error affects the whole sky there are a few places where correcting for it is tricky because of which direction the alignment moves the view o the sky. The zenith is one area and looking East or West the other.

My best attempt so far was OK in altitude but underestimated the movement in azimuth. None the less a few iterations seemed to do the trick.

Jasem and I have been looking at this, so far I've got an all sky way of determining the mount error.

Please Log in or Create an account to join the conversation.

1 month 2 weeks ago
maxthebuilder
Platinum Boarder
Platinum Boarder
Posts: 343
More
Strange result during Polar Alignment #54575
OK. Now I am convinced : )
- Aligned with mount's polar scope.
- Checked with the main scope. Error - 39'09"
- Checked with the guide scope. Error - 37'32"
- Aligned with the main scope. Error - 07'36"
- Checked with the guide scope. Error - 07'47"
Looks good to me!

maxthebuilder wrote: Oh... This is great!
Still a bit skeptical since as far as I remember when I PA aligned with the main scope and then repeated with the guider - the results were different.
It was a while ago.. Will double check...

Ihoujin wrote: Yes It's true. If the FOV is slightly off it can still work. But you should try to at least have the celestial pole in frame. Also a camera assisted polar alignment does not require being on the axis of rotation, only roughly parallel to it. That is why it is possible to use a guide scope, or mount a QHY polemaster on the dovetail bar of a telescope.


Max S.
RPI 3 B+ with ABerry and RPI 4 B (4Gb) with SMate.
Nikon D5500 H-a modified. 180 mm f/2.8 antique Nikon. Tamron AF 28-75mm f/2.8. Rokinon 135mm f/2.
AT72EDII.
ZWO ASI294MC Pro
ZWO ASI120MC-S on ZWO 30F4 MiniScope.
SW AZ-GTi.
SW HEQ5.
iOptron SkyTracker Pro.

Please Log in or Create an account to join the conversation.

Time to create page: 0.692 seconds