Star and Polar alignment

[SECSIO]

Hi

I have always taken photos using my DSLR in prime, 30s exposures. I got a new CCD camera (ZWO ASI 533 MC Pro) and testing it for the first time tonight I noticed that if I want to take advantage of long exposure times, I would need a better calibration. I always use 3 star alignment, I think it is the most accurate for my EQ6R PRO mount. However, reading some posts, an user suggested that before star alignment and after polar allignment you should set Polaris in the center of your scope. I always put my telescope at home position after polar alignment but I never check if Polaris is in the center of the field of view of my telescope. Is this really necessary? 

 

Some tips to improve the calibration are welcome too ^^

[Ouroboros]

No. It’s not necessary.  

[MarkAR]

I just do PA then plate solve on my target.

[endless-sky]

For astrophotography, the only required thing is a correct polar alignment. You don't need any star alignments, they don't have any influence in the tracking/guiding capability of your mount.

What I usually do is this:

- polar align with the polar scope, using an app on my phone, to roughly eyeball the position of Polaris in the polar scope (I found this is plenty good enough for my 1-2 minutes exposures, at 300mm focal length)

- slew to a bright star to focus, plate-solve if it is not in the frame of the camera, until the mount pretty much centers it (I also sync the mount to the star position, once centered - so this technically counts also as a 1-star alignment)

- slew to the object I am interested in capturing and plate-solve until it is framed exactly as I want (I also rotate the camera, in this step, to frame the object to my liking)

- start my imaging sequence

No star alignment required, only plate-solving for focusing and for framing.

Edited July 30, 2020 by endlessky

[SECSIO]

1 hour ago, endlessky said:

For astrophotography, the only required thing is a correct polar alignment. You don't need any star alignments, they don't have any influence in the tracking/guiding capability of your mount.

What I usually do is this:

- polar align with the polar scope, using an app on my phone, to roughly eyeball the position of Polaris in the polar scope (I found this is plenty good enough for my 1-2 minutes exposures, at 300mm focal length)

- slew to a bright star to focus, plate-solve if it is not in the frame of the camera, until the mount pretty much centers it (I also sync the mount to the star position, once centered - so this technically counts also as a 1-star alignment)

- slew to the object I am interested in capturing and plate-solve until it is framed exactly as I want (I also rotate the camera, in this step, to frame the object to my liking)

- start my imaging sequence

No star alignment required, only plate-solving for focusing and for framing.

Then star alignment is just a way to correct the error made while polar alignment? 

[SyedT]

6 minutes ago, SECSIO said:

Then star alignment is just a way to correct the error made while polar alignment? 

Star Alignment is a way for your telescope to determine where it is in the sky more accurately i.e. it's pointing where it should be. Platesolving is a much more robust alternative to this, but of course for this you need to plug the mount into a laptop and run a platesolving program. Star alignment on the other hand can be done from the hand controller or your PC.

[michael8554]

To help guiding the mount needs RA to accurately  follow the arc the stars trace  across the sky - that's good Polar Alignment. 

When you supply date time location to the scope it has a pretty good idea of where the stars are in the sky. 

To improve that you goto one or more stars and centre them, then the mount knows precisely how the stars are oriented at that time. 

You haven't touched the mount azimuth or altitude settings during Star Alignment, so PA hasn't changed. 

Michael 

[endless-sky]

2 hours ago, SECSIO said:

Then star alignment is just a way to correct the error made while polar alignment? 

No, star alignment doesn't correct polar alignment.

Think about it like this: polar alignment is needed so that the axis of rotation of the mount can be parallel to the Earth's axis. This is needed so that when the mount tracks in AR, it can describe an arc in the sky that follows the apparent motion of the stars. If polar alignment was perfect, in theory you wouldn't even need to ever touch the declination axis during tracking/guiding.

On the other hand, star alignment is needed so that the mount can "know" where it is pointing. Each night, for a given time, the positions of the objects in the sky change with respect to the night before, at the same time. Giving the mount the location and time should be enough for the mount to "roughly" know where everything is. This, however, is not entirely true, because there can be many compounded errors (longitude, latitude, date, time, polar alignment, tripod levelling, etc.). So, you give a few "points" to the mount to better determine where everything is. If you plan to do visual observation, the more points you give, the more accurate the model becomes and the more likely it will be that the next object you tell the mount to go to will be inside the field of view of the eye-piece. For astrophotography all this precision in pointing is not needed, as the field of view of a camera is - usually - bigger than the one offered by the eye-piece. Furthermore, you can use a technique, called plate-solving, which consists in taking a picture of where the mount and camera are currently pointing, comparing it to online (or offline, if you download them for later use) maps (using the calculated field of view of your camera and lens combination). The software then tells the mount / planetarium software exactly where it is pointing and the mount can do the appropriate corrections to center the object (if it isn't already in the center) that you asked it to slew to, before the plate-solving.

Edited July 30, 2020 by endlessky

[Martin AB Cohen]

Hi there.  Polar alignment and Star alignment are easy to confuse.  I fully agree with the responses you have received from endlessky, explaining that star alignment has nothing to do with good tracking, and is only about finding the objects in the sky. You might also find my polar alignment tutorial on YouTube helpful...  

  

[Martin AB Cohen]

Hi again.  I just spotted your request for tips re calibration.  As it happens I have done a tutorial on that as well.  Here is the link....