Polar alignment doesnt work

bergenk

I have a celestron SE6 with a focal reducer which gives me a focal length of 945mm.

I have a Canon 450D which when connected gives me a fov of 1.4x 0.9 deg. This is within the 0.4-33 deg requirement for polar alignment and plate solving.

The app is unable to solve. Is there something im doing wrong?

w7ay

There are a couple of things to check by performing Plate Solving in the Preview window, before you even try to Polar Align.

Make sure you are in perfect focus. Plate solve involves first detecting the center of stars by comparing a pixel on the plate against the nearby surrounding pixels. A defocused star will spread over too many pixels, and that can confuse star detection. To learn and get used to plate solving, first try with a good star field that does not have a single very bright star -- do not start with a field that includes Vega or Sirius, for example.

After stars are brought into focus, make very sure that the brightest stars do not saturate the plate. Look at the MAX ADU number in the histogram. Make sure the max ADU value is nowhere above 65000. Reduce the exposure time and the gain of the camera if you are saturating the plate.

You do not need (or want) too many stars. When you start plate solving, ASIAIR will report the number of stars detected; this will tell you if you are exposing for too long -- don't worry if you cannot see many stars in preview window. The worst thing you can do is to overexpose. 10 stars are too few, but more than 100 will not be useful and may in fact confuse the pattern recognition algorithm. Adjust the exposure and gain so that ASIAIR tells you it is seeing between around 20 and 100 stars.

Once you can successfully plate solve in the Preview window, you can note down the exposure time and gain, and use approximately those same values when you do polar alignment.

By the way, if the back focus of your reducer (the distance between the reducer and the camera sensor) is not set perfectly to specification, your actual focal length may not be what you think it is. With ASIAIR, you can enter a focal length of zero (yes, zero), and ASIAIR will attempt to figure out the correct focal length for you when you perform plate solving. It will take a little longer to plate solve, perhaps 20 seconds instead of 1 or 2 seconds, for example. ASIAIR will replace the zero that you entered by the true focal length of your optical path.

You can also take an image, and then submit your image to the web app at astrometry.net to plate solve for you, and the web app will also report the plate scale, from which you can calculate the focal length. astrometry.net also has a good explanation of how plate solve works by using trapezoidal asterisms.

Clear skies,

Chen

bergenk

W7AY wrote:

There are a couple of things to check by performing Plate Solving in the Preview window, before you even try to Polar Align.

Make sure you are in perfect focus. Plate solve involves first detecting the center of stars by comparing a pixel on the plate against the nearby surrounding pixels. A defocused star will spread over too many pixels, and that can confuse star detection. To learn and get used to plate solving, first try with a good star field that does not have a single very bright star -- do not start with a field that includes Vega or Sirius, for example.

After stars are brought into focus, make very sure that the brightest stars do not saturate the plate. Look at the MAX ADU number in the histogram. Make sure the max ADU value is nowhere above 65000. Reduce the exposure time and the gain of the camera if you are saturating the plate.

You do not need (or want) too many stars. When you start plate solving, ASIAIR will report the number of stars detected; this will tell you if you are exposing for too long -- don't worry if you cannot see many stars in preview window. The worst thing you can do is to overexpose. 10 stars are too few, but more than 100 will not be useful and may in fact confuse the pattern recognition algorithm. Adjust the exposure and gain so that ASIAIR tells you it is seeing between around 20 and 100 stars.

Once you can successfully plate solve in the Preview window, you can note down the exposure time and gain, and use approximately those same values when you do polar alignment.

By the way, if the back focus of your reducer (the distance between the reducer and the camera sensor) is not set perfectly to specification, your actual focal length may not be what you think it is. With ASIAIR, you can enter a focal length of zero (yes, zero), and ASIAIR will attempt to figure out the correct focal length for you when you perform plate solving. It will take a little longer to plate solve, perhaps 20 seconds instead of 1 or 2 seconds, for example. ASIAIR will replace the zero that you entered by the true focal length of your optical path.

You can also take an image, and then submit your image to the web app at astrometry.net to plate solve for you, and the web app will also report the plate scale, from which you can calculate the focal length. astrometry.net also has a good explanation of how plate solve works by using trapezoidal asterisms.

Clear skies,

Chen

Thank you for this great check list hopefully we get a clear night and i can give it another go 😁

bergenk

2nd attempt failed.

I had 69 stars detected and good focus. Tried different exposure times and iso settings.

Only thing that has come to mind this morning is the diagonal. Im using the dslr with the prism diagonal. Should this be removed

w7ay

>> Only thing that has come to mind this morning is the diagonal. Im using the dslr with the prism diagonal. Should this be removed

Is the diagonal mirror in between the reducer/flattener and the camera?

If so, did you compensate for its extra optical path when attaching the camera?

See my prior comment about how the path between the reducer and the camera sensor will affect the resultant focal length of the optical system. (Tell ASIAIR that your focal length is zero and it will plate solve (albeit a little slower) and tell you what the actual focal length of your system is.)

You mentioned that you are using a Celestron 6SE together with a reducer. If this is the Celestron or Meade 0.63 x reducer, the back focus needs to be 105mm (the manual that is specific to your reducer should mention it). Every reducer and flattener is designed for a specific back focus and you need to religiously abide by what the manufacturer tells you.

You also mention using a DSLR. If this is a Canon DLSR (EF mount), the flange to sensor distance is 44mm (a Nikon is I believe 46.5mm). Subtract this number from the 105mm, and that should be the precise optical distance between the reducer's back surface and the camera flange's front surface (e.g., for EF mount, it is 61mm). I suspect that your diagonal mirror is contributing more than that value, making it impossible to maintain proper back focus.

If this distance is incorrect, not only will you get a different focal length from what you expect, but you will also get images with field curvature (especially ugly at the corners of APS and full frame sized sensors). Again, tell ASIAIR that your focal length is zero and after a successful plate solve, ASIAIR will replace the zero with the true focal length.

An inexpensive diagonal will also produce image tilts (top of image has slightly different focus than bottom of image). Because of this, I do not know of anyone that does serious imaging who use diagonals.

Chen