Baader (Multi Purpose) Coma Corrector

and CG5 All-Star Software Polar Alignment


Some information on using The Baader Coma Corrector on my F/4 Astro-Tech 8" Newtonian.

I also wanted to post information on using Celestron's All-Star Polar Alignment on my CG5.


First the Baader Coma Corrector.  I've been reading online and hearing hat some coma correctors do a very

poor job of correcting coma in fast Newtonians. Some also increase the focal length and therefore the F Number.

For those of us that don't guide, the F Number is king.  The last thing I want in a corrector is for it to increase Focal Ratio.

For this reason I chose the Baader Coma Corrector, as it specifically states it does not increase focal ratio.


Here's the corner of an image of M31 taken with my F/4 Newtonian without any correction.




Image 1



F/4 Newtonian



Image 2



F/4 Newtonian - with Coma Corrector




This is the bottom right corner of an image of Andromeda

You can see the stars around M32 are blurred and

enlarged.




This is a similar region of a later image of Andromeda.

This was taken with the same equipment as the

first photo - but with the coma corrector attached

to the DSLR





In Image 2 with the coma corrector in place, not only are the stars in the corner of the frame fully corrected,

but some stars are visible in the cloud of M32 which are invisible in the first image.  The difference in orientation

is due only to the DSLR being oriented differently on the telescope and not to any change in focal length.


This coma corrector can be fitted to eyepieces with additional T2 rings.  I'm not concerned with this, my

requirement is that images are corrected.  I therefore leave the corrector attached to my camera's T-ring.

I simply fit this assembly to the camera for imaging.







CG5 All Star Software Polar Alignment


I have always used a Polar Scope to set up equatorial mounts for imaging.  When I heard about

Software Assisted Polar Alignment I was skeptical.  I was skeptical because during star alignment I have often found

alignment stars to be fairly off centre and occasionally outside the field of view.  However this was happening

with my C8 partly due to having to rotate the 2" Diagonal every time the OTA moved to a new star. 

When I rotated the Diagonal, the star would shift noticeably in the Field Of View. 


After upgrading my Hand Controller to Version 4.16, I decided to give the new All-Star Polar Alignment a try.

I performed a 2 star alignment with 1 Calibration star.  The stars were Sirius, Procyon, Rigel.  Procyon was the calibration

star and I was pointed at Procyon when I initiated the Polar Align Routine.  The routine is now accessed by pressing

The Align Button on the hand controller.  The hand controller prompted me to centre Procyon in the eyepiece then press Align.

I did and the scope then re slewed to Procyon.  But this second slew moved the scope to where Procyon should be if the mount

were accurately polar aligned.  Now I moved the scope back to Procyon by using only the Altitude and Azimuth adjustment knobs on the mount.  Its best if you put the hand controller down during this phase - the temptation is too great to tap the direction buttons as you normally do when aligning on stars. 


I had initially used the polar scope to put Polaris in the centre of the Polar Scope.  This way I knew my alignment was already close so there should not be any large correction required.  When the scope re slewed to Procyon, it was off target by at least as much as two moon widths.  I scoffed at this and thought the calculation must be way off.  But I thought I would complete the test anyway.

I gradually moved the scope back on to Procyon using the mount adjustment knobs.  This took around 5 minutes.  I took off

the Polar Scope cover and looked for Polaris.  I did not expect to find it in the field of view.  But in fact it was positioned

just outside the small circle for Polaris in the Polar Scope.  I was surprised at this mainly because it seemed I had moved the

telescope a lot to align on Procyon.  I looked at the orientation of Cassiopeia in the sky and compared with the Polar Scope

and realized if I rotated the mount to match the sky view - Polaris would indeed be inside the small circle. 


From this I realized the Polar Scope has a much wider field of view than I expected.  This means in turn that any error in

placing polaris using the Polar Scope is much larger than it appears.  The opposite is also true, that the main scope has a

much narrower field and therefore potentially a much higher accuracy in placing Polaris.


I did then take some images and found tracking to be good.  My timer had a low power malfunction and set all my exposures

to 30 sec instead of the 60 sec I had programmed.  I was therefore not able to compare directly with previous longer image

runs.  Out of 40 images though, 38 were un trailed at 800mm and 30 seconds. 


This test has caused me to reverse my instinct on this one.  I had been confident the Polar Scope would be more

accurate.  But now having seen how wide the FOV is in the Polar Scope it should be possible to easily improve on

this by carefully using the SW Polar Alignment.





All Content Copyright © James MacWilliam