190 Mak Newt 1st light

[almcl]

Apologies in advance, this is going to be image heavy.

So after only two days the newly arrived SW 190 Maksutov Newtonian got to see some stars!

First, after two days of fiddling with the collimation, here's a star test:

The next step was a Bahtinov mask check.  I didn't notice till afterwards that while the bright star (which was centre frame) appears in focus, the little one to the left is clearly not.  Not sure if this is tilt? Or something else?

Went looking for a star field and happened on Albireo.  Again didn't notice the reflections till afterwards.  My Canon 700d was connected directly to the scope, no filters, no coma corrector (obviously!) so not sure where this red reflection is coming from and does its slightly offset position suggest that we haven't quite nailed Collimation?

The slightly more worrying aspect is the view of a small star cluster (single 20 sec image at ISO800):

In case it's not clear, here are full size crops from the corners:

top left

bottom left

bottom right

top right

centre

This looks like quite bad coma to me.

I ran the image through Maxpilote to get a sense of where the error might lie and the results suggest that while the centre of the field looks OK,  the right and left edges and corners are a different matter, suffering badly from curvature, lack of focus and tilt.

Not sure how much faith to put in these figures but is this what one should expect from a Mak-Newt?  One of the claims made for this model on the Optical Vision Website is: 

" Stars appear as classic pinpoints across the entire field. Perfectly optimised for modern DSLR cameras. " 

Clearly this isn't the case here, but could this be operator error causing camera tilt?

I'd really welcome fellow SGLr's views.

If anyone would care to run the single image through alternative software, there are .fits, .cr2 and .tiff versions here:

https://www.dropbox.com/sh/eq2wub9q49rdpv8/AABnoazj6OQALRWRkq8SEa2ya?dl=0

 

Edited August 1, 2020 by almcl

[tomato]

I’ve seen images taken with a MN 190 and a KAF 8300 sensor  and the star shapes are fine into the corners. A full frame DSLR is, of course a more demanding test, but not by that much and, as you point out, should not be a problem for this scope according to the sales pitch.
 

The aberrations do look to be radially orientated from the centre so I agree it looks more like coma rather than sensor tilt or a sagging focuser.
 

I’m certainly no corrected reflector expert but is the distance from the corrector plate to prime focus critical on these scopes? Is it possible this distance has been altered during collimation?

[almcl]

Thanks for the thoughts.

I believe that the corrector to main mirror distance is important but it seems pretty fixed; the corrector is nailed pretty solidly to one end of the OTA and the mirror cell is fixed at the other end.  

I have had another go at collimation, although I can't believe it would make so much difference.

My APS C sized sensor gave this result for curve/tilt:

 

but this for FWHM:

 the scores were 61.4 for curvature, 37.0 for tilt and 8.65% for collimation.  Not sure what the first two figures mean though.

[tomato]

If the secondary mirror was moved up or down the tube during collimation this would affect the distance of the corrector from the point of prime focus, but again I’m not sure how sensitive the optics are to this dimension with regard to how effective the corrector plate is in fixing coma.

[almcl]

I see your point, but like you, I am not sure if the distance at which the secondary (which is flat) intercepts the light cone is significant?

Anyway, with a slightly different collimation and a change of connector to the focuser, last night's 'quick 'n dirty' (pre-nautical dark) shots were a lot sharper and the field a lot flatter:

  

 

What I should have done, of course, was to rotate the camera through 90° and compare the results.  I may be blaming the scope for something that is camera caused.  Next clear night, I'll give that a go.