Tilt and curvature on my new 183 MC PRO astro camera

[Astro_mark_c]

Hey,

My current scope / camera setup is the WO Redcat V1.5 and the 183 MC Pro cooled camera.  When I used it for the first time on the 29th of September I noticed that my stars near the edges didn't look right.  I've measures the distance and i have 55mm back spacing, but with a filter drawer and filter inside.  

I got someone on one of my regular fb groups to run one of my raws through a program that looks for tilt etc and the results are show below.  seems i have 1% tilt and 16% curvature.  How do i go about fixing these?  I've never heard of tilt and i expected my field of view to be flat as that was the whole point in buying the recdcat. 

Any help would be appreciated.

[symmetal]

You asked a week or so ago what tilt was here and I thought I gave a reasonable answer. 🤔

Your two CCDI images are looking pretty good. I've never worked out what the precentage curvature figure actually refers to as a reference but all the figures are related to the FWHM of the stars in the image. Stars towards the edge of the frame will tend to be worse than those in the centre. You need to spend many thousands for a scope where the edge stars appear as good as in the centre. This change in FWHM across the image is expressed as a curvature figure. The max and min values tell you the actual range of FWHM and yours are good in that respect.

The 3D  (bottom) image displays this 'curvature' by plotting the FWHM value in the Z direction, (up/down) so worsening FWHM makes the Z plot more positive hence the 'bending' up of the plot corners. You are aiming for the 3D plot to be as flat as possible, with all 4 corners, if they are rising to rise uniformly, and again what you have is good.

The image is showing very slight tilt top to bottom as the best focus stars are above the centre, with the bottom corners the worst, but you'll find a refocus and another test image may show it in a different position. You need the average of several images to judge whether it really is a tilt issue, though as mentioned it's so small it's really not worth worrying about.

The Redcat will often exhibit some coma towards one or more edges, being a Petzval design. Some early versions had terrible coma all over and had to be returned for lens re-alignment. Coma will smear the star toward the edge, although the brightest star area will often still be fairly sharp, and so show a reasonable FWHM figure. If you have coma, there is really nothing you can do yourself to fix it, on a refractor.

CCDI seems to pretty much ignore coma in its assessments and only use FWHM, which is best for showing focus variations. Variations in focus across the image are caused by a combination of sensor tilt, and lens aberrations creating 'curvature' issues.

The spacing distance of a separate Field Flattener will affect the 'curvature' but as the Redcat is a Petzval, the image field of focus should be flat by design and no flattener is required.

Alan 

[david_taurus83]

I agree with Alan. That doesn't look too bad at all. CCDI should only be used as a guide really. Do you have an image you can post up showing your concerns? The 183 isn't a big sensor so it should be pretty good for star shapes on the Redcat.

[Astro_mark_c]

On 14/11/2021 at 01:52, symmetal said:

You asked a week or so ago what tilt was here and I thought I gave a reasonable answer. 🤔

Your two CCDI images are looking pretty good. I've never worked out what the precentage curvature figure actually refers to as a reference but all the figures are related to the FWHM of the stars in the image. Stars towards the edge of the frame will tend to be worse than those in the centre. You need to spend many thousands for a scope where the edge stars appear as good as in the centre. This change in FWHM across the image is expressed as a curvature figure. The max and min values tell you the actual range of FWHM and yours are good in that respect.

The 3D  (bottom) image displays this 'curvature' by plotting the FWHM value in the Z direction, (up/down) so worsening FWHM makes the Z plot more positive hence the 'bending' up of the plot corners. You are aiming for the 3D plot to be as flat as possible, with all 4 corners, if they are rising to rise uniformly, and again what you have is good.

The image is showing very slight tilt top to bottom as the best focus stars are above the centre, with the bottom corners the worst, but you'll find a refocus and another test image may show it in a different position. You need the average of several images to judge whether it really is a tilt issue, though as mentioned it's so small it's really not worth worrying about.

The Redcat will often exhibit some coma towards one or more edges, being a Petzval design. Some early versions had terrible coma all over and had to be returned for lens re-alignment. Coma will smear the star toward the edge, although the brightest star area will often still be fairly sharp, and so show a reasonable FWHM figure. If you have coma, there is really nothing you can do yourself to fix it, on a refractor.

CCDI seems to pretty much ignore coma in its assessments and only use FWHM, which is best for showing focus variations. Variations in focus across the image are caused by a combination of sensor tilt, and lens aberrations creating 'curvature' issues.

The spacing distance of a separate Field Flattener will affect the 'curvature' but as the Redcat is a Petzval, the image field of focus should be flat by design and no flattener is required.

Alan 

Missed it...either way thanks for the detailed reply. 

[newbie alert]

Any chance that you're using delrin spacers? 

I had what looked like tilt on my setup and on investigating one of my delrin spacers was thinner on one side that the other.. replaced with baader shims and the difference was quite amazing... so the delrins went in the bin 

[Astro_mark_c]

On 20/11/2021 at 15:52, newbie alert said:

Any chance that you're using delrin spacers? 

I had what looked like tilt on my setup and on investigating one of my delrin spacers was thinner on one side that the other.. replaced with baader shims and the difference was quite amazing... so the delrins went in the bin 

Just used what Came with the camera. I’ll take a photo and post it.