6 replies to this topic

[odddiv]

Setting up my first newt, a CarbonStar 150 w/cc. I think I've got the collimation close using a 3d printed 2in collimation cap and a astronomania laser that when chucked into my lathe has a fixed spot at 15'.

 

 

I'm having some vignetting problems even with a small IMX585 sensor and I'm trying to isolate the root cause.

[kgb]

Collimation looks good/very close. Does the laser spot move at all when racking the focuser in/out?

[Vic Menard]

Setting up my first newt, a CarbonStar 150 w/cc. I think I've got the collimation close

I'm having some vignetting problems even with a small IMX585 sensor and I'm trying to isolate the root cause.

The secondary mirror looks pretty close (green circle is optimal). With a small sensor, I don't think the small secondary mirror placement error would be the cause (unless the vignetting is also small or the intercept distance is pretty long). A little bit either way can cause illumination issues with these small aperture fast focal ratio imaging Newtonians.

 

How far is the native focal plane from the center of the secondary mirror?

Does the primary mirror have an edge mask (no mirror clips)? If so, what is the working aperture?

When the imaging system is focused, how far is the CC field lens from the center of the secondary mirror (and what is the CC field lens diameter)?

Is the clear aperture of the secondary mirror 62mm?

 

The High Point Scientific CarbonStar 150 illumination profile for a larger (IMX533) sensor seems to indicate ~100-percent illumination across a 16mm diameter field (without the 0.95X reducing CC). From what I could find online, your sensor is ~13mm. If you can provide the parameters I requested above, I'd like to model the system using the Bartels calculator.

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[odddiv]

The secondary mirror looks pretty close (green circle is optimal). With a small sensor, I don't think the small secondary mirror placement error would be the cause (unless the vignetting is also small or the intercept distance is pretty long). A little bit either way can cause illumination issues with these small aperture fast focal ratio imaging Newtonians.

 

How far is the native focal plane from the center of the secondary mirror?

Does the primary mirror have an edge mask (no mirror clips)? If so, what is the working aperture?

When the imaging system is focused, how far is the CC field lens from the center of the secondary mirror (and what is the CC field lens diameter)?

Is the clear aperture of the secondary mirror 62mm?

 

The High Point Scientific CarbonStar 150 illumination profile for a larger (IMX533) sensor seems to indicate ~100-percent illumination across a 16mm diameter field (without the 0.95X reducing CC). From what I could find online, your sensor is ~13mm. If you can provide the parameters I requested above, I'd like to model the system using the Bartels calculator.

 

I'm a newb to this,but I'll do my best to answer accurately. Without tearing down the telescope and acquiring larger metrology tools than I have on hand I'm limited to tape measures and eyeballs. My eyeballs lost their factory calibration certification in my early 40s.  

 

The measurement from the center of the secondary to the camera side face of the Apertura CC when focused is ~185mm, and the distance from that face to the field lens is ~72mm, which would make the distance from the center of the secondary to the field lens ~113mm. The CC is 2" diameter - it's the Apertura .95 PRCC and they list it at 50.75mm. I have the required backfocus from the PRCC of 55mm to the camera lens (OAG, EFW, camera).

 

A tape measure measurement from the surface of the primary to the center of the focus tube is ~355mm. (okay, I didn't touch the tape to the mirror, I measured from the mirror mask to the visual "center" of the focus tube and added to thickness of the mask, as it's a known value (3mm)).  

I do have a mirror mask - actual exposed primary mirror diameter is 145mm.

 

The secondary does appear to be ~62mm of reflective surface.

 

This is a 10 min integration with an ASTF applied to show the vignetting.

 

And this is the background extracted via graxpert to show it more clearly

 

This same base optical train assembly (the OAG, EFW, and camera) were pulled from a small 62mm refractor setup as a unit and moved over to the newt. I've moved it back and forth a couple of times and confirmed there is no vignetting or aberrations in the smaller scope. 

[Vic Menard]

...This is a 10 min integration with an ASTF applied to show the vignetting.

And this is the background extracted via graxpert to show it more clearly

Do the illumination profiles show the full sensor?

If you rotate the camera 90-degrees, does the vignetting stay on the top edge?

If you rotate the base optical train assy 90-degrees, does the vignetting stay on the top edge?

[odddiv]

Do the illumination profiles show the full sensor?

If you rotate the camera 90-degrees, does the vignetting stay on the top edge?

If you rotate the base optical train assy 90-degrees, does the vignetting stay on the top edge?

I'm not sure what you mean by illumination profile?

 

I can't easily rotate the base optical train assy by 90 - it will contact the mount. I did rotate it 45 degrees and the vignetting shifted.

 

 

rotating the camera individually requires complete disassembly of the OAG and EFW, which i'll do if I have to - it's just a pain. 

[Vic Menard]

Well, 45-dregrees shows the profile has rotated from top side to top/right (and a little bottom?).

I wonder if the contributing factor is the 62mm secondary mirror (43-percent obstruction). I just can't understand why it isn't impacting the center--or is it? Maybe we need to see the vignetting profile with a bigger sensor? 

I also find it interesting that your OAG/EFW/camera were fitted to a 62mm refractor with no vignetting...

 

Perhaps some other CarbonStar 150 users will add their thoughts...