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Old 25-02-2021, 11:53 AM
Startrek (Martin)
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Baader MPCC Mk3 Coma Corrector Spacing

I’ve been imaging with a Baader Mk3 coma corrector in my newts for years now and it was only recently brought to my attention by a fellow member that my images have some coma affected stars in the corners. I upgraded last August from a Canon 600D DSLR after using it for 4 years to a ZWOASI2600MC cooled OSC
I looked at a few of my old images captured with the DSLR and using the Baader Mk3 coma corrector ( same scopes as I have now ) and the Star field to the corners were pretty much coma free.
Now using my 2600MC I have some coma affected stars mainly towards the corners of field.
Comparing camera specs

Canon 600D
Sensor size 22.3mm x 14.9mm APS-C
Pixel size 4.3uM
Adapters Canon T ring adapter only
Distance from sensor face to Baader Mk3 CC. Approx 50mm to 51mm

ZWOASI2600MC
Sensor size 23.5mm x 15.7mm APS-C
Pixel size 3.75uM
Adapters 1 x ZWO M42M-M48F (16.5mm ) 1 x ZWO M42M-M42F (21mm )
Distance from sensor face to Baader Mk3 CC Approx 55mm to 56mm

NB: Baader Mk3 coma corrector specs advise a spacing from sensor to coma corrector of 55mm +- 1mm

According to the above when reviewing both old DSLR images and my latest images using the ZWOASI2600MC , I should move my Baader Mk3 CC closer to the sensor by around 5mm.
I would have thought bringing it closer would introduce curvature or something like that.
Before I start any trial and error testing , I would appreciate some advice from more experienced imagers if bringing the coma corrector closer to the sensor may solve my coma issues
NB: I use a 6” f6 newt and 8”f5 newt

Thanks in advance
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Old 25-02-2021, 01:37 PM
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multiweb (Marc)
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Specs say 55mm +/-1mm. I had it at 58mm on my newt 5" f/6. Years ago I got peter tan to make me a helicoidal spacer. to tweak spacing. I have attached the picture I provided for him to build it. It was much easier to test and get the perfect distance. It's all M42 threads.
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Old 25-02-2021, 02:56 PM
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Karlz (Karl)
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Your Canon would've had 44mm from flange to sensor, so with T-mount added would give 55mm not the 50mm you have written. Only thing I can think of is that your sensor is fractionally larger that it's showing the outer limits now.
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Old 25-02-2021, 03:31 PM
Startrek (Martin)
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Quote:
Originally Posted by Karlz View Post
Your Canon would've had 44mm from flange to sensor, so with T-mount added would give 55mm not the 50mm you have written. Only thing I can think of is that your sensor is fractionally larger that it's showing the outer limits now.
I used a thin feeler gauge blade to measure from the sensor face to the outside edge of the Canon t ring adapter. The shutter was open so I could get past that and down to the face of the sensor ( being careful not to touch the sensor itself)
It measured 50mm ( checked it twice) with the Canon at ring adapter locked in.
Unless there is another glass plate over the sensor ( which I couldn’t see using a torch ) but even if there was it would only be another 1 or 2mm
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Old 25-02-2021, 04:36 PM
RyanJones
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Quote:
Originally Posted by Startrek View Post
I used a thin feeler gauge blade to measure from the sensor face to the outside edge of the Canon t ring adapter. The shutter was open so I could get past that and down to the face of the sensor ( being careful not to touch the sensor itself)
It measured 50mm ( checked it twice) with the Canon at ring adapter locked in.
Unless there is another glass plate over the sensor ( which I couldn’t see using a torch ) but even if there was it would only be another 1 or 2mm
Hi Martin,

If I remember rightly your DSLR wasn’t modified so when you measured down you’ll be hitting the I\R filter which is likely about 5mm from the sensor it’s self.
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Old 25-02-2021, 04:37 PM
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There's two filters ( glass plates ) under that black screen which will give you that extra, here's a snippet for flange to sensor distance. I modified my camera last week, it was quite interesting having the camera completely apart.
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Old 25-02-2021, 04:43 PM
RyanJones
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I was out testing last night and this image LOOKED pretty good to me. I’ve taken a few different shots with different spacers. This is one with a distance of 58.5mm. I’ve also taken some at 59.5 but I think that is too much. Camera is a ZWO 071mc APSc sensor on a 200mm F/4 Newtonian.

Note : All measurements were taken outside the camera plus the 17.5mm claimed by ZWO. I haven’t verified that measurement.

Cheers

Ryan
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Old 25-02-2021, 05:43 PM
Startrek (Martin)
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Karl
Thanks
Forgot about the IR cut filter
So yep 55mm

Ryan
Looks like you have nailed it ? I wouldn’t fiddle any more
I’ve ordered a M42 space adapter kit from Bintel today so Ive got some testing to do to eliminate the little bit of coma left in the corners

Thanks all for your prompt replies
Cheers
Martin
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Old 25-02-2021, 06:35 PM
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Peter Ward
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No surprises there.

55mm is the standard for spacing of T-Thread (M42) adapters.

Many SLR camera T-adapters were made to conform to that standard, the which was also adopted by Baader.
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Old 25-02-2021, 09:22 PM
RyanJones
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So I downloaded CCD inspector to see what it said. I must say I’m pretty happy with it. Not only the field flatness but also my collimation says it’s pretty close to spot on and my tilt is negligible. Looks like all the modifications and work was worth it. I have no doubt there are improvements that could be made but from what I’ve read, seen and experienced with working with an f/4 newt I must say I’m pretty happy.

Cheers

Ryan
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Old 25-02-2021, 11:37 PM
Startrek (Martin)
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Ryan
An f4 newt not an easy beast to tame
Well done !!

Martin
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