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Paracorr + SonyA7III on f3.5 Dobsonian

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#1 PeterWar

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Posted 29 February 2024 - 06:48 PM

53559427147_071962fb42_z.jpg

These are the results of just one 15 second ISO12800 exposure on my DocTelescope LaserMAX III 20" f3.5 using the Paracorr type 2 and an unmodifyied Sony A7III camera. I coupled the camera on a Celestron Sony E-Mount 42mm T-Ring and used the Paracorr tunable top. I could not dial-in any setting that would remove the coma, as aparent in the photo. However I find this preliminary results very promising for the following reasons:

 

-As explained in this great video optimal SNR exposure times on modern CMOS RGB photography (not narrowband) are really short for suburban and rural skies, requiring no equatorial base. At F4 with my other Sony A7S camera I can do optimal multi 10 second exposures integrations while keeping noise below 5%. The only drawback is more computing time really.

 

-While coma makes this difficult to assess, when looking in the center of the field, I think I might have demostrated that Servocat tracking in Alt-AZ at 2000mm focal lenght with my DocTelescope allow for 15 second exposure times without significant field rotation, as stars seem to look relatively round. I could have aimed for 20-30 second maybe but aligment wasn't perfect nor was seeing.

 

So need help in finding out the best way forward, do Paracorr type 2 adapters for Sony E mounted cameras exist? or should I buy a Baader Mark III coma corrector? Any other options to eliminate coma and reduce vignetting would be appreciated.


Edited by PeterWar, 29 February 2024 - 06:49 PM.


#2 maadscientist

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Posted 29 February 2024 - 08:19 PM

The spacing from the chip in the camera to the paracorr should be around 55mm +or- 4mm. The chip to the bayonet is 17.5 if I remember correctly, then you add a few mm for the Tring. So you will have to order a spacer(s) to make up the difference, approx 35mm.

 

Dan



#3 PeterWar

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Posted 03 March 2024 - 06:51 AM

Strange because I had the tunable top mounted and I dialed every setting and did not find optimal correction… setting A or B should have worked isn’t it?

#4 PeterWar

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Posted 03 March 2024 - 12:32 PM

EDIT: Nevermind that, the tunable top is 56mm in lenght so I'm past the optimal distance. I'll probably order a set of 6 accesory tubes from televue (TLS-2245).




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