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Old 25-02-2019, 12:47 PM
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Lognic04 (Logan)
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Unhappy Tak MT160 reducer doesn't correct for coma?

Hi all,
i have tried to nail the spacing with my tak MT160 and 0.8 dedicated reducer, and have gone through the entire range around 55mm, only to discover that, this thing doesn't even correct coma!
Is there something wrong with my reducer? VERY annoying. I'd rather not have to buy a dedicated CC that doesnt even reduce. CC's worth more than the scope are out of the question
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Old 25-02-2019, 01:42 PM
LewisM
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Ill say it again - where are you measuring from and to?

It’s from the rear non-threaded surface of the reducer to the sensor plane of the camera. Any other BFD will be wrong
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Old 25-02-2019, 01:54 PM
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Yes exactly. I went thru 50 to 60mm, nowhere within that range corrects properly.
heres a screen snip:
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Old 25-02-2019, 02:28 PM
LewisM
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I could be very wrong, but that looks more like tilt/sag.

Also, what sensor size are you using, as these early reducers only corrected for film/35mm.
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Old 25-02-2019, 02:50 PM
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Micro 4/3. unacceptable really for a tiny sensor. Also heaps of vingetting. Totally dumb from an expensive telescope, takahashi noneteless.
The pixinsight graphs disagree, a clean circle of FWHM change.
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Old 25-02-2019, 06:14 PM
LewisM
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Logan,

Take me a photo with the camera attachedd to the entire rig so I can check some things.

If you still have it set up ex-Ellis Jury-Rigged r Us, you will have issues... (as per your photos at https://drive.google.com/drive/folde...2dSZWxTYlBmaU0)
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Old 25-02-2019, 06:45 PM
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Hi Lewis,
Nope, fully threaded now.
https://drive.google.com/folderview?...WTssPGP9Y44sHi
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Old 25-02-2019, 08:29 PM
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That's WAY too long Logan.

In front of the filterwheel I see at least 31mm - the 20mm spacer and what looks like an 11mm spacer. The FW width is looking like at least another 20, so there is 51mm, leaving 5mm...

Distance from the front of the cam to the sensor for the ASI1600 is 4mm for the front ring plus 6.5 from the larger base to the sensor plane, making 10.5mm.

So, by my eyeballing, you are 5.5mm over-long.

I could be wrong, but I do believe this is the case.

EDIT: ok, the 4mmon the face is internally threaded to the FW, so maybe 1.5mm long. Send it here to measure

Last edited by LewisM; 25-02-2019 at 08:41 PM.
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Old 25-02-2019, 08:43 PM
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I suspect your vignette is from "throttluing" the vlight cone too early after the reducer - thats a large adaptor ring, stepping from IIRC 56mm to 42mm. you should be wide at the reducer exit as per the Tak diagrammes.

Heck, with the FSQ, I don't step down from 92mm exit to M42 until the camera face, and that's with a 70mm extension (all 92mm wide until the interface with the camera - overkill, but no vignette either)
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Old 25-02-2019, 08:53 PM
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This guy says MINOR coma (with the MT-200 reducer/corrector) in his MT-160: https://www.astrobin.com/383169/?nc=all
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Old 25-02-2019, 09:00 PM
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Lastly , the Tak system chart has the reducer as part 18.

http://www.takahashijapan.com/ct-pro...MT160_main.pdf
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Old 25-02-2019, 09:23 PM
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Thanks lewis. ATM i;m within about 1mm of 55mm.
I'd say that "minor coma" is just as bad as my pic. Zoom in. Its pretty bad!
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Old 26-02-2019, 11:11 AM
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Keep persevering. A modern coma corrector like the Baader MPCC etc aren't very expensive really if you need to go that route.

Or sell it and buy a used imaging refractor like an Esprit 80.
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