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Old 19-07-2015, 09:26 AM
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ThatGuy (Guy)
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Field flattener and focus issue

Hi all,

I recently made the jump to AP and I've hit a small problem achieving focus.

My set up is Megrez 90 > WO pflat4 feild flattener > T ring > canon 550d

My problem is that when i wind the focuser all the way in I am still not quite in focus.

Anyone know why I can't achieve focus? I'm not sure if it's the flattener (WO states that it works well with the Megrez 90) or if I just need an extender or reducer or something.

I should also mention that i can happily achieve focus with a diagonal and eyepiece.

imgur.com/a/DGTDZ

Any help would be great, thanks!

Guy
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Old 19-07-2015, 01:19 PM
jase (Jason)
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Are you hitting the right back focus requirements for the WO field flattener with your DSLR? WO recommend the following from their web site.
FLT 98 - 76.8 mm
FLT 110 - 73.5 mm
FLT 132 - 71.5 mm
GT 102 - 76 mm
M90 - 77.3mm

So you need to ensure that your Megrez 90 has a space of 77.3mm between the rear element of the flattner and the CMOS chip of the camera for the corrector to work optimally. This in turn will alter the imaging train's focal point. It sounds like you need more space if you are racking the focuser all the way in.

http://www.williamoptics.com/accesso...BFL122~142.pdf
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Old 19-07-2015, 04:59 PM
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ThatGuy (Guy)
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Thanks for the help so far, unfortunately still haven't worked it out.

So distance that is optimal for the Megrez 90 is 77.3mm.

Correct me if I am wrong here, but with the WO field flattener I should be able to adjust it to ~77.3mm?

http://www.williamoptics.com/accesso...AT4_Layout.jpg

At present even with the adjustment set to 66mm of back focus I still can't reach focus racked all the way in.

If I'm looking at trees about 70m away it is just out of focus.

If I'm looking at my fence 10m away i can achieve focus.

Could it be that my Megrez 90 is an older version (and the pflat 4 just won't work?

It certainly feels like the chip needs to be closer, not further away... but I'm probably wrong.
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Old 19-07-2015, 05:04 PM
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ThatGuy (Guy)
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Also, I found this archived thread that discusses the problem.

http://www.iceinspace.com.au/forum/a...p/t-72680.html

From the thread it sounds like the older scope may not be compatible...
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Old 20-07-2015, 04:50 PM
HarryD (Greg)
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The distances given by William Optics for their reducer/corrector did not work with my FLT 132. The reducer to camera distance as given was too long for my set up. The adjustment on the reducer gives round stars at much less than 72 for me. No focus problems though.

In my experience the reducer/corrector works but prepare yourself with lots of experimenting with different length T spacers.

I wish it would be clear in Sydney.
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Old 21-07-2015, 09:23 AM
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ThatGuy (Guy)
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Update: Problem solved.

After a lot of research... The original Megrez 90's (I think the ones that are f6.9) aren't compatible with the PFLAT4

Hotech SCA FF works with the scope with a little adjustment.

I've also heard that the TV TRF-2008 will work as well.

Thanks to all that helped me out with this one.
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