View Full Version here: : William Optics GT81 Flattener

20-10-2017, 05:59 PM
Hi All
I am looking for advise from the astro community. I am a newbie looking at the William Optics GT81 telescope as an upgrade from my ED80. I cant help but notice that there are no straight flattener's for this scope. Willaim Optics sells you a flattener / 0.8 reducer. Why would they sell a reducer with this scope and not just a flattener. I am worried about loosing focal length on an already short OTA. can anyone recommend a straight flattener , can i re-use my orion field flattener from the ED80 or are there good reasons why William Optics only sell a 0.8 reducer.

Kind Regards

21-10-2017, 07:41 AM
Hi Orion,
I can't tell you why they don't sell a 1 x field flattener but your Orion non-reducing flattener should work fine. They are designed for telescopes from 400-660mm. The GT81 is 480mm.

I have to say, the Orion ED80 is a nice instrument, and William Optics make nice glass too, I own a William Optics M70 and an ED80. I just don't know if, aesthetics aside, it is much of an upgrade going from the ED80 to the GT81 unless you got a bad example of the ED80. Aperture will be the same, focal ratio is a bit faster, the William Optics is probably better optical quality but not a huge leap in quality. Maybe you should think about something else for an OTA or if you buy a 0.8x reducer for the ED80, it will give you the same aperture, focal ratio and focal length as the GT 81 for a fraction of the upgrade price.

Just a thought!

Good luck whatever you decide.


23-10-2017, 08:56 PM
Hi Joe

Thank you for taking the time to reply, i really appreciate it. I will give the flattener a go with the GT81, as you mentioned it should work as its within spec.

24-10-2017, 01:37 PM
I have a GTF-81. It has a built in field flattener. So I suppose different to what you are looking at?

Email William Optics and ask - I found them to be really good at responding to such queries.

24-10-2017, 01:46 PM
Hi Chris

They dont sell the GTF-81 any more else i would have bought one....the earlier WO scopes focuser werent up to scratch, i belive in your model and later they fixed these issues. Are you happy with the GTF-81, is the focuser ok?, nice and tigh with a camera on it?

Kind Regards

24-10-2017, 01:56 PM
Mine is fine but I'm no expert - its the first refractor I've had. Bought it here second hand a while ago and am unlikely to ever sell it.

I have an ASI071 (500g) hanging off it on a 50mm extension tube. I have the focus tightened and it seems OK - no wobble or slip. But I've only used it for live imaging so far, not longer astrophotography shots.