ED80 Focal Reducer

multiweb (Marc) · #1 18-09-2009, 02:40 PM

Hi guys, I'm trying to delve in the new world of widefield so I'm starting to ask stupid questions - bear with me.

My ED80 is F/7.5 650mm FL. Lovely little scope. Let's say I want to use it as a widefield lens. Can I get it down as far as F/3? Is there a focal reducer that would do this?

Am I likely to get real bad field going down to this kind of F Ratio even if I stop it? What do you guys suggest? Going the other way by getting a DSLR lens? The good ones arent' cheap though. Just testing the waters to see if I can get away with what I already own. The ED80 isn't a bad scope at all. As usual thanks for any pointers.

Octane (Humayun) · #2 18-09-2009, 03:39 PM

Marc,

I'm not sure of what reducer you'd need to use, or if anyone makes what you're after, but, I can imagine the extreme amounts of vignetting that will be present. Your flat fields would have to be so critical. Might just be easier to invest in a Canon 200mm f/2.8L II, or Canon 300mm f/4L or an equivalent Nikkor 300mm f/4 ED, and then get a relevant adapter.

Regards,
Humayun

TrevorW · #3 18-09-2009, 05:16 PM

I use a WO Flat 2 with my ED80 works a treat

Cheers

Octane (Humayun) · #4 18-09-2009, 06:00 PM

Trevor,

Marc was asking how to get his scope down to F/3, not f/6.

Regards,
Humayun

multiweb (Marc) · #5 18-09-2009, 06:03 PM

Had a look at this NextGEN WideField 0.50X . That would work for my C11 too apparently to get it down to F/5? So in theory I could get the ED80 to F/3.7? I think Mike Sidonio has one. Maybe he can chime in?

DavidU (Dave) · #6 18-09-2009, 06:10 PM

Marc, have a fiddle around with this
http://timosastro.1g.fi/tools/focalreducer.html

DavidU (Dave) · #7 18-09-2009, 06:16 PM

A Meade .33x 40mm reducer gives f2.68

multiweb (Marc) · #8 18-09-2009, 06:17 PM

Quote:

Originally Posted by DavidU

Marc, have a fiddle around with this
http://timosastro.1g.fi/tools/focalreducer.html

Wow!

Very cool! Thanks David

DavidU (Dave) · #9 18-09-2009, 06:19 PM

see my 2nd post, a usefull CCD frame of 17.5mm may be ok

RB (Andrew) · #10 18-09-2009, 06:43 PM

Agreed, don't waste your time/money trying to get it down to f/3 to image with a DSLR Marc.
It will vignette horribly.
Get a decent lens instead for wide field shots.

I'd only push the ED80 down around f/6.

What lenses/DSLR do you have?

multiweb (Marc) · #11 18-09-2009, 06:54 PM

Quote:

Originally Posted by RB

Agreed, don't waste your time/money trying to get it down to f/3 to image with a DSLR Marc.
It will vignette horribly.
Get a decent lens instead for wide field shots.

I'd only push the ED80 down around f/6.

What lenses/DSLR do you have?

I have a Pentax FA-J 18-35mm - AL (67mm diameter)

TrevorW · #12 18-09-2009, 09:08 PM

The best I've heard is a .5 or .6

check this

http://webcaddy.com.au/astro/DSLR_Focal_Reducer.htm

Robbie · #13 23-09-2009, 02:54 PM

Wasting your time just piggy back the dslr on top of your ota, if you have a few different camera lenses use that for your wide fields.
I have used 0.5 reducers on my ed80 without great success.

I find the use of any focal reduction comes at a cost somewhere along the line with vignetting etc.
Just my humble uneducated opinion.

multiweb (Marc) · #14 23-09-2009, 06:19 PM

Thanks for all the info guys. Food for thoughts.

mldee (Mike) · #15 23-09-2009, 06:40 PM

Quote:

Originally Posted by multiweb

Thanks for all the info guys. Food for thoughts.

More food? You can get good, older and non-auto, fast Pentax, Canon, etc lenses reasonably cheaply on Ebay. Get an adaptor from Steve Mogg or Ebay to use them with your QHY8. Then you have a fast, 6Mp cooled wide field RGB piggy backer.