Free Shipping on ALL Products
camera reviewsbest cameraslens reviewsphotography tipscamera forumvideo toursphotography bookssupport me
It is currently Wed Jul 23, 2014 6:52 pm

All times are UTC




Post new topic Reply to topic  [ 15 posts ] 
Author Message
 Post subject: Using focal reducer
PostPosted: Sun Jan 03, 2010 1:35 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
I'm probably doing something really stupid, but I finally got a chance to fit the focal reducer, and I'm not sure exactly how best to use it.

Test setup:

Scope: Maksutov-Cassegrain with erecting diagonal on its rear port (no mirror).

To that I have the 50D, t-ring and t-mount adapters.

I have a generic focal reducer branded Opticstar, nominal 0.5x, marked as such on the part with lens. It also comes with an extension tube which is unmarked. The bundle is sold as 0.5x and 0.3x.

Image
The view with the above kit without focal reducer.

Image
This is what I get if I screw the focal reducer only into the end of the t-mount adapter. Note the diagonal has a bit of a stop inside it, so I can't insert the whole lot in as far as without.

Image
This is what I get if I screw the extension tube between the focal reducer and t-mount adapter.

What confuses me is that 2 and 3 seem... wrong? Without measuring, #3 looks about 0.5x, and #2 looks maybe 0.25x.

Should a focal reducer invert the image? Or is my spacing totally wrong? The contrast seems significantly reduced too.

I tried removing the barrel from the t-mount adapter and that didn't give anything usable. All I saw there was a donut of light regardless of focus.

I think I'll give it another try with the scope's side port (with mirror) if I can make the space.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Sun Jan 03, 2010 2:10 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
As before, using side port.

Image
Camera directly to scope.

I tried putting on the reducer but the scope didn't have enough focus adjustment range to get focus.

Image
This is with reducer and extension tube. Much the same size as before, although it suggests the mirror might not be in ideal alignment.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Mon Jan 04, 2010 7:36 pm 
Is it this one?
http://www.opticstar.com/Run/Astronomy/ ... 10_5_0_3_9

If so, that seems to be designed with visual astronomy in mind. :(


Top
  
 
 Post subject:
PostPosted: Mon Jan 04, 2010 7:45 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
Yes, it is that set. But they do say:

Quote:
Exposure Times
When attached to a telescope the f0.5 focal reducer will typically reduce exposure times to near a quarter when imaging extended objects like galaxies, nebulae and the Moon.


I'm just wondering if I'm missing something obvious. From theory elsewhere, I understand that focal reducers only work at their magnification factor when placed at a set distance from the focal plane. At the moment I suspect I'm not at that distance, but I don't know what it would be.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Tue Jan 05, 2010 1:01 am 
They may mean CCD imaging, with a DSLR sized sensor and a mirror box this might be the result. I don't see how a 1.25" lens could cover such a large sensor. This is way outside my knowledge of optics though...

I've tried to find some kind of focal reducer/corrector for the 4SE but can't find any. Bah....


Top
  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 4:15 pm 
Offline

Joined: Thu Jul 31, 2008 4:31 am
Posts: 296
Did you use a 2inch tube spacer after the focal reducer ? I have use my 0.63 focal reducer on my C8 plenty of time and never experience any of theses issue. the FR does 2 thing : Reduce Focal Lenght and flattend the field a little. I think you might experience coma on the edge of the image 0.5 FR.

This is how am I setup.

Focal Reducer 0.63-- > T-Adapter -- > T-ring -- > Canon

_________________
Canon T2i (550D)- Tamron 17-50 F2.8 Non-VC, Sigma 70-200mm F2.8 EX DG OS HSM, Canon EF-S 55-250mm F4.5-F5.6
http://www.astronomyphotography.com


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 5:24 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
I tried with and without the extension tube as above.

I'm guessing the FR on C8 is the official matched one? Here I'm using a generic. My guess is still that the reducer is at the "wrong" distance from the sensor so it is not giving the expected effect, but I've been too lazy to investigate further.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 5:35 pm 
Offline

Joined: Thu Jul 31, 2008 4:31 am
Posts: 296
popo wrote:
I tried with and without the extension tube as above.

I'm guessing the FR on C8 is the official matched one? Here I'm using a generic. My guess is still that the reducer is at the "wrong" distance from the sensor so it is not giving the expected effect, but I've been too lazy to investigate further.


I use an Antares FR 6.3. I always believe that FR 5.0 are too aggressive. The picture is my C8 is always inverted without or without the FR. You have a Mak right ? you may want to make sure that your FR is compatible with Mak.

_________________
Canon T2i (550D)- Tamron 17-50 F2.8 Non-VC, Sigma 70-200mm F2.8 EX DG OS HSM, Canon EF-S 55-250mm F4.5-F5.6
http://www.astronomyphotography.com


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 5:46 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
I wouldn't think the various Cassegrain scopes would be that different if you're introducing a modifying element relatively close to the focal point.

I need to do the calculation at the bottom of here to see where my reducer should go, and then see if that is where it is. Or conversely, what its impact should be where it does sit.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 5:47 pm 
Ohh, it isn't a flattener. That's the reason for the distortion then I guess...

After doing some searching on the 'net, I found that the distance from corner to corner on an APS-C sized sensor is about 30.1mm, while your telescope has a baffle tube that's 1.25" (31.75mm) across - This means the sensor just avoids being clipped. With a focal reducer, that size goes down as it 'compresses' the light cone leaving you with an image plane that's too small for the sensor. Now the clipping finally makes sense to me...

With that said, the results I'm seeing in the images you posted make sense. Annoyingly, this means that you'll end up with clipping no matter what focal reducer you use. Only option is to get a scope with a bigger baffle tube. :(


Top
  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 5:56 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
Ooh, that kinda makes sense. Maybe I was mistakenly expecting the image circle to be bigger for some reason - as I made the unfounded assumption it'll be ok for full frame.

If that's the case, that only makes me want to get a bigger scope :D

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 6:11 pm 
Offline

Joined: Thu Jul 31, 2008 4:31 am
Posts: 296
popo wrote:
Ooh, that kinda makes sense. Maybe I was mistakenly expecting the image circle to be bigger for some reason - as I made the unfounded assumption it'll be ok for full frame.

If that's the case, that only makes me want to get a bigger scope :D


There is always a reason to get a bigger scope :-P

_________________
Canon T2i (550D)- Tamron 17-50 F2.8 Non-VC, Sigma 70-200mm F2.8 EX DG OS HSM, Canon EF-S 55-250mm F4.5-F5.6
http://www.astronomyphotography.com


Top
 Profile  
 
 Post subject:
PostPosted: Wed Jan 13, 2010 6:59 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
Hey, I'm having enough trouble with camera lens lust to look at scopes too. :D Maybe I should combine the two - for example, how good is the 300mm f/2.8 for astrophotography? :P

On that note, I'm off to google...

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject: Re: Using focal reducer
PostPosted: Sat Jun 02, 2012 1:47 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
Ok, 2 and a half years since I started this post, I finally bothered to figure out the generic focal reducer I picked up then.

Generically, it helped me understand the relationship of the focal reducer and its positioning. In normal use, the magnification of a reducer varies from 1 when placed on the focal plane, to 0 at a distance equal to its focal length. The design operating point will be somewhere in there. Now if you take the spacing past the focal length of the reducer, you get an inverted reduction increasing in magnitude to -1 at double the focal length of the reducer. That's the region I was operating in with my earlier setup. Beyond double the reducer's focal length, you get magnification so that's way out of spec!

As a ball park estimate, the focal length of the focal reducer is around 80mm. This is a rounded approximate since the reducer is quite thick and I'm not really sure where the nodal point of it is. The spacing between this reducer and sensor should therefore be 40mm. So the distance I was using it at is totally wrong. The Canon EF mount is 44mm already, and you're adding perhaps another 70mm by the time you got a standard T-ring and eyepiece adapter in place. If I want to use this, I'll need a low profile T-ring and eyepiece adapters. And it'll still be too long. Still, it might make more sense when I get an imaging device without mirror.

Note the above ignores consideration the reducer will be optimised in optical design for its rated reduction factor, so using it outside that will likely cause problems to some degree. Also it's a bit theoretical and I can't claim with confidence to have got it all correct!

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
 Post subject: Re: Using focal reducer
PostPosted: Sun Jun 03, 2012 2:22 pm 
Offline
User avatar

Joined: Sun May 25, 2008 12:48 pm
Posts: 8022
Location: UK
I noticed something earlier today while fiddling with my eyepiece adapter. The inside of the barrel is exactly the right diameter to take a 1.25" filter! This means I can put the filter close to the T-ring. So close in fact, it sticks out camera side. I compared it against an EF-S lens which does the same, and as the distance was similar I decided there was minimal risk of mirror collision and gave it a go.

Image
A quick test of the camera (600D) on scope without reducer. A pigeon happened to be standing on the TV aerial of the houses opposite.

Image
By the time I got the focal reducer in place, the pigeon had flown off though. I thought this didn't look right with a defocus pattern.

Image
This is better. I turned the reducer around. It appears to be direction sensitive. This is with the exposed threads pointing sensor side, as it would be if screwed into an eyepiece or similar.

Image
I thought I'd pick a more defined subject for comparison to get a feeling for the reduction. This is a rooftop without reducer.

Image
Same position with reducer. I estimate the reduction is slightly more than 2x (or, magnification <0.5x). There is only a small gain in field of view before the image circle becomes inadequate to cover the sensor.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 15 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  

All words, images, videos and layout, copyright 2005-2012 Gordon Laing. May not be used without permission.
/ How we test / Best Cameras / Advertising / Camera reviews / Supporting Camera Labs

Webdesign by Alphabase IT
Powered by phpBB® Forum Software © phpBB Group