I want to use 2 focal reducers in my image train, but after searching the whole net I cannot find anywhere that lets me know distances they need to be apart for optimum performance.
I want to get my FOV even wider.
Can someone please assist?
I have a 150mm f5 Refractor
an Astro-Physics 2.7" 0.75x FR
and a 1.25" 0.7x FR
What would be the optimal distance between the two FR's?
I'm just being obtuse maybe but why bother?
With all those lens elements you will get significant aberations in the image- pincushioning etc and a small field.
Ken,
That gives you the back fous distance for the first reducer...you then need to apply the same calculation for the second reducer...
Where it sits relative to the "new"focus ( ie behind the AP reducer) and the final back focus (ie the distance from the second reducer to the CCD plane) will depend on it's focal length.....
Just thinking about what I posted above:
So if I keep the distance between the two Focal Reducers at around 175mm I should be ok (I hope)
I still don't think that's right.
It would be correct if the focal point was hitting the camera chip, but it isn't. It is hitting the 2nd Focal Reducer. By the time it passes through the 2nd one it will be out of focus by the time it gets to the camera chip
Ken,
That gives you the back fous distance for the first reducer...you then need to apply the same calculation for the second reducer...
Where it sits relative to the "new"focus ( ie behind the AP reducer) and the final back focus (ie the distance from the second reducer to the CCD plane) will depend on it's focal length.....
Ahhhh, so I'm almost correct
With the 2nd FR is there a way to measure focal length by focusing light and measuring it?
I just measured the distance from a sheet of white paper to the Focal Reducer. I moved it up and down until the lighting fixture on the ceiling was at best focus and it is 140mm (5.5")
Probably close enough.
Using the 140mm fl. So if that if it were placed 56mm inside the "new" focus for a further reduction of x0.7 the back focus would be 40mm.
This means that the distance between the reducers would be (175-56 = 119mm) and the final focus would be 40mm behind the second recucer.
I have a spreadsheet I use Reducers V2.
When you insert the focal length of the reducer it gives you all the answers!!
The spreadsheet meant nothing to me. All it did was confuse and frustrate me. Sorry.
I really appreciate the help you are giving but I can't wrap my head around anything mathematical.
I can't do maths.
In your post you say "Using the 140mm fl. So if that if it were placed 56mm inside the "new" focus for a further reduction of x0.7 the back focus would be 40mm.
This means that the distance between the reducers would be (175-56 = 119mm) and the final focus would be 40mm behind the second recucer."
Are you saying the distance between the 2 FR's should be 56mm or 119mm?
Neither.
The distance between the reducers is 119mm
and the distance from the rear reducer to the focus is 40mm
oh well, between the 2 images I got it right. Half of each one
Thanks heaps for your help
P.S. Not that it matters, but I just remembered that the 2nd FR (1.25") is 0.6x, not 0.7x.
But that doesn't matter because I tested and measured the focal length and it is 140mm.
Now all I have to work out is what the final Focal Ratio will be.
f5 scope with 0.75x plus 0.6x
No, exactly the opposite.
I will get a wider field.
Sorry, my error. What I meant was that the size of the image circle that will be produced at the CCD will be very small. This is OK for a very little sensor but not much else. Because of this vignetting on the CCD may result in only the central part of the sensor being useable. This is why the meade 0.33x extender was only useable with very small sensors.
