Double Focal Reducer distances please?

[ballaratdragons]

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?

The RED lines show the distance I need :thumbsup:

[Terry B]

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.

[Merlin66]

Do you have the focal lengths of the reducers??

[ballaratdragons]

Because I want to




No, exactly the opposite.
I will get a wider field.

[ballaratdragons]

No I don't. :help:

All I know is the Focal Reduction of each.
0.75x and 0.7x :)

[ballaratdragons]

I found the Focal Length of the Astro-physics one :thumbsup:

Focal length is 700mm (27.6")

Distance needed to achieve 0.75x FR - 175mm (6.9")

Found it here: http://www.docstoc.com/docs/52188756/Astro-Physics-Telecompressors---27TVPH-CCDT67-Compression-and

So if I keep the distance between the two Focal Reducers at around 175mm I should be ok (I hope)

[DavidU]

This will be interesting.
Did you get the PC connected to the internet?

[ballaratdragons]

No Dave :(

[Merlin66]

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.....

[ballaratdragons]

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 :sadeyes:

Am I way off here?

[ballaratdragons]

Ahhhh, so I'm almost correct :P

With the 2nd FR is there a way to measure focal length by focusing light and measuring it?

[ballaratdragons]

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")

Would that be the focal length?

[Merlin66]

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!!

[ballaratdragons]

I apologise but I don't get what you are saying.

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?

I'm lost :sadeyes:

[ballaratdragons]

Which one of these is correct? No. 1 or No. 2?

(If any of them are) :P

[Merlin66]

Neither.
The distance between the reducers is 119mm
and the distance from the rear reducer to the focus is 40mm

[ballaratdragons]

:rofl:

oh well, between the 2 images I got it right. Half of each one :lol:

Thanks heaps for your help :thumbsup:

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

[ballaratdragons]

f5 scope with 0.75x plus 0.6x

All spacings being correct, that should give me a focal Ratio of f2.25 :thumbsup:

[Terry B]

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.

[ballaratdragons]

Ahhhh, yep, fully understand what you mean Terry.

The Camera chip is 1/2". Could be a problem. Only one way to find out ;)

If it vignettes I think I can move the camera up the light cone closer to the 2nd FR and re-focus :shrug:

[Barrykgerdes]

Hi Ken
Can I buy in on this?

Focal reducers are designed specifically for one type of optical system. They reduce the focal length of the original arrangement with some form of correction for vignetting etc.

While a second reducer if placed inside the now modified (shortenned) focal length will certainly reduce the focal length even more, its correction elements will no longer be valid and the usefull FOV will not be improved. Just made to occupy a smaller area on you camera chip or in your eyepiece. The area outside this will most likely be of no practical use even if it is a wider field and subject to all sorts of distortion.

However you should experiment with the positions of the two correctors making sure that the overall focal length can still be focused on the camera or in the viewing eyepiece. This may not be possible if you can't move the focuser far enough forward in the OTA. However the result may be good enough to suit your purpose.

Barry