6.3 reducer help

[Johnboy]

Hi folks,

To get the magnification of a given eyepiece I know I have to divide the Focal Length of the telescope by the MM of the eyepiece. My CPC1100 has a FL of 2800 which means an eyepiece of say 7mm will give me a mag of 400x, but if I use a 6.3 reducer (which will change my scope from f10 to f6.3) does that change the magnificaton of my eyepieces? If so how do I calculate it?

The reason I ask is when I had my Lightbridge (FL 1524) the aforementioned 7MM gave me a Mag of 217x. Now I'm getting 400x with the same eyepiece. Will a Focal Reducer change things again?

Sorry if it's obvious but I'm not sure if using the Focal Reducer changes the FL of my scope.

Thanks as always.

John

[brantuk]

These formulae from Andrews primer should help:

Focal ratio = focal length / aperture

focal length = f/ratio x aperture

magnification = telescope f/l / eyepiece f/l

Just substitute your figures here and you should be able to work it out. The reducer as I understand it reduces the focal length and hence the f-ratio.

Bit too early to do the maths for you - I need a few more coffees first lol.

Cheers

[Johnboy]

These formulae from Andrews primer should help:

Focal ratio = focal length / aperture

focal length = f/ratio x aperture

magnification = telescope f/l / eyepiece f/l

Just substitute your figures here and you should be able to work it out. The reducer as I understand it reduces the focal length and hence the f-ratio.

Bit too early to do the maths for you - I need a few more coffees first lol.

Cheers

Thanks Kim,

That's very helpful and just what I needed. I'll try the maths later on - you're right about the coffee. Just a few more cups and my brain may be up to it! LOL!

Being just into visual at the moment I might just attach it to the scope and leave it there for a while and see what happens.

Cheers,

John

[Johnboy]

Hi folks,

I have attached my 6.3 FR to my CPC 1100 and tried it out in the daytime looking at a nearby chimney pot but had some strange results. I used a 20mm (66 degree) eyepiece. Firstly there wasn't enough focus travel which meant the chimney was almost clear but not quite. I am assuming that the chimney was too near and I may have more luck with a celestial object. Is my assumption correct? Also there was a circular shadow in the middle of the view. Is this the secondary and will it disappear when it focuses correctly?

The strangest thing was when I changed eyepieces to a 2" 32mm Panaview. (70 AFoV). It's hard to explain but all I could see was the chimney pot in the middle of the view, which was surrounded by a black circle which in turn was surrounded by more of the background (more sky etc.) Is the reducer + 2" eyepiece + a close object causing the odd effect?

Thanks for your thoughts.

John

[michael.h.f.wilkinson]

The F6.3 reducer does not change the effective field of view of the scope. After all, the only information the reducer is getting is from the about 48 mm diameter circle in the rear cell of the scope. The reducer simply images that on a smaller scale. Thus, the largest useful field stop diameter to be used is just 0.63 x 48mm = 30mm. Your panaview has a field stop well in excess of that (some 46mm presumably). This leads to the vignetting you observe. The focal reducer is mainly useful if you want to image, or if you do not have a 2" visual back.

[Johnboy]

Thanks Michael,

Very useful info. So anything over 30mm would be useless with the 6.3 reducer I guess.

The problem I'm having is all my eyepieces are giving me much higher magnifications with my CPC1100 which has a FL 2800 than I had with my 12" Lightbridge which had a FL 1524. For instance with the Lightbridge I enjoyed viewing the moon with a 16mm eyepiece. This gave me a mag of 95x, but using the CPC with the same eyepiece I get 125x. I was hoping the 6.3 FR would help reduce the mag. If not then I assume I'll have to buy long FL eyepieces to get the 95x I want.

Hope this makes sense!

John

[michael.h.f.wilkinson]

The Lightbridge simply has the larger FOV at the field stop. I can still get the full moon in a 14mm (at F=2000, 143x mag) with my C8, but mainly because I have an ultra wide angle EP. With the focal reducer, you can still use a 35mm EP, provided it is a Plossl or something with just 50 deg FOV. Most 16mm EPs should be fine. Also remember that in the Lightbridge, a 38 mm EP gives an exit pupil of 7.6mm, which means you are losing light, unless you are younger than about 35. For us oldies the longest useful focal length for an EP is about 5x F-ratio (or at most 6 x F-ratio), for a 5 to 6 mm exit pupil.

[Merlin66]

With the reducer in place, the "new" focal plane is 110mm behind the reducer....maybe you didn't have enough spacers behind the reducer...

[brianb]

The F6.3 reducer does not change the effective field of view of the scope.

Indeed ... there is no point whatsoever in using a focal reducer visually.

When used for imaging, the reduction you get (and the correction for field flattening etc) depends critically on the distance between the FR and the focal plane, it should be approx. 110 mm for the standard 0.63x unit.

[Merlin66]

FOV = eyepiece apparent FOV/ magnification.

So if the effective focal length is reduced, the magnification is reduced, the FOV increases...yes/no??

[Rossco72]

You definately need to use spacers if you are going to try to get an EP to focus with the reducer in place.

As has been said, the reducer is mainly intended for smaller areas such as CCD or CMOS chips rather than large, low power EP's.

[Johnboy]

Indeed ... there is no point whatsoever in using a focal reducer visually.

Hi,

Now I'm confused! (Doesn't take much. )

Before I bought the reducer I asked this question on SGL: http://stargazerslounge.com/equipment-help/111087-celestron-cpc1100-option.html

As you can see a few members recommended the reducer for visual.

Not sure now...

Cheers,

John

[michael.h.f.wilkinson]

A 0.63x reducer is OK visually IF you do not have long focal length, or ultra-wide angle mid focal length EPs, in the same way as a barlow is useful if you do not have short focal length EPs. I used it visually when all I had were 1.25" EPs, which only use the inner 30mm of the image plane anyway. Since I got 2" EPs I have not used my 0.63x at all.

[brianb]

A 0.63x reducer is OK visually IF you do not have long focal length

But it doesn't increase the angular size of the illuminated field, in fact it can cause vignetting on its own account.

Since I got 2" EPs I have not used my 0.63x at all.

Precisely ... extra glass is in general Not A Good Thing ...

[michael.h.f.wilkinson]

But it doesn't increase the angular size of the illuminated field, in fact it can cause vignetting on its own account.

Precisely ... extra glass is in general Not A Good Thing ...

My point is: like a barlow (which I also use only photographically), it is of use when your budget for EPs is low.