# FOV and focal reducer

Started by
Guest_**DONOTDELETE**_*
, Feb 18 2004 10:43 AM

9 replies to this topic

### #1 Guest_**DONOTDELETE**_*

Posted 18 February 2004 - 10:43 AM

On an 8" SCT, how much will the focal reducer (F/6.3) increase the FOV? With the standard EP (25mm Plossl), Celestron claims a FOV of .64*. Is there a calculation for this in general? Thanks.

### #2

Posted 18 February 2004 - 11:06 AM

Currently, you have a TFOV of about .615 degrees. With the focal reducer, you have about .97 degrees.

TFOV = AFOV/mag

There are some great definitions and information in the Astronomy FAQ thread in the "Off Topic" category.

Enjoy!

TFOV = AFOV/mag

There are some great definitions and information in the Astronomy FAQ thread in the "Off Topic" category.

Enjoy!

### #3

Posted 18 February 2004 - 11:57 AM

the f6.3 means, I think, that you'll get 63% of the scope's unaltered focal ratio. So, if f10, you get f6.3. If your scope was f12, you get 63% of 12 = about f7.6.

From the new focal ratio, you can calculate your new effective focal length. Instead of 2000mm (assuming 8", f10 scope), your focal length is now 1260mm (63% of 2000).

Put in your 25mm Plossl, and mag = 1260/25 = 50x. Like Dennis said, take AFOV/mag = 52 deg/50x. You get 1.04. A little different than Dennis said, but we're close.

1 degree is A LOT more than .64 degrees; you'll see about 2.5x more area of sky.

From the new focal ratio, you can calculate your new effective focal length. Instead of 2000mm (assuming 8", f10 scope), your focal length is now 1260mm (63% of 2000).

Put in your 25mm Plossl, and mag = 1260/25 = 50x. Like Dennis said, take AFOV/mag = 52 deg/50x. You get 1.04. A little different than Dennis said, but we're close.

1 degree is A LOT more than .64 degrees; you'll see about 2.5x more area of sky.

### #4 Guest_**DONOTDELETE**_*

Posted 18 February 2004 - 02:35 PM

So if I get the Scopetronix MaxView 40 (40mm, AFoV = 44) in combination with the focal reducer, my resulting TFoV would be 1.4*?

1260mm/40mm = 31.5x mag

44*/31.5 = 1.4*

Also, would a 2x barlow effectively give the 25mm EP without the focal reducer a TFoV of 0.32* due to doubling the focal length? Just trying to wrap my head around all this. Thanks guys.

1260mm/40mm = 31.5x mag

44*/31.5 = 1.4*

Also, would a 2x barlow effectively give the 25mm EP without the focal reducer a TFoV of 0.32* due to doubling the focal length? Just trying to wrap my head around all this. Thanks guys.

### #5

Posted 18 February 2004 - 02:46 PM

you got it.

### #6

Posted 18 February 2004 - 03:29 PM

As a newbie, I would like to correct any misunderstanding I may have. Here's my assumptions:

8" = 2032mm (8 x 25.4)

F/6.3 would yield 1280mm

Plossl has 50* AFOV.

Jeff, your calculations are based on 2000mm and a 52* AFOV. Are my numbers wrong? Please understand, I don't mean to split hairs. I just want to learn this info properly. Thanks!

8" = 2032mm (8 x 25.4)

F/6.3 would yield 1280mm

Plossl has 50* AFOV.

Jeff, your calculations are based on 2000mm and a 52* AFOV. Are my numbers wrong? Please understand, I don't mean to split hairs. I just want to learn this info properly. Thanks!

### #7

Posted 18 February 2004 - 04:18 PM

Guys, you're leaving out the effect of the maximum light cone you can get into an eyepiece/reducer. i.e. the calculations are correct but they assume you've got an unlimited light cone to work with.

Once you use up all the light coming into a 2" eyepiece, you can't get any wider TFOV (True FOV). You'll just get a vignetted image, like looking into a tube.

There's a really good table of formulas at:

http://www.televue.c...page.asp?ID=107

TFOV can be calculated as:

TFOV in degrees = eyepiece / focal length x 57.3

Televue lists the largest 2" field stop as being 46mm (basically the entire eyepiece barrel), so:

TFOV = 46 / (8 x 25.4 x 10) x 57.3 = 1.3 degrees

The Celestron focal is slightly larger than a 2" drawtube, so if you measure the telescope side ID and plug into the above formula you'll get the maximum TFOV through the reducer.

For example, a 50mm Plossl has the same 46mm field stop as a 41mm Panoptic, so they would both give a TFOV of 1.3 degrees, despite the usual TFOV/AFOV calculations.

The TFOV/AFOV calculations can be used with impunity once the field stop is smaller than the maximum light cone.

Suk

Once you use up all the light coming into a 2" eyepiece, you can't get any wider TFOV (True FOV). You'll just get a vignetted image, like looking into a tube.

There's a really good table of formulas at:

http://www.televue.c...page.asp?ID=107

TFOV can be calculated as:

TFOV in degrees = eyepiece / focal length x 57.3

Televue lists the largest 2" field stop as being 46mm (basically the entire eyepiece barrel), so:

TFOV = 46 / (8 x 25.4 x 10) x 57.3 = 1.3 degrees

The Celestron focal is slightly larger than a 2" drawtube, so if you measure the telescope side ID and plug into the above formula you'll get the maximum TFOV through the reducer.

For example, a 50mm Plossl has the same 46mm field stop as a 41mm Panoptic, so they would both give a TFOV of 1.3 degrees, despite the usual TFOV/AFOV calculations.

The TFOV/AFOV calculations can be used with impunity once the field stop is smaller than the maximum light cone.

Suk

### #8

Posted 18 February 2004 - 04:40 PM

Thanks for the explaination, SUK. And that's why they call it a field stop.

### #9

Posted 18 February 2004 - 04:56 PM

Dennis...you got me right. I was just assuming 2000mm and 52-deg EP.

Suk...thanks for clearing all that up. Yes, I knew I was ignoring the issues you mentioned.

jeff

Suk...thanks for clearing all that up. Yes, I knew I was ignoring the issues you mentioned.

jeff

### #10 Guest_**DONOTDELETE**_*

Posted 18 February 2004 - 05:10 PM

Sweet, thanks.