Help with TFOV LX90 8 inch

[BarneyMaroon]

I am a bit confused about TFOV. I bought a 2'' 30mm 80 degree FOV Moonfish EP to put on on my LX90-8. I am aware that the LX90-8 does not have a full 2'' barrel (is that the right word). I understand it is 1.5''. Ceteris paribus this would knock 25% of the TFOV from a full 2 inch set-up. I also know that this is not a wide field scope.

The true FOV (ignoring the small barrel) should be 80/66=1.2 degrees where 66=2000/30 is mag. I reckon I have read many posts on all sorts of forums saying people can get 1.0 to 1.2 degrees TFOV from their wide EPs.

However, I looked at the newish moon last night and reckon the moon occupied about 80-85% of the diameter of the FOV (which was fabulous btw). So if the moon is 0.5 degree, I am getting a TFOV of 0.5/0.8=0.625 degrees.

So given the fact that I have a wide EP, and 2'' diagonal and am getting 0.625 degrees, I think that this is the maximum (not the 1+ degrees claimed by others). I did not observe any darkening around the edges.

Am I missing something? My error guesses are:
1. The 1.5 inch barrel is killing the TFOV, and others have it wrong.
2. The eye relief is not 22mm of the EP, and I am not seeing the full FOV.
3. The moon is in fact 1 degree not 0.5 degrees ;-)
4. My estimate of the moon as 80-85% of the FOV was an illusion. (Actually I'm being conservative - I thought it was 90+%)
5. My calculations are wrong.

Anyone with more experience able to help :shrug::confused2:



Focal length: 30 mm
Apparent Field of View: 80°
Fully multi-coated
Barrel: 2" / 50.75 mm
Rubber eyecup
M49 camera thread
Elements: 5 elements in 3 groups
Eye relief: 22 mm
Field stop: 40.8 mm
Weight: 570g (1.2lb)
Length: 114 mm (4.5")

[anj026]

To calculate the actual field of view of a telescope you need to know the actual focal length of the scope and the field stop of the eyepiece.

Then the formula is; field stop/focal length times 57.3.

So for your example you might go something like this;

40.8/2000*57.3 = 1.16 degrees.

However when you put a 2 inch diagonal on a SCT you effectively make the focal length longer. The nominal focal length of 2000mm is when fitted with a 1.25 inch prism diagonal. The SCT focuses by moving the primary mirror closer to or further away from the secondary mirror. From what I have read an 8" SCT with a 2 " diagonal is more like 2200mm focal length.

So our calculation is now;
40.8/2200* 57.3 = 1.06 degrees.

Still doesn't agree with your findings but you can see there is a bit more to it than you might think.

[anj026]

Have a look at this thread from Cloudy Nights. Particularly the comments from EdZ.

http://www.cloudynights.com/topic/331842-high-power-ep-for-c8/?hl=%2Bhigh+%2Bpower+%2Bep+%2Bfor+% 2Bc8#entry4261825

[BarneyMaroon]

Thanks, I did not think about the focal length - and this is certainly part of the puzzle. I will check what the moon looks like through my 1.25'' 25mm and a GSO 32mm 2'' EP I have - and have a think about it.