3 inch focusers - why?

[Knighty2112]

Silly question maybe, but what is the advantage of having a 3 inch focuser on some scopes? I am not aware of EPs or diagonals that come in this size range, so what is the point of this larger focuser really, if you then need to use either a 2" or 1.25" adapter to use any equipment with it? 

[Alan White]

I am sure there is a valid reason, but the synic in me says 'so folks upgrade and buy one' but I clearly could be wrong ?

[ollypenrice]

It will avoid vignetting for imagers using large format chips. 

Olly

[Knighty2112]

1 minute ago, Alan White said:

I am sure there is a valid reason, but the synic in me says 'so folks upgrade and buy one' but I clearly could be wrong ?

But what would the upgrade achieve? Do you get better/wider views or something? Are 3 inch focusers better for astrophography or what? Not seeing a real obvious reason for them myself! ??

[Knighty2112]

2 minutes ago, ollypenrice said:

It will avoid vignetting for imagers using large format chips. 

Olly

OK. So this is purely something required just certain kinds of astrophotography then. Thanks!  

[Alien 13]

There are eyepieces that are four inch or greater which would be good for me, I like widefield even if it means huge exit pupils, it is also more rigid which always helps with AP. 

Alan

[John]

ES do a 30mm 100 degree eyepiece in the 3" fitting and a matching diagonal:

https://www.explorescientific.co.uk/en/Eyepieces/Explore-Scientific-100-Ar-Eyepiece-30mm-3-76-2mm.html

https://www.explorescientific.co.uk/en/Optical-accessories/Explore-Scientific-Diagonal-Mirror-76-2mm-3.html

 

 

[Knighty2112]

3 minutes ago, Alien 13 said:

There are eyepieces that are four inch or greater which would be good for me, I like widefield even if it means huge exit pupils, it is also more rigid which always helps with AP. 

Alan

Really? Wasn't aware of that at all. Thanks! Bet they cost quite a bit of dosh though! ??

[Knighty2112]

2 minutes ago, John said:

ES do a 30mm 100 degree eyepiece in the 3" fitting and a matching diagonal:

https://www.explorescientific.co.uk/en/Eyepieces/Explore-Scientific-100-Ar-Eyepiece-30mm-3-76-2mm.html

https://www.explorescientific.co.uk/en/Optical-accessories/Explore-Scientific-Diagonal-Mirror-76-2mm-3.html

 

 

Nice prices there John! Hehe!  

[Alien 13]

1 minute ago, Knighty2112 said:

Really? Wasn't aware of that at all. Thanks! Bet they cost quite a bit of dosh though! ??

I think the rigidity and lack of vignetting are the big plus points, you can actually use camera lenses as eyepieces if the focus tube was wide enough.

Alan

[John]

There are 4" eyepieces as well (the small one in this pic is a 2" Pentax 40mm):

 

 

[AndyH]

Some more insanely massive goodies ......

Japanese monster 3" ep + 3" diagonal.

http://www.kkohki.com/English/kkohkiparts.html#Masuyama 3 inch eyepiece

 

Andy

[Knighty2112]

1 hour ago, John said:

There are 4" eyepieces as well (the small one in this pic is a 2" Pentax 40mm):

 

 

Wow! Just.......Wow! ???

[Knighty2112]

1 hour ago, AndyH said:

Some more insanely massive goodies ......

Japanese monster 3" ep + 3" diagonal.

http://www.kkohki.com/English/kkohkiparts.html#Masuyama 3 inch eyepiece

 

Andy

For such big EPs the FOV seems very small? Also, they do taper in quite a lot at the top like a volcano? 

[John]

At 80mm focal length, even in a 3" barrel, the FoV will max out at 52 degrees. The field stop is probably close to the full internal width of the 76.2mm barrel to get that.

The tapering top makes the eye lens more accessible.

[AndyH]

It's a simple-ish 5 element ep, so 52 deg is about right, in what I think is a modified plossl design. To get 52 deg, a 70mm field stop is needed, hence the 76.2mm barrel size.

Perhaps the tapered volcano top is to aid in binoviewer use........... ahem

[DaveS]

The 3.7" focuser on my 130 f/7 allows the 0.75x APM Ricardi reducer to fit inside, limiting the overhang.

[Knighty2112]

25 minutes ago, DaveS said:

The 3.7" focuser on my 130 f/7 allows the 0.75x APM Ricardi reducer to fit inside, limiting the overhang.

OK, not quite sure why that's a good thing, but great!