Focal Reducers - fact v fiction

[AstroJOE]

Okay so I've had my CPC1100 now for about a month and first things first, I love it!

I'm currently using it with the standard 1.25" diagonal and whilst I was prepared for a restricted FOV with this SCT, I was perhaps a little surprised just how limiting this was going to be for some of the larger DSO's, for example open clusters, Andromeda etc, even through the 40mm e/p.

I'm therefore considering getting the Celestron F6.3 reducer to open things up a bit. Problem is I've read lots of reviews all saying different things about just how effective these are for visual use.

Anyone got any experience of using one of these with the CPC1100 or similar? Also what would be the effect of adding a 2" diagonal and or an 82 degree ultra wide e/p be for example?

Am I going to get an appreciable return on these additional investments?

Thanks in advance all

James

[E621Keith]

What kind of 40mm eyepiece do you have? You will not gain anything apart from increase distortion, internal reflection and vignetting if you are using 40mm 70deg wide field eyepieces with a max out field stop like an 40 Aero ED, Panoptic, XW or 42 LVW.

You should see a increase field if you are using a 40mm narrow field eyepiece such as a 1.25" 40mm plossl.

For visual observation, a 2" eyepiece with a max 2" field stop (40mm 70deg / 30mm 82 deg / 25mm 100deg) and a 2" diagonal will be a much better investment than using a reducer.

[AstroJOE]

It's just the standard 40mm that came with the scope. Not a 70 degree widefield.

[John]

There are just some objects that need a wide field scope and an F/10 2,800mm fl scope is never going to be that. I think a 1 degree of true field of view is about the max you can get whatever approach to go for.

[AstroJOE]

There are just some objects that need a wide field scope and an F/10 2,800mm fl scope is never going to be that. I think a 1 degree of true field of view is about the max you can get whatever approach to go for.

I'm confused (not that difficult). How do I calculate what e/p, diagnoal and focal length combination will offer the widest visual FOV?

The standard specifications for this scope say it offers a 0.6 degrees. How do I work out how to get this up to 1 degree?

[E621Keith]

The 0.6 deg max FOV was probably written for 1.25" eyepiece (max 27mm field stop).

You should be able to get around 1 deg if you use a 2" eyepiece (max 47mm field stop)

You can calculate yours scope's FOV here.

http://www.davidpaulgreen.com/tec.html

[John]

I'm confused (not that difficult). How do I calculate what e/p, diagnoal and focal length combination will offer the widest visual FOV?

The standard specifications for this scope say it offers a 0.6 degrees. How do I work out how to get this up to 1 degree?

The tool that Keith has linked to is good.

I assumed the widest apparent field of view eyepiece currently available in the 2" fitting which is the Vixen NLVW 42mm / 72 degrees AFoV. I've assumed you will buy the 2" diagonal to fit it too !.

My calculation is not 100% exact but I take the magnification given by the above eyepiece in your scope, 66.67x and divide the AFoV of the eyepiece by that magnification to get the true field of view (TFoV) = 1.08 degrees. Around two moon diameters as it happens.

[AstroJOE]

The 0.6 deg max FOV was probably written for 1.25" eyepiece (max 27mm field stop).

You should be able to get around 1 deg if you use a 2" eyepiece (max 47mm field stop)

You can calculate yours scope's FOV here.

http://www.davidpaulgreen.com/tec.html

That's very useful thanks

[AstroJOE]

The tool that Keith has linked to is good.

I assumed the widest apparent field of view eyepiece currently available in the 2" fitting which is the Vixen NLVW 42mm / 72 degrees AFoV. I've assumed you will buy the 2" diagonal to fit it too !.

My calculation is not 100% exact but I take the magnification given by the above eyepiece in your scope, 66.67x and divide the AFoV of the eyepiece by that magnification to get the true field of view (TFoV) = 1.08 degrees. Around two moon diameters as it happens.

Hi John it's all starting to make sense now i think, thanks!

[Dunkster]

Another EP that doesn't break the bank quite so much is the Baader Aspheric 36mm...it's ever so slightly off the max FOV of the XW40 (which I use) and the LVW42 but a fraction of the price! You'd also need a 2" diagonal and a decent visual back - I can recommend the Baader clicklock 3.25". The stock vb that mine came with was restricted to a 38mm aperture, which isn't really tennis, if you know what I mean

For much wider views than 1 degree, you're probably better off piggybacking an ST80 or similar and having your cake and eating it

[Dunkster]

Almost forgot... I also use an Explore Scientifc 82 degree model 30mm EP with my scope and although it's not as close to the max FOV, what you do get is a nice wide and immersive view. Depending on your pocket depth, that or the TV Nagler, or SW Nirvana 28mm should all do a great job