EPs,fov, vignetting, widevision and long focal length refractors

[Thales]

Hi all

I'm new to the game but I've been having an interesting pm with Charl about long focal length refractors e.g. 5"/1200 etc. We were discussing the new 2.5" hex focuser in the Bressers and Explore Sci scopes as compared with the standard 2" ones. The blurb says that in faster scopes the 2.5" gets rid of any vignetting issues.  I am interested in the long focal length frac  but had read on here somewhere that the widevision ES EPs (70. 82 degrees etc.) were ok/good in long focal length scopes - this is right?

So I'd be interested to know what people make of this - I've heard that you get vignetting in fast reflectors etc. - but I hadn't heard about the diameter of the focuser itself being key.

cheers good folks

Jeff

[John]

Hi Jeff,

Are we talking about imaging or visual use here ?

As I understand it, for visual use, the only time you will get vignetting of a wide field eyepiece is where something in front of it in the optical train has a smaller diameter than the eyepiece field stop.

An example of this would be if you used an adapter to put a 2" eyepiece into a 1.25" focuser or diagonal. The inside diameter of the 1.25" drawtube would be smaller than the field stop of, say, a 30mm eyepiece with an 82 degree apparent field so there would be some vignetting of the field of view.

Hope that makes some sense !

[Thales]

Hi John,

Thanks, yes that does make sense!

I'm thinking mainly visual yes - with maybe taking some point and shoot as a quick record rather than proper AP

So, if I've got you right ,a 1.25 EP into an 1.25 diagonal into a 2.5"  fat hex drawtube causes no probs; similalrly a 82 deg 1.25 EP into a 1.25 adapter, into a 2" diagonal into a 2" drawtube also causes no probs??

I'm thinking of only 1.25 EPs for the time being, but I've read that some 70 and 82 degree ones are nice! I have the option of the 2.5 drawtube or the standard 2" and was wondering what the advantage of the 2.5" was.  The ad blurb talked about avoiding vignetting with the fatter darwtube which all sent me into a tailspin!!

Your thoughts are very helpful and most appreciated

cheers

Jeff

[Moonshane]

There's a chance with fast scopes that as the light cone is wider at the focuser you might vignette the light cone. This is unlikely to make a visible difference but may theoretically reduce the aperture and increase the focal ratio, bizarrely this might improve the view aesthetically in a fast achromat.

This won't affect the views in any eyepieces though as they will show all the available field whether this is 40 or 110 degrees afov, albeit with or without the aforementioned vignetting.

[Thales]

Hi Moondhine, that is sounding good, thank you.  Yes, from what I've read, I'm picking up that the higher f ratios in the less pricey scopes tend for better images - my thinking was to put more of the money into the EPs so that if I get different scopes I can use the same EPs as far as possible. But I didn't want to go for a 70 or 82 field if it wasn't going to do the biz in a long focal length tube right now.  I also read a discussion on here about an ES EP 68* 24mm 1.25 which suggested it would sit well in a f/9. 

thanks for your thoughts, appreciated

[John]

Hi John,

Thanks, yes that does make sense!

I'm thinking mainly visual yes - with maybe taking some point and shoot as a quick record rather than proper AP

So, if I've got you right ,a 1.25 EP into an 1.25 diagonal into a 2.5"  fat hex drawtube causes no probs; similalrly a 82 deg 1.25 EP into a 1.25 adapter, into a 2" diagonal into a 2" drawtube also causes no probs??

I'm thinking of only 1.25 EPs for the time being, but I've read that some 70 and 82 degree ones are nice! I have the option of the 2.5 drawtube or the standard 2" and was wondering what the advantage of the 2.5" was.  The ad blurb talked about avoiding vignetting with the fatter darwtube which all sent me into a tailspin!!

Your thoughts are very helpful and most appreciated

cheers

Jeff

I think they might be referring to imaging with reference to the 2.5" drawtube. My Vixen ED 102 F/6.5 refractor was intended to be the imagers choice of the Vixen range and that has a 60mm drawtube whereas the slower ones have a 50.8mm (2") drawtube or even smaller with some models.

I'm not an imager though so I can't be sure of that.

My ED120 F/7.5 refractor has a 2" drawtube but handles 21mm 100 degree and 31mm 82 degree eyepieces without vignetting.

[Moonshane]

Great eyepieces work well in any scope. Not always the case with lesser quality.

[Thales]

I think they might be referring to imaging with reference to the 2.5" drawtube. My Vixen ED 102 F/6.5 refractor was intended to be the imagers choice of the Vixen range and that has a 60mm drawtube whereas the slower ones have a 50.8mm (2") drawtube or even smaller with some models.

I'm not an imager though so I can't be sure of that.

My ED120 F/7.5 refractor has a 2" drawtube but handles 21mm 100 degree and 31mm 82 degree eyepieces without vignetting.

Ah right! That's good to know about your ED120.  So the Vixen, like the ES models now, use the 60mm drawtube to try to prevent cutting the light cone that Moonshane is talking about?! That makes sense, which may mean that Bresser is using the 62mm focuser on all models, including long focal length, maybe just because they can scale up manufacture.

Involved stuff this telescope business!!

Thansk for that, very useful

cheers

Jeff

[xtreemchaos]

which one you going to keep then jeff, i like the look of the bigger one but i wouldnt fancy adding EX tubes for vis but it may be better for imaging, i hate them kinda  choices id end up keeping them both. good luck. charl.

[Thales]

which one you going to keep then jeff, i like the look of the bigger one but i wouldnt fancy adding EX tubes for vis but it may be better for imaging, i hate them kinda  choices id end up keeping them both. good luck. charl.

Hi Charl! Yes, spot on. It's been raining for days so not had a chance to see how I find them  to use - I still really don't know what to do...yes, keep both !! I don't think I'll be allowed to do that!  The narrow draw tube like yours one comes with better accessories, but I can defintely see advantages to the fat tube for photography.  To be honest I thought I was ages away from any AP, but now am becoming tempted especially after seeing your April moon shot through the 5"!  I am learning a lot just by having the scope here and getting used to it and asking you questions! I guess the choice thing is a nice dilemma to have - but you've put it in my head to keep both now!! I'll get killed!  cheers

[xtreemchaos]

hi jeff, ive been only taking pics since last november ive still got loads to learn,after being a diehard vis man for 30 or so years.   the guys on here will freely give you the benifit of there knowhow and make things a lot easyer to get into AP. you got a choice to make, if it was me i think id go for the newer model just for the AP side and put up with having  to use ex for vis. good luck mate  charl.

[Thales]

hi jeff, ive been only taking pics since last november ive still got loads to learn,after being a diehard vis man for 30 or so years.   the guys on here will freely give you the benifit of there knowhow and make things a lot easyer to get into AP. you got a choice to make, if it was me i think id go for the newer model just for the AP side and put up with having  to use ex for vis. good luck mate  charl.

Hi Charl...hmmmm yes....hmmmm tricky.  I've got a feeling I'll be able to see the moon tonight so will test out both and see how they go - I may try using both for point and shoot and see what happens.  I've got a feeling the fat tube one will be more stable with a camera bracket as you say...I'll let you know how I get on tomorrow! clear skies, Jeff