low power wide field in an f5 refractor

[rdfwtf]

Anything higher than a 7mm exit pupil is pointless so the max is 35mm. I'm in my 30s, with no need for glasses but mostly observing in relatively light polluted suburbia. Should I really be going down to 30mm as a comfortable maximum? Is there a formula for how much fov would be lost relative to exit pupil size?

As for recommendations: panoptic seems to be the rolls royce but that's an awful lot of money. In the less than €150 bracket, there's maxvision 34mm, ES 70degree 30/35mm. Any other suggestions?

ES seems to be a premium eyepiece in all but the 70degree range, how bad are they?

[YKSE]

30mm Skywatcher Aere can be a candidate.

Maxvision 34mm weighs over 800 g, if you're fine with the weight, it'll surely be the best one in your price-range.

[rwilkey]

The Tele Vue 32mm Plossl will give you a good quality low power view.

[ronin]

Would be useful if the scope was specified. An f/5 refractor makes me think of the ST-80 and if that then CA and SA are more relevant. Also does FoV go down with exit pupil to any significant amount? The light level is lost but the FoV shouldn't really suffer.

[rdfwtf]

Would be useful if the scope was specified. An f/5 refractor makes me think of the ST-80 and if that then CA and SA are more relevant. Also does FoV go down with exit pupil to any significant amount? The light level is lost but the FoV shouldn't really suffer.

 

My misunderstanding on the FOV, thanks for the info. It's a sky watcher star travel 120 so yes CA may be an issue. I thought it should be more pronounced at higher mag and brighter objects rather than wide fields. SA is a new one on me - spherical aberration? What does that look like at the eyepiece?

[LunaSwift]

I have a SW 120ST, and for observing I dont worry about CA , different for imaging though but I fit a semi APO filter and that reduces it a fair bit, , As far as a wide angle ep is concerned,  My Meade 26mm gives me a mag of 23x ,  field of view of 2.26deg and exit pupil of 5.2mm, I also have a 40mm EP but the exit pupil is 8mm  far too wide for by 66 yr old eyes and fov of 3.46

[John]

If you are happy to buy used then this 28mm Nirvana would work superbly with your scope and would show around 3.8 true degrees of sky while retaining a reasonable 5.6mm exit pupil and almost Nagler-like sharpness across the field of view:

http://www.ebay.co.uk/itm/Skywatcher-28mm-Nirvana-2-telescope-eyepiece-with-82-degree-field-of-view-/331065111995?pt=UK_Telescope_Eyepieces&hash=item4d1504b5bb

It's just a little over budget though 

I don't normally recommend e.bay as a place to buy astro items but I believe the seller in this case is possibly an SGL member ?

(it's not my advert I ought to add !)

[bingevader]

(it's not my advert I ought to add !)

We all know your secret identity is wasps1000! 

[John]

We all know your secret identity is wasps1000! 

Darn, my secret is out !

[LunaSwift]

You may find this link helpful http://www.csgnetwork.com/telefov.html, gived mag ,AFOV  and TFOV for scopes of varying FL and different FL eps, 

As a suggestion for a not expensive but good EP, look at them result for the Celestron 32mm ep.  AFOV is 46deg, mag 19x and a TFOV of 2.45 , I am thinking of getting this one for my ST120

[John]

You may find this link helpful http://www.csgnetwork.com/telefov.html, gived mag ,AFOV  and TFOV for scopes of varying FL and different FL eps, 

As a suggestion for a not expensive but good EP, look at them result for the Celestron 32mm ep.  AFOV is 46deg, mag 19x and a TFOV of 2.45 , I am thinking of getting this one for my ST120

To be honest, 46 degrees AFoV is rather narrow for a 32mm eyepiece, even a 1.25" format one. Most 32mm plossls will give you 52 degrees and a TFoV of 2.77 degrees in your ST120. Thats about as much TFoV as the 1.25" format can deliver. Moving to 2" will give you somewhat more.

[rdfwtf]

You may find this link helpful http://www.csgnetwork.com/telefov.html, gived mag ,AFOV  and TFOV for scopes of varying FL and different FL eps, 

As a suggestion for a not expensive but good EP, look at them result for the Celestron 32mm ep.  AFOV is 46deg, mag 19x and a TFOV of 2.45 , I am thinking of getting this one for my ST120

 

Thanks for the link. Working off the Backyards Astronomers guide, I was using afov/magnification for TFOV and I found this yesterday http://www.skyatnightmagazine.com/field-view-calculator which seems pretty useful.

Used is out as this is a gift. It's for me but I don't think the missus will see past secondhand. I've just seen the Nirvana 28mm for €300+ locally, I think I'd buy a panoptic 35mm for that. What are the Skywatcher SWA-70 or Panaview 32mm like?

[rdfwtf]

Also does FoV go down with exit pupil to any significant amount? The light level is lost but the FoV shouldn't really suffer.

This is confusing me since. I think I've misunderstood exit pupil issues. If what's coming out of the eyepiece is too big for your eye, what exactly is wrong at the eyepiece? Say, I bought a 40mm piece giving 15x magnification but an 8mm exit pupil, do I see the full fov at 15x but less bright than it could be? So a 60 degree 40mm eyepiece would allow me to see 4 degree fov but less bright than it could be?

[AlexB67]

Easy way to think of it is that you'll be wasting aperture since you are not capturing all of the light  from the exit pupil when the exit pupil diameter is larger than your own eyepupil diameter, so yes, there is light loss, though a little bit too large will not be that noticeable I expect.  You will see all of the FOV.

Also, a very large exit pupil may not be ideal when observing from light polluted areas, since the sky background becomes brighter with increasing exit pupil, this will give a washed out look to the view and result in lack of contrast. This may or may not be that important to you, depending where you observe from. If you observe from a light polluted area often it would be better to go for something with a bit more FOV and smaller exit pupil ( i.e. higher mag)  compared to smaller FOV and greater exit pupil, though both will bag the same amount of sky, or better put, result in the same true FOV that you see when you put the eyepiece in the telescope.

[John]

Used is out as this is a gift. It's for me but I don't think the missus will see past secondhand. I've just seen the Nirvana 28mm for €300+ locally, I think I'd buy a panoptic 35mm for that. What are the Skywatcher SWA-70 or Panaview 32mm like?

Understand about used not being a possibility.  The 28mm Nirvana lists at £245 from First Light Optics but I know there is some cost associated with shipping outside mainland UK.

The Skywatcher SWA-70 and the Panaview's are decent eyepieces although they will show some astigmatism (distorted star shapes) in the outer parts of the field of view at F/5 but many folks live with that quite happily. The Skywatcher Aero ED's are a bit better again in my opinion, but at more cost as well.

[rdfwtf]

So for a simplified example with 600/24=25x mag, a 24mm 100 degree eyepiece would give a 4 degree field with a very manageable exit pupil. And a penny drops, thanks a million.