A wide field EP for a 150 f5 'scope?

[Alfian]

Hi all, I'm apologise if this is a bit of a regular chestnut. I'm pretty close, I hope, to buying a 150 f5 reflector (see separate topic) and I'm considering a 2" wide field EP to go with it. This is a bit of a two parter:

1) I've tended to accept the given logic that its a bit pointless using an EP that gives an exit pupil that is larger than my 60+ No 1 dark adapted eyeball can can fully utilise. However in trawling around the forum I've found several examples of well experienced members who I am guessing are slightly past the first flush of youth (I am trying to be as diplomatic here, although its not always one of my strengths!) using 32mm EPs, even a 38mm EP in an F5 scope. The latter gives a 7.6mm exit. I remember trying my 32mm plossl in my old Celestron 130 and got the expected hole in the view. Am I missing a trick or something? Please be gentle.

2) Right, with that over, at the moment I'm looking at just a couple of options. My ES  20mm maxvision will give 37.5x and 1.8 degrees which I think will prove useful so  a 26mm Panavision will give me 2.4 degrees and the ES 28mm (68) will give me 2.5 degrees. Not much between - but will the ES give a better performance in the f5 scope? I really don't want to go to an 82degree EP. I find the maxvision a big enough port hole. I think I could push to 70 but not 82! Any suggestions?

[John]

I would expect the ES / Maxvisions to be better corrected at F/5 than the Panaview would be in the outer 1/3rd of the field of view. A 40mm / 68-70 degree eyepiece will give you as much sky as the 2" format can deliver. According to Al Nagler a large exit pupil is not an issue with a refractor. The "hole" with the reflector was the shadow of the secondary mirror. No secondary with a refractor of course.

Edit: I've just noticed that you are thinking of a relfector so ignore the 40mm stuff above and stick to 30mm - 32mm as a max focal length.

[pjsmith_6198]

I have a Orion Starblast 6" and a Zhumell Z10 scope.  Both are F5 or close to it .  The Z10 is F4.9.    The  longest FL eyepieces I have are a 32 mm Plossl and the 2 inch GSO 68 deg EP that came with the Z10.    My skies are fairly light polluted.  I found that I was able to find objects easier using my 24 mm ES 68 EP.   The extra magnification darkened the background and it was much easier to see dim objects.   For example I had trouble seeing M65 and M66 in Leo with the 32 mm EP,  but I could with the 24mm.   If you have to deal with light polluted skies, I would suggest the ES 24 mm 68 EP.   It will still give you over 2 degrees FOV plus it's cheaper than the ES 28.  I agree with you about the 82 deg EP's.  To mee they're just too much.  The 68-70 FOV is mjuch more comfortable to  me.

[SpaceWalker]

Hi all, I'm apologise if this is a bit of a regular chestnut. I'm pretty close, I hope, to buying a 150 f5 reflector (see separate topic) and I'm considering a 2" wide field EP to go with it. This is a bit of a two parter:

1) I've tended to accept the given logic that its a bit pointless using an EP that gives an exit pupil that is larger than my 60+ No 1 dark adapted eyeball can can fully utilise. However in trawling around the forum I've found several examples of well experienced members who I am guessing are slightly past the first flush of youth (I am trying to be as diplomatic here, although its not always one of my strengths!) using 32mm EPs, even a 38mm EP in an F5 scope. The latter gives a 7.6mm exit. I remember trying my 32mm plossl in my old Celestron 130 and got the expected hole in the view. Am I missing a trick or something? Please be gentle.

2) Right, with that over, at the moment I'm looking at just a couple of options. My ES  20mm maxvision will give 37.5x and 1.8 degrees which I think will prove useful so  a 26mm Panavision will give me 2.4 degrees and the ES 28mm (68) will give me 2.5 degrees. Not much between - but will the ES give a better performance in the f5 scope? I really don't want to go to an 82degree EP. I find the maxvision a big enough port hole. I think I could push to 70 but not 82! Any suggestions?

If your skies are light polluted you need an eyepiece that provides an exit pupil close to 3mm to darken the background, but at least not wider than 4mm. Therefore your 20mm MV should work perfect as a wide angle eyepiece at f/5 and 1.8*TFOV is more than enough, IMO.  Forget about the 26mm Panaview, I have it branded as the UO WS70 and even in my f/6 Dob it shows significant off-axis astigmatism, I can only imagine how poor it can perform at f/5. The 28mm ES68 will give exit pupil over 5mm so the background will look washed grey which I believe you don't want. Although, if you have access to a dark sky at least from time to time this eyepiece can be beneficial for low contrast DSOs.

[John]

The 26mm focal length in these budget 2" wide angle ranges always seems to be the weakest. The 26mm Meade QX was awful

[Charic]

Alfian, This is one article I have  saved as a favourite, its a  helpful reminder for myself.  http://www.garyseronik.com/?q=node/117

Having an exit pupil greater than the entry pupil has one positive, in as much as the image formed by the system is as bright as it could possibly be, its just that we can`t use/receive the full benefit, due to the limitations of our own pupils. 

[alan potts]

The 26mm focal length in these budget 2" wide angle ranges always seems to be the weakest. The 26mm Meade QX was awful

I believe they used to sell all their Dobsonian scopes with one, maybe the only way of getting rid of them.

Alan

[John]

I believe they used to sell all their Dobsonian scopes with one, maybe the only way of getting rid of them.

Alan

Yep - I got one with my Meade 12" Lightbridge. It did not show the scopes abilities off shall we say

[Alfian]

Hi guys and  thanks to all for the comments and interest. Charic, that article by Gary Seronik is a good read and seems to cover all aspects, he does do some good stuff. I must confess I  really like wide field views and my binos are real favourites for this. The pull of trying to better resolve objects using higher magnifications versus  seeing them in a more contextual way is always something of a dilemma. Fortunately having several bits of equipment gives me the choice but it doesn't seem to take away the urge to want it all ways! 

PJ' - the skies here are not too bad light pollution wise, some city glow on the horizon that creeps higher than I would like but compared to many folk I'm lucky. On a good night seeing can be quite startling, but being on a hill in a damp climate I tend to get plagued by hill mists that come and go, that's when its not cloudy or raining or snowing of course!  I'd considered the 2" 24mm Maxvision (the 1.25" seems not to be available any more) but its a bit too close mag and fov wise to the 20mm to make it worth it, I think. I had also considered the 25mm/60 degree ED Starguider which is reputed to be good at f5 and with being a 1.25" would be useful in my other 'scopes too and just "settle" for 2 degrees. I do appreciate though the difference  that magnification makes to contrast. Going back to the binos, the difference between what I can see with my 20x60s compared to the 10x50s is very striking despite the relatively small difference in aperture.

The lure of the 28mm Maxvision though, may prove too much (assuming I can raise the cash) and just hope my pupil will stretch to it. I must get round to measuring it if I can before splashing out.

[nightfisher]

If you would like to borrow my 25 starguider to try in your new scope you would be welcome

[Alfian]

Hi Jules, that's really kind, thank-you. When the scope arrives and I get it set up to my satisfaction, I'll drop you a PM and get something arranged.