GSO 40mm or 32mm?

[Steve777]

Dear all,

Some advice please! I want a low power eyepiece for my Skyliner 150p dob-in particular, for things like the seven sisters & Andromeda. So looking at GSO, was all set for 32mm, but wonder how 40mm would perform 'cos would fit more of the Pleiades in?

Thank you all!

[nightfisher]

The GSO 32mm is a really nice eyepiece, i dont think you would notice any extra FOV with a 40mm

[bingevader]

The 40mm has a smaller FOV than the 32mm.

I think you'll end up seeing about the same amount of sky.

[happy-kat]

I am happy with mine which is a 32mm it will invariably come with revelation on the side not gso. Same thing.

Some choose the 30mm vixen npl as an alternative. Almost same fov but it my telescope the gso was bang on the maximum fov my telescope 130p can show so I went for that.

[John]

Another vote for 32mm over 40mm here. My 1st eyepiece purchase, many years ago, was a 40mm plossl. I was dissapointed in the narrow field of view and soon did the maths and realised that a 32mm would show as much sky and have a more pleasing apparent field of 50 degrees v's the 40mm's 43 degrees. We live and learn !

I think the Skywatcher 150p dob will take 2" eyepieces. If yours can then you can go wider with a 2" eyepiece than a 1.25" can.

[Reeny]

Note: you will need the 2-inch 42mm or 30mm GSO / Revelation branded eyepieces

The 40 and 32mm versions are 1.5-inch fittings, and restrcit the field of view.

http://www.telescopehouse.com/acatalog/Online_Catalogue_Revelation_Eyepieces_34.html

I have the 200p dob and Peades does not fit in the 42mm anyway (at 1200mm focal length) so the 30mm would have been better for me.

The main problem I have with the 42mm is the 7mm exit pupil for my combination.

The background sky is washed out grey instead of black, therefore a lot of the contrast and clarity is lost - it is not an ideal situation.

You will need an eyepiece program which can calculate the exit pupil, and some advice from somebody cleverer than me.

The safe bet is the 30mm Revelation / GSO 2-inch fitting (70-degree), and not the 42mm (only 65-degree, and larger exit pupil)

[Reeny]

The exit pupil is coming in at a massive 8.4mm with the 42mm 70-degree eyepiece (at 150mm dia meter scope x 750mm focal length)

The 30mm would be slightly better at 6.0mm exit pupil.

The 30mm will be less terrible than than the 42mm.

So the 30mm will be the one to go for.

[Steve777]

Thank you all, for preventing a likely mistake!

[John]

The exit pupil is coming in at a massive 8.4mm with the 42mm 70-degree eyepiece (at 150mm dia meter scope x 750mm focal length)

The 30mm would be slightly better at 6.0mm exit pupil.

The 30mm will be less terrible than than the 42mm.

So the 30mm will be the one to go for.

I think the Skywatcher 150P dob is F/8 so 1200mm in focal length. The exit pupils would be useable at either focal length I reckon.

[ronin]

The 32 and the 40 will have very much identical fields of view, the 40mm has a field stop the reduces this.

A 32mm would give: 1.33 degrees - I have assumed 50 degree view.

Strangely the BST Starguider will have the same FoV as a 30mm plossl.

BST: 1.25 degrees

The widest view comes from the Antares W70 eyepieces, their 25mm will give (fractionally under) 1.46 degrees.

For the Pleiades I have read of it being 1, 1.5 and 2 degrees across.

Never simple is it. :grin:

[LeeB]

for the 42mm 2inch 70deg. exit pupil is 5.25mm with a 150 dob at 1200mm focal length, and they are nice eyepieces at f8. 

[Earl]

Most of the 32mm Plossls perform well and its always worth having 1 in the arsenal if your a visual observer, 2 matched  if considering bino viewers.

I have had the 32 Televue and 40mm and they are not a huge leap up from the GSO, Revelation, Celestron offerings.

[John]

The widest field I've managed to get in the 1.25" eyepiece format was the Japanese Widescan III 20mm which delivered a whopping 84 degrees. It's a great eyepiece in F/8 or slower scopes but the field edges get very messy in faster scopes. They have been out of production for some time now but do crop up on the used market from time to time.

[Reeny]

for the 42mm 2inch 70deg. exit pupil is 5.25mm with a 150 dob at 1200mm focal length, and they are nice eyepieces at f8. 

Oooops - I read 150P in the signature - but missed the dob description in the original post.

At 1200mm the 42 Revelation can make a very nice finder scope (28x mag).

Pleades won't fit everything in, and Andromeda will also spill over the edges of the view.

With a smaller exit pupil than I have with the 200p dob, it should be more usable than I originally indicated.

[Floater]

For my 150 Skyliner (see sig.) I went for the 2" low power option and am happy with the 30mm and 40mm EPs I have - although I'd be happier if I'd had the opportunity/conditions to use them more often! Most recently I added a 2" Barlow just to play around with the possible permutations. But, again, haven't had many chances for 'playing' so cannot yet comment on whether or not it's worthwhile.

As implied in the posts above, there are two types of Skyliner 150 scopes, with different focal lengths. I did the sums to calculate the exit pupil figures for mine and, as I wrote earlier, I'm happy.

[Steve777]

Hi all,

Just went ahead & ordered the 32mm 1.25 inch as per original plan-no doubt will be fine. Thanks for the advice!

[Reeny]

I think the 30mm will be more usable, more of the time.

When use the 42mm to find stuff, I still need to put the 24mm back in to see any detail.

It's nice to get the 28x binocular type of view out of the 200p scope.

But the 30mm will better, giving you the best of both worlds - a wide 70^o view and 40x magnification.