Low mag eyepieces

[Unicronicus]

We currently have a Vixen NPL 30mm eyepiece as our wide angle view of the night sky with our Explorer 130p, but even with the Andromeda Galaxy it cannot fit all of it in completely. We would like to go for a lower mag eyepiece if possible but what is the limit with this scope and what is the formula of calculating it? I would also like to piggy back another question on top of this. We do want to upgrade to a 12" dob soon, but the focal length of this particular scope is circa 1500mm compared to our 650mm now. Based on mag= focal length / EP this would make our current 30mm even worse at viewing Andromeda in all of its glory, so how do you go about buying bigger scopes but retaining a good FOV?

[RikM]

The Andromeda galaxy is best seen in binoculars really.

The only way to gain field of view with a larger telescope is to spend vast amounts of money on widefield eyepieces. But even so, large telescopes are not widefield instruments.

The thing to remember is that once you reach the maximum exit pupil for your eyes, a larger telescope won't make the object any brighter. E.g your 130P is f/5, a 300P is f/5 so a 35mm eyepiece will give a 7mm exit pupil in both scopes. The object will appear the same brightness but it will be much larger in the 300P.

[Paul73]

You need to consider wide field eyepieces to get the wide views. Your Vixen (good eyepieces BTW) only has a 50° ish field. Sadly to get a wide angle eyepiece of that image quality, you need to spend a bit more.

The Max Vision 68° range are very good ( or the 82° version, if you don't mind spending a bit more).

Sorry.

Paul

[John]

The only way to go much wider is to move to a 2" eyepiece but I don't think your 130P can use them. I have a 31mm Nagler eyepiece which gives a true field of view of 1.7 degrees with a 12" F/5 scope - very wide for such a scope but not enough to fit M31 in and thats about as wide as even 2" eyepieces can go. With my much shorter focal length 102mm refractor though the 31mm nagler gives a 3.8 degree true field which is getting there although M31 still flows over the edges of the field on a dark night.

[Stu]

To answer one of your questions about how to calculate the actual field of view, the simplest method is to divide the apparent field of view of your eyepiece by the magnification it gives in your scope.

For instance, your 30mm NPL has an apparent field of view of 50 degrees, and gives 650/30 ie 21.7 times magnification.

This makes the actual field of view 50/21.7= 2.3 degrees.

Andromeda is around 3 degrees by 1 degree so will more than fill your field of view.

I think the only slightly wider option available to you in 1.25" format would be the 24mm 68 degree maxvision or TV Panoptic but that would only give you 2.5 degrees.

Bear in mind that there aren't that many very large objects up there; Andromeda Galaxy, the Veil, North America Nebula etc these are perhaps best seen in their entirety with binoculars or a widefield refractor under very dark skies. I have a 4" f5 refractor which will give 5 degrees with a 31 Nagler and is wonderful in the right conditions.

As has been mentioned, larger dobs have many things going for them, but very widefield views is not one of them. Inevitably as aperture increases, so does the focal length, even if you have a very fast f4 or faster mirror. The benefits are obviously higher resolution and brighter images but at higher magnification. A 16" dob will show detail in the Veil that I can only dream of with my 4", but it will only show a portion of it at once. I cheer myself up by being able to take in the entire complex, but in much less detail and requiring darker skies and more patience at the eyepiece.

They are two very different views, but each have their merits.

I guess all that rambling leads to to conclusion that no one scope can do it all, so feel free to use this post as an excuse to build up a collection :-)

Stu

Sent from my iPhone using Tapatalk

[Unicronicus]

Cheers for the tips guys, duly noted for when we get the next scope, which is hopefully going to be a Revelation premium 12" dob so can take a 2" eyepiece. I like trying to resolve the fainter and larger objects in a smaller aperature as well in a wide field of view, it is pretty spectacular when get your eye in and the conditions are perfect. But I think my desire for more detail outweighs that argument for me, I would also like to spot low magnitude DSO's as well. Of course, we could always keep the 130p for lunar observations, larger nebulae etc... But we need the money to part fund the new scope, so I think a pair of decent cheap binos might suffice, any suggestions for a pauper like myself?

[swamp thing]

 so how do you go about buying bigger scopes but retaining a good FOV?

You don't.

The advantage of aperture is image scale. You don't want to be observing wide field with a big scope thats not what they are about. 

Aperture provides you with a much bigger image at the same brightness. For wide field use a small scope

Big scopes excel at giving close ups. Check out the dust lanes in M31, Look for HII regions in M101. Get inside M13 This is what big scopes do. for wide field views of objects like M45 the double cluster etc, use a small scope. 

[bish]

You don't.

The advantage of aperture is image scale. You don't want to be observing wide field with a big scope thats not what they are about. 

Aperture provides you with a much bigger image at the same brightness. For wide field use a small scope

 

Big scopes excel at giving close ups. Check out the dust lanes in M31, Look for HII regions in M101. Get inside M13 This is what big scopes do. for wide field views of objects like M45 the double cluster etc, use a small scope.

Good point. To me the double cluster looks best in some decent bins (under a dark sky). I prefer M31 in my 15x70s than in my scope.

[bingevader]

The 130p-DS has a 2" focuser, I don't think the others do, so your 30mm is about the limit as the 40mm plossl doesn't really provide much of an improvement in field of view.

The Maxvision eyepieces are a possibility, but the longest focal length at 1.25" appears to be the 24mm.

This is a wider field of view eyepiece at 68°, but at a lower magnification, so actually ends up almost the same size in degrees of observable sky as the 50° 30mm plossl.

[Moonshane]

I don't necessarily agree. I have a 12" f4 dob and it is the best of most worlds. Easily fits in M45, double cluster,beehive and the M31 trio to the 1.94 degree field.

[Moonshane]

Obviously not all at the same time :0)

[jetstream]

Use the field stop (FS) diameter to calc the true field of view (TFOV). TFOV = FS/Focal length x 57.3 . This is independent of the apparent field of view. I think the max FS for a 1.25" is 27mm and for a 2" 46mm. Televue has great info http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=79#.VCyYZGctB9A

For most galaxies get an EP between 2mm & 2.5mm exit pupil and let the mag fall where it will, I also like the view of the Double Cluster etc in my 10" much better than my smaller scopes and don't see much in binoculars. The Double Cluster is great in the 10" f4.8 and will be super in the 12" f5, with a nice widefield EP.

The 12" will blow the 130mm away in terms of resolution on the planets and the moon, seeing and optical quality considered.

You are on the right track of searching info before you buy