Most useful mag/EP size for finding and viewing DSOs?

[Dazzyd]

Hi all

Not sure if this is a common thing with newbies but if I'm trying to find a DSO I initially thought high magnification because they're so far away sort of thing. I'm starting to get the idea that this really isn't the case and I should be backing off on the mag a fair bit. The 2 EPs that came with the scope are 25mm and 10mm, 25 giving me 48x and the 10 giving me 120x. I'm going to be getting better EPs next month and was initially looking at maybe 15mm and a 6mm but for finding and viewing DSOs should I maybe look at a 30mm ish sort of EP?

I'm still learning to star hop so don't really want to make things harder for myself by using an EP that is hampering me.

Thanks guys

[Jon Sales]

You're better off with a wider field of view at a lower magnification to locate the object first.

Once found centre the object in the eyepiece and then use higher magnification.

Star clusters, globular clusters and galaxies I find are better at around 15mm max as they cover a wide patch of sky.

[Tinker1947]

DSO's won't jump out of the EP at you, there little grey smudges, found best with a low power EP, once found center them increase the mag power, Cluster are the best to practice on as there bigger and brighter and some actually show as a bunch of stars...

[twotter]

Last night I was hunting DSOs with my 24mm Hyperion. With the 68 degree field of view I had M81 & M82 in the frame at the same time...lovely sight

[Uhwaz]

http://www.12dstring.me.uk/fov.php

priceless site it should give you fair idea

[AlBoning]

Personal experience doesn't support this but from what I've read there are two things to keep in mind.

1) It's easier to see a low contrast object when it is larger, so add magnifiication, and

2) A magnification that produces an exit pupil that is larger than one's eye pupil reduces the effective aperture thus raising the contrast threshold. So there is a lower limit to magnification that going below makes things worse not better. In this case it is very much dependent on the individual and their telescope. For instance, in my case the lowest useful magnification works out to be about 50X which produces a 4 mm exit pupil; but then I'm a bit of a geezer. As one's maximum eye pupil tends to decrease with age a younger person could make use of lower magnifications than an older person.

[Admiral Crispy]

It depends. For DSOs there are a few considerations. Firstly, there is exit pupils, which is a primary consideration. Divide your telescope aperture by magnification. Depending on age, you never want an answer of above 7 really, as your pupil will not eb able to grasp all of that light, and images will be fainetr. But this depends on age. Depending on whether this will give you an appropriate exit pupil or not, you want to make finding it easier, so a 2" EP, around 30mm, would be a good choice for scanning the sky. Lower magnification gives you brighter images as well, so it should help. But, there is the catch that the sky will aslo be brighter, so once you locate an object, you may want to switch between EPs to find the best contrast, as higher magnifications darken the sky.

My final point will be that your 25mm will not be much of a handicap, since it could provide a useful magnification for DSOs, by providing a darker background sky than a 30mm.

[ronin]

If your 25mm gives decent views then I would say stick with that for locating the things.

What you are looking for is a small smudge, basically not a point, somewhere in the area you are pointing at. Hopefully the little smudge will be in the centre.

Once you locate what you hope is the correct little smudge then centre it and start applying more magnification. For more magnification I would suggest 12mm and 8mm - I will say that my thoughts are on the BST eyepieces and sizes.

The initial lower magnification will concentrate the light into a smaller image, idea being that this small image of the DSO is therefore bright enough for you to see it. High magnification makes the image too dim to be initially recognisable, also the view is smaller so more chance of just not having it in view.

If the 25mm is not good, then you have to decide if you buy 2 eyepieces or 3. Usually people say the 25mm isn't that bad.

[umadog]

Image brightness is proportional to the area of the exit pupil. There is a limit to how small the exit pupil can be but I will disagree with the others: in practice you won't run into exit pupils that are too large. Yes, even if it's larger than your eye's pupil.

If you progressively increase the exit pupil so that it becomes increasingly larger than your eye's pupil you will see the image magnification decrease. You won't see the image get any dimmer because the effective exit pupil is always the same size: the diameter if your eye's pupil. It's true that you're effectively using a smaller aperture, but that's not the point: the image won't become dimmer. Yes, telescopes with smaller apertures produce a lower contrast image. But that's only a problem for high powers. You won't see a contrast reduction at low powers because the magnification is too low to show the instrument's point spread function. If you want a really wide field of view, by all means use an eyepiece that provides an over-sized exit pupil. The only minor problem you will run into is that you may see a darkening in the field centre (the shadow of the secondary) when you look at the moon.