Help with observing technique please.

[mental4astro]

Hi all,

I have a problem in using anything but my lowest power while observing. I find it difficult to change EPs to enlarge an image. I know that uping the power can improve certain details, but I'm unsure on which EP will do what to whatever object.

I'm after a few obs. tips to help improve my skills. I'm a little reluctant to 'have a go', so a little technical help would be most welcome.

If it helps, my gear includes an f/4.9 10" dob, an f/4.5 17.5" dob & a Celestron C5.

Thanks in advance,

Mental.

[Paddy]

To me this seems best addressed through experimentation - playing with eps and seeing which you like for what.

For myself, this tends to be low power for larger objects like open clusters, but otherwise 130x to 180x depending on how good the seeing is. I find faint DSOs eg galaxies and PNs come up quite well with higher mags as the smaller field seems to enhance contrast. For planets I again go for the highest power that seeing allows.

But play, play, play and see what you like! I'll be interested to hear what others think.

[Lismore Bloke]

I'd agree with Patrick. I start with the Nagler 22mm for a general look at the object and to check out its surrounding area. It is also easier to find objects with the wider angle EP. If more power is needed I try the 12mm. Increasing magnification does aid contrast but the seeing plays a big part. With the moon and planets, use as much as you can until the image starts to deteriorate. I know that details seem clearer and sharper with the new TV's compared to my old eyepieces.

[ngcles]

Hi Alexander & All,

I think you might find the information in this thread useful:

http://www.iceinspace.com.au/forum/showthread.php?t=28240&highlight=exit+pupil

and I'll extract what I wrote in it here:

Mbaddah wrote:

"Will the above apply for DSO as well such as globular clusters?"


While this is not a really "vexed" question the answer is complex and something like the answer to that age old conundrum "How long is a piece of string?"

I said above that Saturn is a high-surface brightness object -- it is, but I'm now going to re-define that. Saturn is an extremely, extremely, extremely high-surface brightness object -- compared to even the highest surface brightness DSOs -- they just ain't in the same ball-park!

Perhaps the highest SB of all non-stellar DSOs are small, bright planetary nebulae (PNe) -- for example the Blue Planetary in Centaurus (NGC 3918), or the Homunculus Nebula in Eta Carinae, and they descend from there all the way to tiny (ie < 2 arc-mins) faint galaxies, faint PNe and the really difficult, ultra-faint, small, unresolved globulars etc.

This is what _I think_.

First, for DSOs that can be mostly resolved into stars (open clusters and bright globs), your first concern is to use a magnification that resolves as many stars as can be seen in your aperture, while nicely "framing" the object. If say a cluster is 30 arc-minutes diameter and assuming you want to see all of it in one go, you use a magnification that produces a field large enough to take the whole thing in, plus a frame around it (if possible) so it can be seen in context. Using the above example, I'd be thinking something like about a 45 arc-mins field or perhaps a bit bigger. My 20mm Nagler in the 18" is perfect, producing x111 and a 44 arc-min field If it is a globular you could also use a bit more magnification (ie smaller pupil) to bring out the faintest stars (for reasons below).

Second, for very large (ie super-huge) extended objects like big nebulae (Eta Carinae -- NGC 3372 or M42 or the Veil nebulae complex), the biggest galaxies (like NGC 253 or NGC 55) etc you would use all the field you can that will effectively frame it, _but_ I wouldn't exceed an exit pupil that approximately matches your fully dilated pupil. If there are bits in that extended object that you'd like to scrutinise, lift the magnification for a higher contrast view of smaller areas.

But, the bulk of Deep Sky observing (well, my Deep sky observing and I think most deep sky observer's observing anyway) are either small, very low SB objects like small faint galaxies, small faint globs or small to tiny (relatively) high surface brightness objects like many PNe.

Third, in the case of small, unresolved (ie not stellar) LSB objects, there is I feel, that there is a "magic" exit pupil that maximises the contrast between object and background sky. I believe it is a bit either side of 2.0mm. For a 10" ''scope, say _about_ x130, 12" x150 and for my 18", a bit over x200. This exit pupil range seems to darken the background while not stretching-out the little patch of fuzz so much it disappears entirely. In short it means maximum contrast. Magnification can be your friend!

If you happen to glance at some of the observing notes I post here occasionally, you will see with my 18" observing small galaxies I almost exclusively use either x185 or x247 -- which bracket that 2mm exit pupil range. That approx 2mm exit pupil is enough magnification to significantly darken the background sky, while rendering somewhat fainter stars visible, while enlarging it enough to be comfortable to see easily, while not stretching it too much to "dissolve" into the background sky. A dob of butter will only cover so much bread!

Fourth, if the object is small _and_ has good SB, like a bright planetary, you can use somewhat more magnification I regularly use x247 and x317 and sometimes x371 and x465 on PNe like NGC 1535, NGC 2392 and IC 418 with the 18" which is getting down near the 1mm size -- if the seeing allows. In say a 10" for example, this would translate to somewhere between x180 and about x260

Fifth, in the case of things like your "average" globular (lets say 6 arc-mins diameter and 8th magnitude) you are aiming for a field big enough to see it and frame it, while also using an exit pupil small enough that the faintest stars your aperture can show, can be seen. This usually translates to about 1.5mm exit-pupil or even a bit less. If you want to maximise the number of faint stars visible (or perhaps just see _some_ stars in a faint or really faint glob) go a fraction higher again.

Lastly, if you are only feeling your way, the best way to go about it is to start at low magnification and increase slowly in increments through the eyepieces you have. As you are observing and as part of this process, you will reach a point where you will say something like "I think it looked better in the last eyepiece I tried a moment ago" and you go back to that one -- and that is the best view!

But bear in mind this also works a bit like the "pirates code" -- its not a hard and fast law. Its more of a guide ...

The exit pupil a telescope/eyepiece combination produces is approximated by dividing the aperture expressed in mm, divided by the magnification the eyepiece produces with that telescope.

Worked example: 200mm aperture of 1200mm fl (ie f/6) + 16mm eyepiece = x75 magnification

200mm aperture / x75 magnification = 2.66mm exit pupil

Give a hoy if anything need clarification! Have a search through the threads on Exit pupil and you will get some answers.


Best,

Les D

[mental4astro]

Thanks for the replies.

I've reviewed the EPs I have & they seem to provide the range of values described by Les.

I'll prepare a plan for my next session out in the dark & have a crack.

I'll certainly chase down more PNe's. Not an object I've sought out too much.

The current discussions on reports has also served to enthuse to "play, play, play" & seek out detail.

Ta,

Mental.