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Thread: How do you determine what EPs or Power to use for DSO viewing?

  1. #1
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    Default How do you determine what EPs or Power to use for DSO viewing?



    Not sure if this is better here or over in the beginner's forum but here it goes.

    I am becoming very interested in DSOs from my NY suburbs. It is rather challenging to see them . I would like to make sure I have a reasonable assortment of EPs to cover most situations.

    This is my thinking so far. You may want to skip to the questions section

    The scope we have is a 8" dob F/5.9
    I have a 30 mm, 25 mm, 9 mm and a 6.5 mm along with a 2X Barlow (giving me equiv. EPs of 15 mm, 12.5 mm, 4.5 mm and 3.25 mm respectively).
    Their powers are: 40X, 48X, 133X, 185X (Power with Barlow: 80X, 96X, 267X [basically unusable], 369X [basically unusable])

    Since I do not have electronic assistance to find celestial objects, I proceeded to calculate mathematically the actual field of field of all the EPs and also when the Barlow is in use.
    EP : aFOV
    30mm: 102 arc min
    25mm : 75 arc min
    30mm+2X = 15mm: 51 arc min
    25mm + 2X = 12.5mm: 37.5 arc min
    9mm: 23.4 arc min
    6.5mm: 9.75arc min

    Doubt I will use Barlow with 6.5 and 9 mm unless I need to split double stars

    What immediately came to mind was the size of DSO. Many appear to be smaller than 25 arc min in diameter. and surface brightness is below 12 Mag/arc min^2

    Also I figure having a pupil diameter in the range between 2-3 mm would give me greatest detail which would be when I use my 30 and 25mm EP with the Barlow (15 and 12.5 mm respectively). This yields the best compromise between brightness and detail. Optimum Magnification is found, I am told, when the pupil diameter is between 2-3 mm with ideal at 2.4mm. Max brightness is found at max pupil diameter which maxes out for some at 7mm and others at 5mm. 7mm PD will yield the lowest usable power. 5mm will give a balance of performance for Deep Sky Observing and using wide field magnifications. This turns out to be the 30 mm EP. For my scope, max magnification is when the pupil dia. is at 1mm which is with a EP of 6mm (hence why I purchased 6.5 mm for my max mag.)

    I have also heard that as the power goes up, image size goes up and everything darkens BUT sometimes the larger size will in fact make a DSO more visible.


    SO I finally come to my questions (sorry it took so long):

    1. When searching, does the size of the object influence which EP to use to search? For example, If I am looking for the Crab Nebula (6' x 4'), It might be futile using the 30MM other than to initially get you in the ball park but to actually find it, I would think you would need to switch to a 9-12 MM EP so that it will be large enough to find?

    2. Does the size of the object influence which EP to use for pinpointing a DSO? I could get in the ballpark with wider fields of view but will I really be able to find them when my available amount of contrast (due to light pollution) is low?

    3. Finally, which EP do you use to enjoy your found prize? Is a larger size necessarily improve enjoyment? In the case of the Crab Nebula, if the field of view is 60' and the Crab Neb. is at most 6', it will at most fill 10% of the area. Is this typical?

    4. Do any choose to usea EP that has a much larger FOV for viewing such a small object or is it determined by the DSO's characteristics and the current conditions play an even bigger part in which to use? I understand that if conditions are poor you can only magnify only so much and this also can limit which DSO one can find. In this case, I am more concerned about once I have found the DSO, how does one choose the best EPO to view the DSO.

    5. How do you select which EP to use to hunt down and view DSOs? Do DSO hunters try to avoid using Barlows since some small amount of light is lost?

    Any other information and the sharing of knowledge that might help this newbie is GREATLY appreciated.

    thanks in advance.
    Frank
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  2. #2
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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    For me, FOV never really enters in to the equation for DSOs. Most of them (at least the observable part) are small (or tiny). It is nice to have a wider FOV helping you search them out around a faint star field that may not show up sufficiently in a finder to hone in on the area of the object. What is really important is the exit pupil given by your scope and EP combination. For DSOs, I find that the exit pupil of 2 to 3 works very well giving more contrast. I believe this works particularly well against the light pollution as well. My method is to start with an EP such as my 22mm giving me a decent FOV and usually it will show up but perhaps be quite indistinct and then move up to my 13mm to get more detail. For brighter, smaller objects you may be able to push the magnification even more. Here's it's a bit of try it and see with your EP collection to see what works best for you regarding different types of DSO.
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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    For me, that advantage of wide fov means I don't have to move the scope as much. Plus there's a greater chance that the object will be in the eyepiece field of view. Once I find it though, it's not that typical that I need to go wider. Only a few objects are so wide that you "need" a wider eyepiece.

    I typically start at low power to find the object and the go as high as it will take. All my eyepieces are wide though (2 @ 82* and 2 @ 100*). Even with just 4 eyepieces, I rarely use the barlow, mostly because I don't want to bother with it. The only thing I might use it for is planets or the Moon.

    What you really need is a good star atlas. I use manual setting circles combined with my phone. Sometimes it gets you close but not always. Plus, I may not know what the object is supposed to look like. I might be looking right at it and not know it. This is especially true with Herschel 400 objects as some of the cluster objects are pretty small and the galaxies are faint. However, when you know what stars are supposed to be around the object, you'll know where it should be in your field of view and that will probably make it easier to find objects.
    Rob

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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    With my 10 and 12 inch dobs, I tend to use my ES 82° 18mm as my DSO finder eyepiece. This yields the following:

    10 inch - 69x, 3.7mm exit pupil, 1.2° true field of view (TFOV)
    12 inch - 84x, 3.6mm exit pupil, 1.0° TFOV

    While I also have the ES 82° 24mm, I prefer the additional magnification bump of the 18mm, while still having an ample TFOV along with a nice roomy exit pupil for bright images. Now in the 17.5 inch, I do prefer the 24mm as my DSO hunting eyepiece as it yields 83x with a 5.3 exit pupil and 1.0° TFOV. Of course I could use the 18mm with this scope but I prefer to have a little lower magnification and slightly wider TFOV for the finding process, therefore I use the 24mm in that scope. I also like to keep my maximum exit pupil as close to 5mm as possible given my age. I find that is more comfortable for myself personally. That is why I don't use something like the ES 82° 30mm in the larger scope.

    Depending on the nature of the object and where I am observing from, I may often see the DSO in the initial finding eyepiece, but not always. I will then run through the gamut of my eyepiece compliment. Starting with the 18mm (or 24mm as the case may be) for finding the object, I then move through the 14mm, 11mm, 8.8mm and sometimes 6.7mm, but rarely the 4.7mm, making notes at each step. As the magnification increases and the exit pupil shrinks, the background darkens, and the object brightens slightly, up to a point. However, there is a tipping point where you find the best balance between image brightness and detail and achieve the most pleasing view. Moving beyond that the image starts to deteriorate. However, I will qualify that with the fact that sometimes going beyond that tipping point can be beneficial. There can be cases when a specific detail is suspected at the point of best image quality, and you are required to move higher to try and confirm that detail. While the overall image quality my deteriorate slightly, the increase in magnification may be enough to confirm a specific detail's presence. It is always best to experiment, and don't get boxed in by imagined limitations. Simply put, you never know until you try.

    As for barlows, I personally don't own one. However, that has nothing to do with perceived loss of light. With typical name brand barlows nowadays, one would be hard pressed to notice any light loss. Rather, my dislike of them stems from not wanting another piece to fiddle with in the field. I just don't want to be swapping out anything else in the focuser other than my next eyepiece in sequence. Thus I prefer to have enough eyepieces to hit the magnification spots I want. I find that the simplest and easiest way to observe for myself, and I can run through my eyepiece series easily and efficiently and have no need of the barlow. But understand that is purely my personal preference, and I realize not everyone has the luxury of having enough eyepieces to do that with, and thus having a barlow can greatly extend their capability.
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  5. #5
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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    Hello Frank,

    for the navigation and star hopping, an EP yielding about 2° true field of view does a very good job.
    Most of the deep sky objects within the reach of my scope can be nailed down with the hunting EPs, yielding the exit pupil of 3mm or slightly higher.
    Small galaxies, details inside the bright galaxies and bright diffuse nebulae, are best through the exit pupils 2 mm down to 1.4 mm.
    Globular clusters, close double stars, and small planetary nebulae are best viewed through the exit pupils of 1 mm and down to 0.8 mm.

    So, it is basically the exit pupil ruling the choice of the eyepieces.

    Best,

    JG
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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    I have the luxury of pretty dark skies (blue zone), so contrast is seldom an issue. I start with my widest eyepiece (27mm FL = 7mm Exit Pupil) to maximize the chance of finding the object in the FOV. Once I have found it and centred it, I might move up to shorter eyepieces to see more details.

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    Default Re: How do you determine what EPs or Power to use for DSO viewing?

    For DSOs & Faint Fuzzies, I find that low/medium power is needed to find the object, and then medium/high power can be used for a closer look. I recommend UHC filers for planetary or emission nebulae beyond the Messier Catalogue. I have a 10" f/4.7 Dob, focal length 1200mm. I like using a 15mm (80x) or 20mm (60x) 1.25" Orion Expanse eyepiece for hunting, then switching to my Celestron X-Cel LX 12mm (100x) for a better view. The Night Sky Observer's Guide, in my reading, generally recommends 100x mag for 8-10" scopes on most DSOs, except wide-field open clusters like M44 or M45. Just my 2 cents.

 

 

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