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Hello. True beginner given an Orion Skyquest XT8 (1200mm focal length; f5.9). I have had to collimate (all sorts of fun that was) as the scope had been moved quite a bit over time. Believe I have it very near perfect but will star test. Scope came with an Orion 25mm Plossl eyepiece so I am exploring what range of additional eyepieces I would like. From what I’ve read this scope is capable of a theoretical 400x magnification. Again in theory that would take me to 3mm as limit of eyepiece. But then I read about exit pupil limitations and scratching my newbie head. The majority of what I see suggests .7mm exit pupil minimum...? But it appears I would need to buy a much larger eyepiece focal length to avoid too small exit pupil. I wear eye glasses so would be buying longer eye relief pieces in case that is relevant. Advice truly welcome. Neill
This text considers the previous thread http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/ but aims to show how to build the collection using the exit pupil explicitly. I tried to report the information in a more systematic and clear way, adding explanations and details which were under the lines. To me this thread should be considered as a complement of the previous document (which is an evaluable one), not a replacement. Introduction The reason for building up a collection using the exit pupil is that this is independent of the telescope (therefore different from the magnification which is specific). The exit pupil is the size of the image exiting from the eyepiece and coming to our eye. Because of our eye pupil can shrink up to about 0.5mm and enlarge up to about 7mm, we are interested in eyepieces within the range 0.5mm-7mm. Alternatively, the exit pupil can be interpreted as a measure of brightness. Why are we focusing on image brightness, instead of object magnification? First of all, there is an important relationship: the larger the exit pupil, the brighter the image, the lower the magnification. We focus on image brightness because targets in the sky can be somehow divided by groups according to their brightness. To observe very dim objects one should consider eyepieces showing a bright image (therefore we use an eyepiece which has a large exit pupil or alternatively has low power). If the exit pupil is too small, the object might not even be visible. The useful exit pupil range for this class is 3mm-7mm. Examples of targets belonging to this class are extended nebulae and faint galaxies. When observing moderately dim objects, one could push the magnification a bit more. This magnification comes to the cost that the overall image becomes dimmer. In this case the observer is using a medium size exit pupil or medium power eyepiece. The useful exit pupil range for this class is 1.2mm-3mm. Examples of these targets are bright galaxies, open and globular clusters and planetary nebulae. When observing bright objects, the observer can magnifying even more. Although the overall image becomes dimmer, the object will still be bright enough to show some detail. In this case, the observer is using a high power eyepiece or using a small exit pupil. The useful exit pupils for this class are <1.2mm. Examples of this targets are planets, moon, double stars and certain planetary nebulae. This is the general reason why it is said that one would need only three eyepieces to observe a lot of targets.Choosing an eyepiece set The next point is to use the previous information and select a set of eyepieces. This will require some basic mathematics, but nothing more than multiplication or division. Let’s start by defining the telescope focal ratio. This is the telescope focal length divided by telescope aperture (e.g. telescope aperture: 200mm, telescope focal length: 1000mm, telescope focal ratio: 1000/200=5 or simply F5). NB this has been corrected from f6 Now, an easy way to calculate the exit pupil is dividing the eyepiece focal length by the telescope focal ratio. For instance, an eyepiece with focal length of 25mm, will have an exit pupil of 25mm/5=5mm when used with the telescope proposed in the previous example. Now it is the time to add some constraints. As mentioned above, the human eye pupil (whose diameter controls the amount of light we can detect) can shrink / enlarge in the range 0.5mm-7mm. Of note, some reports show that the exit pupil decreases with age, and that the maximum exit pupil in old people is about 5.5mm. I will not consider this latter point in this discussion though.Most of us live under moderately light polluted skies. A large exit pupil could increase the sky background brightness so much that it appears light grey. Distinguishing between a light grey object and a grey sky is a difficult task for our eye. Our eye would work out contrast differences a bit better when the sky is darker. In obstructed telescopes, a large exit pupil under a light polluted sky could also reveal the shadow of the secondary mirror which is better to avoid. Therefore, under moderately light polluted sky, it would be better not to have eyepieces with more than 4mm-4.5mm exit pupil.Astronomy is an outdoor hobby and obviously affected by atmospheric conditions (seeing). Unfortunately, not everyone lives under pristine dry skies which are not affected by wind and other kind of atmospheric turbulences. Seeing can limit the maximum useful magnification. In the UK, most observers rarely go beyond 200x-220x. For a Dobson 8” F6, this means that an eyepiece with exit pupil <1.0mm (200x) might not be used very much.The above constraints show that we are interested in a range of eyepieces giving exit pupils between 0.5mm and 4mm-4.5mm, with a maximum power of about 200x.To build this eyepiece selection, we start from an exit pupil of 1mm and use a factor of 1.4x. This factor is the minimum to see appreciable differences in terms of image brightness (or magnification). Therefore we obtain the following range: 0.5mm - 0.7mm - 1mm - 1.4mm - 2.0mm - 2.7mm - 3.9mm For a Dobson 200mm F6, these exit pupils will give the following eyepieces focal lengths (up to 200x): 6mm - 8.6mm - 12mm - 16.8mm - 23.5mm. Of course one does not need to get all these eyepiece focal lengths, but could simply select the lower power 24mm, the medium power 12mm and the high power 6mm (3 eyepieces). Alternatively, s/he could get a barlow 2x in combination with the 24mm and 17mm to get an additional medium power 12mm and medium-high power 8.5mm (2 eyepieces + barlow 2x). For a Maksutov Cassegrain 127mm F11.8, the above exit pupils will result in the following eyepiece focal lengths: 8.6mm - 12.1mm - 16.9mm - 23.6mm - 33.1mm - 46.3mm . Again, not all these eyepieces are needed, but one can play a similar game as for the dobson 200mm case. Other considerations: Consider eyepieces with similar apparent field of view (afov, as indicated on the eyepiece). E.g. a Tele Vue Nagler 13mm (afov: 82 degrees) and will show about the same real field of view of a Plossl 20mm (afov: 50 degrees). These two eyepiece will not work nicely together.Eye relief. This is the distance between the eye and the eyepiece lens. This was not really an option in the past, but thankfully nowadays many eyepiece companies are becoming quite sensitive to people wearing spectacles. Eyepieces with focal lengths above 20mm are generally comfortable in terms of eye-relief. Below that, the eyepiece eye relief could be tight or really uncomfortable (e.g. plossls) if one needs to wear spectacles to observe (e.g. people suffering from astigmatism). For these people, eyepieces with 20mm eye relief should be considered.If an observer lives under dark skies and is interested in very faint targets, the investment in a very low power eyepiece could be worth being considered (e.g. 6.0-6.5mm exit pupil).If an observer is predominantly interested in planetary or double star observation, it could be worth considering closing between eyepieces gaps at high power or evaluating the option of a zoom eyepiece.
lets imagine I wasn't to see a nice DSO about 15' size and I think it should look good nicely framed with a 1 deg field of view in the EP.. Which would give the better (or higher probability of seeing anything at all ) view from a semi urban light polluted home site (e.g Bortle 6)? a) an 100mm f/6 refractor (fl 600mm) and a 10mm EP (60 deg afov, gain 60x = fov pf 1 deg) (and exit pupil of 100mm / 60 = 1.6mm) or b) a 200mm SCT with focal reducer to give f/6 (fl 1200mm) and a 20mm EP (60 deg afov, gain 60x = fov of 1 deg) (and exit pupil of 200 / 60 = 3.3mm) My gut feeling is that the SCT should give a better view just based upon its 2xaperture - but Im not sure I understand fully the maths why. Is the larger exit pupil going to result in a better / brighter / more successful view? Or will the view be 'roughly' the same ? Or have I got it all wrong..... Thanks.
Hi all, I am thinking about getting a panoptic 41mm for my VX 16 F4 dob (which has an FL of 1600/1840 with paracorr). I worked out though, that the Exit Pupil would be 10.25mm, or 9mm with the paracorr. That's pretty huge, and definitely bigger than anyone's maximum pupil dilation. It would give me the widest true field possible in my scope though (AFAIK). Are there any diasdvantages to having an Exit Pupil this big? Does it rob the view in any way? If anyone has experience of using a similar combo, that would be great info. Cheers!
Had alot of fun observing galaxies last night, despite poor transparency. Picking up dozens of galaxies in Virgo, Leo triplets, and M51, M63, M94. I did however fail to detect the Galaxy cluster in Leos mane. (ngc 3185, 3189, 3193 etc) I`ve observed them before (3 of them), but I cant remember the eyepiece focal length used. Can you recommend the best optimal Power / exit pupil for these faint Objects? I use 8"dob F6. Regards Rune