Minimum exit pupil

[FPM]

Hello

i have a skywatcher 6” f8 dob. The most powerful eyepiece is an 8mm TV Radian.

i had thought that a 6mm or 6.5mm EP would be a useful addition for planetary viewing, but am concerned that this would give an exit pupil of less than 1.

Any advice on this? Also- would the drift across the view be a major problem at these higher magnifications? Presumably, an EP with a wide FOV would fix this?

Thanks!

[DirkSteele]

I would not worry too much about this if you are considering a 6mm eyepiece. 0.75mm exit pupil is still very useable for planetary viewing and the 203x very much an appropriate power with a well collimated scope when seeing conditions permit. If you go smaller still, eye defects like floaters can start to become apparent but some observers (me included) are not that bothered by it.

As for actual field, the wider the apparent field (as long as it is well corrected) the more useable drift time you will have. A 100-deg AFOV would yield 0.49 degrees in you scope which is just shy of the full moon and would give you around 100 secs drift time from field stop to field stop when viewing the celestial equator. A 50-deg AFOV that a typical plossl provides, you can half those figures.

 

[FPM]

Thanks, Matthew. That’s very helpful.

[Chandra]

Right now your Radian 8mm provides you with what is known as resolving magnification, that is, the one that provides an exit pupil of 1mm (the magnification equivalent to the effective aperture). A 6.5mm eyepiece will be around the maximum effective magnification of your optics, that is, the one that optimizes the light collection capacity and the resolution capacity of your aperture. Of course you could continue increasing the power with 5mm and even 4mm eyepieces, but going beyond 200x-250x will always be difficult with any aperture due to atmospheric conditions (seeing). Getting quality views from those magnifications requires good seeing conditions, and there aren't many nights a year that provide them. Perhaps only the Moon, as a very bright object with high contrast details, could support them. However, this also always depends a little on each person's level of demand. On the other hand, as DirkSteele has very well explained to you, the more you reduce the exit pupil, the more likely you are to be bothered by the "eye floaters" (we would talk about exit pupils below approximately 0.6mm), and also the more you get older...

On the other hand, light is dispersed more when passing through higher power lenses, which can result in a blurry or distorted image that is more difficult to focus correctly. In addition, increasing the power of the eyepiece also affects the amount of light that reaches the observer's eye (the image becomes less bright). And as our colleague told you, the field of vision is also reduced. You can compensate for this by using larger apparent field eyepieces, but good large apparent field and short focal length eyepieces are expensive, so you'll probably end up with average quality eyepieces that may give you that larger field of view, but the viewing quality of the object will worsen rapidly from the center of the field towards the edges... 

From my point of view, although with apparent fields around 50º-60º, there are good eyepieces (planetary) with a contained cost, such as the Ortho from Baader, the UWA TMB Planetary II or the BST Starguider. You can also choose to get a good Barlow (for example the Televue 2x), and with your Radian 8mm you would be at the maximum magnification of your telescope, on nights with good seeing you could take advantage of that configuration.

[FPM]

Thank you, Chandra. That is vey clear.

[Mr Spock]

Around x200 will be fine - 6mm. With my 4" apo I use a 3.5mm eyepiece for x211, exit pupil 0.47mm. The moon looks great at that.

For Jupiter I prefer a 4-5mm eyepiece. So x185 to x148, depending on conditions.

With my 12" x304 (5mm eyepiece) gives an exit pupil of 1mm. That's about the minimum I use on the moon. When seeing is decent I use a 3.3mm eyepiece for x461 and 0.66mm exit pupil. I still won't go above x190 (8mm eyepiece) for Jupiter though.