Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Eyepieces How low do you go......


Recommended Posts

I have been reading a good article on eyepieces and did not realise that there is a pratical limit on how low you should go when looking at low power EP's.

If a low power eyepiece yields more than a 7mm exit pupil in obstructed telescopes, such as reflectors and catadioptrics and the sky background is bright you may see a field with a dark hole in the centre.

This does not occur on refractors as the only ill effect is that not all the light collected enters the eye.

This is assuming your pupils can open as wide as 7mm when dark adapted as the older you get the less the pupil opens when dark and this can be as low as 4mm especially for the over 50's.

But to determine the low power limit of any telescope, multiply the telescopes focal ratio by the average pupil size for your age group, for example, my F6 dob and i am 40yrs young so about me pupli should open up to 5-6mm when dark adapted, sothat is (6x5) so the longest eyepiece recommended for me and my scope is 30mm.

I found this very interesting and hope it helps anyone who was wondering what low power EP to buy.

Link to comment
Share on other sites

Ummmm, like i said i have read this from a very good book, written by 2 very well respected astronomer's (Terence Dickinson & Alan Dyer) and copied it nearly word for word, but if this "how low should you go - exit pupil" post is wrong then as the saying goes:

DON'T BELIEVE EVEYTHING YOU READ

I am still going to follow there advice, as who am i to say they are wrong.

Edited by GazofCorra
Link to comment
Share on other sites

I've used an exit pupil of 11mm on my Dob and not had problems with the secondary.

But you will be missing out on quite a bit of light gathering power as your pupil will vignette the light cone.

I'm just over 50 and work on the basis that my max dark adapted pupil is 5-6mm max (probably nearer 5). With my 10" F/4.8 newtonian this means that 25mm is probably the lowest I can go with that scope - I may need to consider moving from a Nagler 31mm to a 26mm for that reason although the 31mm is fine with my other scopes which are F/5.9, F/6.5 and F/8.

Link to comment
Share on other sites

I'm 44, and for me it's not so much about exit pupil really but about light grey sky due to light pollution.

if I go too low in power, the sky is so grey that nothing really stands out.

for this reason I am looking to buy a 27mm Panoptic (5mm exit pupil) as my home widest field and this will supplement my 35mm (6.6mm) which will only really be useful at dark sites.

Link to comment
Share on other sites

I'm 44, and for me it's not so much about exit pupil really but about light grey sky due to light pollution.

if I go too low in power, the sky is so grey that nothing really stands out.

for this reason I am looking to buy a 27mm Panoptic (5mm exit pupil) as my home widest field and this will supplement my 35mm (6.6mm) which will only really be useful at dark sites.

Hi Shane, looks like you have to contend with some pretty poor skies, makes your observations and reports even more impressive.

good luck at SGL6, hope you get some decent skies there! :(

I am now just the one ep short, I think, before I will be content with my collection and am wondering if it should be the 24mm Pan or 22mm Nag, or maybe I should buy both of them :)

Link to comment
Share on other sites

To jahmanson, and to clarify things for the OP, yes it's true that the 55 mm Plossl leads to light loss. I should explain that I wanted a "finder eyepiece" that wasn't going to break the bank: I like using just the scope and a Telrad, I don't like finderscopes. The 31 mm Nagler was too pricey so I bought the 55 mm Plossl instead. About the same true FOV, but it's true that it turns a 12" into a 7". Does the job, though, and I've never noticed the secondary causing a problem. Recently, however, Explore Scientific had a big sale here in the US; I caved and bought their 30 mm 82 degree. It weighs a lot and needs a Paracorr, but one look at M42 and I knew it was worth the trade offs.

Edited by umadog
Link to comment
Share on other sites

I am waiting on my scope to arrive (SW Skyliner 200P) but i also ordered an eyepiece at the same time, because the 200P Dobsonian focuser can accomodate 2" & 1.25" EP's, i ordered a 2" SkyWatcher PanaView 38mm, because my scope is approx f6 and i was aiming for a 6mm dark adapted pupil (6x6=36mm).

Only time will tell, but i have heard good things about the SkyWatcher PanaView EP's.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.