Exit pupil

[Popeye (Bert)]

Well I have been having fun with my PST for some weeks now, in spite of a dreary sun surface. But I am not getting the best out of it due to my lack of suitable EPs.
The 20mm Kellner EP that comes standard is pretty useless as the image, though sharp is very small. I do a lot better with my 15mm Plossl, but still not enough detail. I have been advised that some folk use around 10 - 12mm EPs successfully. So I tried the Kellner with a 2x Barlow. Now I am closer to 68 than 67 so things like eye relief and exit pupil are important to me as my pupils don't contract as much as those of a younger person. I cannot get any joy out of the 20mm plus Barlow as the exit pupil seems to be very large. I have calculated that the exit pupil with the 20mm given the 40mm aperture of the PST is 2.00 and I can handle that. But I don't know how to calculate the exit pupil with the Barlow. I suppose its academic really as I just get to see a blob of orange/red instead of a decent image unless I pull my face away from the EP and look around the edges of it for just glimpses of a much larger limb. Any ideas on how to work out the exit pupil with a Barlow?
Cheers!
Bert

[Dennis]

Hi Bert

Here is a Sky & Telescope article where the author writes:

“In my 12.5-inch, I really can't see the difference between the 180x view in a 10.5-mm eyepiece and the 180x view in a 26-mm eyepiece of the same design with a 2.5x Barlow — except that the latter gives more comfortable eye relief.”

Cheers

Dennis

[Popeye (Bert)]

I can understand that, Dennis. I have no problem with the Barlow on my 200mm LX90. So going to 300+mm should be even easier. But exit pupil is aperture/power and by comparison the PST only has an aperture of 40mm and I am trying to work out if the smaller aperture makes for larger exit pupil using a Barlow. Its how it seems to me as I cannot get a decent view as is. If I take my eyes well away from the EP I can just look into the EP and catch glimpses of parts of the sun. This seems to indicate a larger exit pupil (too large,) situation to me. I might have the chance to try out other EPs next Saturday and if a 9.5 - 12mm EP works for me then I will buy one.
Cheers!
Bert

[OneOfOne (Trevor)]

I think with solar viewing the exit pupil would not be terribly critical as the observing will be done during the day, in full Sun. Therefore the pupil will be virtually fully contracted anyway, so even an exit pupil of 4mm may be excessive. At night time however, it is dark and the pupils will be dilated considerably, making the exit pupil more important.

I have found that putting the Pentax XW-10 or 14 give a larger image, but because of the wider view (70 degrees) you can see the whole Sun (a Plossl is usually 55 degrees, I don't know what sort of view a Kellner gives). The larger eye lens also makes viewing much easier, the eye lens in the Kellner that comes with the scope is pretty small. All of the wider angle eyepieces I have seen have a much larger lens, so this may also help. Maybe you could try a wider angle eyepiece with a shorter focal length and see how you go. I think your thoughts of 9.5 to 12 is probably on the money!

BTW, the Sun is pretty boring at the moment, haven't looked at it for a couple of months now!

[Popeye (Bert)]

Thanks for the idea of the wider angle EP! We (AST) have a meeting at Ross (Tassie) on Saturday starting with some solar viewing in the afternoon so I will see if I can get the use of different EPs from the members to try out.
You're right about how quiet the sun is at the moment. One of my mates described it as bland!
If you have a 2x Barlow I would be grateful if you used it with a 20mm or so EP and let me know how it looked to you. (I just need the clouds to clear at the moment.)
I've mounted mine on an EQ2 without the counterweights and find it very steady and so easy to track!
Cheers!
Bert

[casstony]

I use a Vixen 8-24 zoom in my pst and find that I usually use it at around the 12mm mark. I have XW's 14mm and 10mm, but they don't appear to provide more detail than the zoom and the 10mm often provides too much magnification for the conditions. Carrying the pst and zoom eyepiece outside is very convenient too. If you wanted a single fancy eyepiece the Pentax XF12mm might be worth a try.

[Popeye (Bert)]

Thanks for that, tony. I will see if I can get a look through one on Saturday.
Cheers!
Bert

[NQLD_Newby]

Hi there Bert,

Please remember I am only a beginner and I may be corrected, but I'm almost sure the formula for exit pupil is EP focal length / Telescope focal ratio. Therefore a 10mm EP in a 4.5" f/5 telescope will give you a 2mm exit pupil, while a 10mm EP in a 12" f/5 telescope will give you a 2mm exit pupil. Arpeture does not effect exit pupil if the f/ratio of the two scopes are the same. provided I am correct???

To answer your question about a barlow. Effectively a barlow changes the focal length of the scope and therefore the f/ratio, if it is a 2x barlow the focal length will be doubled, and therefore the f/ratio doubled. This means a barlow should give you a smaller exit pupil for a particular scope and ep. once again, I may be corrected.

If I am wrong someone please correct me, but this is how I understand it.
Hope it helps.

[Popeye (Bert)]

My understanding of exit pupil is aperture/power. So with my 20mm EP and a 400mm focal length of the PST gives a power of 20X. Aperture is 40mm divided by 20 = 2 ie exit pupil of 2mm. This is fine by my ancient eyes! Which is why I cannot see how the Barlow makes a difference since, by its addition the power should effectively be doubled. Therefore 40/40 = 1. Thus an exit pupil of 1mm - even better! Your formula is a different matter and may be the clue to what is bothering me. Yet it makes no difference on the 200mm LX90! I think I will just go away and meditate for a couple of hours to get rid of the headache I have thinking about it!
Cheers!
Bert

[Popeye (Bert)]

It has finally dawned on me why I was having problems with 'exit pupil' on the Coronado PST. I could not understand why, when using a higher power with good eye relief (eg Barlow and 15mm Plossl,) the exit pupil made life difficult for me. Even though I looked down where the EP fits I did not take into account the very small (2 - 3mm) exit aperture on the PST - until prompted by a mate! So, unlike a normal scope where, say, the star diagonal is open to 1.25" all the way this is closed allowing only a very small light source. I bought a 9mm EP with excellent eye relief and still had problems. Calculated that the exit pupil should be .9mm - easy for me. I have now settled for an Orion 9mm 'Expanse' EP with 66 degrees FOV and love it.
Hope that clears it all up!
Cheers!
Bert