My Collection and Potential Expansion

[Branden_Rapp]

Although my only familiarity is with Orion products, I am quite happy with their performance. At present I use a Sirius Plossl 20mm, 10mm, and 7.5mm combined with a 2x Barlow for Lunar and planetary viewing. My only widefield eyepiece as of right now is a DeepView 35mm 2", but I'm looking at getting a 45mm as well as a 25mm and a 2" 2x Barlow for those. I get a good 3 degree field of view with the 35mm and the image is crystal clear (atmospheric conditions dependent). Am also looking into a 2" ultra narrowband filter, 2" hydrogen beta, and OIII filters down the road.

[John]

Interesting collection.

What scope do you use with these ?

[Branden_Rapp]

The telescope in my pic. Orion SkyQuest XX14i truss tube Dobsonian. 355.5mm aperture, 1650mm focal length, focal ratio of 4.6.

[John]

With a focal ratio of F/4.6, a 45mm eyepiece will give an exit pupil of 9.8mm which is far larger than the dilated pupil of your eye. It's not likely to be a success I feel.

If you get a chance to have a look through an Explore Scientific or Tele Vue eyepiece with your scope you might be impressed with the results.

[Branden_Rapp]

I'll have to check that out for sure!!!

[Branden_Rapp]

If I remember correct, you want your exit pupil no greater than 7mm, correct?

[Branden_Rapp]

So even my 35mm is passing just a little bit more light than what my eye can accept

[John]

Well thats the theory !

As you get older the maximum pupil dilation decreases. I'm 55 years old so I generally try and stay with exit pupils less than 6mm in diameter.

[Branden_Rapp]

Okay. That's very helpful! Still a steely-eyed astro-man!!

[Branden_Rapp]

Would a Barlow affect the exit pupil? Say a 2x, would that basically divide the exit pupil value in half?

[John]

Would a Barlow affect the exit pupil? Say a 2x, would that basically divide the exit pupil value in half?

Yes, that would be the effect of a 2x barlow.

[rwilkey]

Hi Brandon, in my experience I have never found that a 2" Barlow works well with 2" widefield ep's, you might want to reconsider.

[estwing]

Hi Brandon, in my experience I have never found that a 2" Barlow works well with 2" widefield ep's, you might want to reconsider.

..this is where a powermate does its magic!

[Branden_Rapp]

On that note, the eyepiece in question that I was wanting to get the 2" barlow for is the Orion DeepView 35mm. It has an AFOV of 56 degrees, f-stop of 37mm. My 1.25" Plossls that I barlow all the time aren't much different.. AFOV of 52 degrees... So given that, would you still recommend something different than a 2" barlow?

[John]

I think the reason that a barlow is not great with a longer focal length eyepiece is that, as well as giving the magnification boost, a barlow has the effect of moving the eye relief outwards. This is not a problem with shorter focal length eyepieces and can be a benefit but with a longer focal length eyepiece (ie: 25mm plus) the eye position will be moved to a point where your eye will need to "hover" some way behind the eyepiece which is not a comfortable or practical experience.

The Tele Vue Powermates, Meade TeleXtenders and Explore Scientific Focal Extenders do the same job as a barlow lens but minimise or eliminate the extension of the eye relief making for more comfortable viewing. They do this through using more lenses and a more complex optical design though so their purchase price is somewhat higher than a conventional barlow lens.

Another issue is that the Orion DeepView eyepieces are a 3 element design similar to a Kellner or Modified achromat. They are really not optimised for a fast (ie: F/4.6) dobsonian scope and using a barlow lens with one will compound these issues I fear.

[Branden_Rapp]

Okay. I think I comprehend that. I have heard of focal reducers and extenders but wasn't sure what they did