Hosted by Chris Beckett & Shane
Ludtke, two amateur astronomers in Saskatchewan who
enjoy teaching astronomy classes and showing the public views
through their telescopes. actualastronomy@gmail.com
This month we talk about the full range of telescopic
magnification powers from Low, Medium to High and how to achieve
them using exit pupil. Topics include using exit pupil as a guide
to power and how to successfully use high powers.
To recap Exit Pupils and Eyepieces:
We have High, Medium and Low powers,
- Low Power is in the Exit Pupil Range 2-4mm, so on my 180mm f6
that’s 12-24mm eyepieces giving 45x-90x
- Medium Power is around 1.5mm Exit pupils, so a 9mm for my scope
for 120x
- High equates to about a 0.5mm Exit pupil which for an F6 scope
that would be a 3mm which on my 180mm would give 360x
In addition to these we have RFT or very low power for me I use
my 40mm and 32mm for 27-35x and Ultra-high which is anything
above 0.5mm. Careful in going to low as beyond 6-7mm exit pupils
the pupil will cut into the light path.
The most favored eyepiece exit pupils are those in the 1.5-3mm
range for galaxies, globular clusters. This is where the eye
performs best under low light conditions and these magnifications
help reduce the sky brightness without reducing the object
brightness to the same extent,,,thus making such objects more
visible. This is why I use my 17mm, 12.5, 12, 10mm’s the most
when making observations of these targets. For low power wide
field nothing beats the 40mm XW which gives me a 2.5TFOV and 27x
and I use my 5mm XW lots on the planets and Moon.
Shane do you have any high power eyepieces? What is the highest
power you’ve successfully used? What are some of the challenges
with using high powers?
My introduction to using high power came by accident. I wanted to
buy all the Pentax XW’s, while designed for spotting scopes, they
work perfectly with eyeglasses and each year Pentax would put
some on sale. The 20mm is arguable the dog of the lot, the 14mm
is a bit better but the rest are of the highest quality so I
ended up with these first but soon a after the 3.5mm went on sale
and I thought…well…I’d eventually get it anyway so even if I
didn’t use it that much it’d be nice to have. Oddly I ended up
using it all the time and it is one of my most used eyepieces and
not only that but it barlows very well with my 1.6x Nikon
creating a 2mm eyepiece.
Eventually I acquired the 5mm, 7mm and 10mm and use my 2x and 3x
barlows on them. Typically you don’t want to exceed the 0.5mm
exit pupil size as on planets floaters and other eye issues
challenge the observer. So why would I go above this threshold?
So wind the clock back to the Nova East Star party in 2003, Mars
was at it’s closest point to earth in 60,000 years and Tony Jones
showed up late at night with his 5-inch Astrophysics APO
refractor. We ended up barlowing a 4.8mm Nagler 2x for
430x. The details were amazing. Roy Bishop, who has authored that
section on Exit Pupils and Eyepieces for the OH was there, he
explained that while no additional detail could be seen past
about 300x making the image this large made the details that
could be seen larger.
I’ve always enjoyed running high power on Mars and the 3.5mm on
my 60mm Tak gives splendid views of Venus in the 100x range. With
the 180mm f6 my 3.5mm gives 308x and on steady nights I have used
up to 500x. After experimenting I determined the optimal
magnification limit is about 430x and that coincides with the
actual theoretical limits discussed in many texts and online
posts.
I found that the Pentax 70-degree AFOV was a bit restrictive and
wanted to use my 84-degree Docter. I ran across the SkyWatcher
Nomad 5x Telecentric Barlow, Telecentrics don’t move the exit
pupil around and I have found sometimes SkyWatcher has a few
hidden gems…it was only about $100EU and not available in NA so I
thought I’d give it shot. It works perfectly, I get 432x and the
field of view is excellent!
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