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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