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Walking on the Moon

Am I mad or a genius?


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I have a Skywatcher MN190 and recently acquired an Oticstar ED80S, both of which I bought second hand. I'm now starting to build up the eyepieces I think I will need and I have read up on eyepieces, to better understand FOV, TFOV, focal length, exit pupil and all the other factors that affect how they behave. As a result of this, I decided to try and find reasonable quality 82 degree eyepieces and to keep the number I buy as low as possible, whilst giving me the best options to choose from.

I played with Excel and created a table that would calculate the eyepiece focal lengths I should consider, by keeping the magnification difference between each eyepiece constant. I have a Hyperion 2.25X Barlow, so the calculations also took this in to account. I then added eyepiece focal lengths to the spreadsheet, so that each increase in available focal lengths provided a 50% change in magnification and ended up with the following:


2.1mm (Barlow 4.7mm) - n/a

3.1mm (Barlow  7mm) - 49%

4.7mm (Explore Scientific 4.7mm) - 51%

7.0mm  (Skywatcher Nirvana 7mm) - 49%

10mm (Barlow 23mm) - 46%

15mm (Barlow 34mm) - 48%

23mm  (Don't have one yet) - 52%

34mm (Meade SWA 34mm) - 48%


is my logic sound or am I missing out of the emotional aspect of purchasing eyepieces or something else more fundamental?


Edited by keybaud
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You wouldn't want to go bellow 4.7mm, that's about the limit.

EPs don't need to be perfectly spread out, just enough to give you a decent range. Observe for a while then work out where the gaps are in your collection. Your plan looks very nice, just remember when you're looking for those eyepieces you don't need to get the focal lengths bang on.

Edited by Joe12345
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48 minutes ago, triton1 said:

Mad Astronomy is supposed to be fun and enjoyable there is no need to be so analytical,clear skies oh hang on let me work that one out.

Unless of course you deem analysis as fun :) 

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It's not madness, it's part of the enjoyment ;)

I don't space my eyepieces out the way many suggest. I have a couple for wide views then a whole series close together for planetary. The idea behind that is I can increase magnification on planets gradually depending on seeing conditions.

I have mine in excel too :)

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30 minutes ago, bomberbaz said:

I am analytical like you Keybaud and looking at that I would say thats a pretty good line up. Not sure if the 3.1 & 2.1 will get used much, maybe on the Moon 

Those are the barlowed values of the 4.7 and 7, so I'm not fussed if I don't use them....but I could if I wanted to  :-)

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I find having closely spaced eyepiece focal lengths much more useful at the higher magnifications. My eyepieces go:

31mm - 21mm - 13mm - 8mm - 6mm - 5mm - 4mm - 3.5mm - 3mm

My main scopes have focal lengths of 1590mm, 900mm and 663mm.

That gives me a range of options at high power which means I can experiment to find what delivers the best results for the target / conditions.

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I spend hours calculating various magnifications with different EP / Barlow combinations. It's something to do on cloudy nights lol. This is my present combination selection for my binoviewer on a 1300mm f/l scope. The eyepieces are Astro Hutech 12.5mm and 25mm ortho's, TeleVue 15mm Plossls, Baader Classic 18mm, 32mm and 20mm WO SWAN's. There are two Barlow choices.

Eyepiece                                     0                               1.6x                  2x      

AH 12.5mm Ortho                     104x                         166x                 208x
TV  15mm  Plossl                       87x                          139x                 173x
BC  18mm   Ortho                      72x                        115.5x                144x
WO 20mm  SWAN                     65x                          104x                 130x
AH 25mm Ortho                         52x                           83x                  104x
BC 32mm Plossl                         41x                              -                       -



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Playing with Excel sheet is fun, I have all my eyepieces combinations, binoviewer+GPC+barlow combinations with telescopes in Excel:smiley:

As others have already pointed out, there're something more you might need to consider:

1. High power eyepieces (exit pupil around 1mm): these're mostly Moon, planetary eyepieces (in addition to double stars), therefore very much seeing-dependant, when seeing is excellent , a 4mm(exit pupil 0.75mm) works in your MN190, but when is bad, a 6mm might be too much, that why people tends to have tighter spacing in high mags.

2. 34mm is 2", while hyperion barlow is for 1.25", even if you find a way to connect it (say via a T2-m48 connector), it will vignette strongly.

3. My own over-analysis resulted in two zooms with an optimized barlow so that I have all the middle to high power easily covered.

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I think I've almost closed the gaps for my non-bino viewing. I have chosen the present selection (contained in two medium-sized cases along with TV 2x, 3x Barlows and a 2.5x Powermate) to be compatible with a star diagonal with a compression ring (i.e. tapered undercut or smoothie). From the shortest f/l:

8mm TV Plossl

9mm WO SWAN (x2)

10mm Delos

11mm TV Plossl

15mm Celestron Kellner

16mm Nagler T5

18mm AH orthoscopic

19mm Panoptic

20mm TV Plossl

24mm Panoptic

25mm TV Plossl

32mm TV Plossl

40mm TV Plossl

There is a gap between the 11mm and 15mm. I am working on this lol.


Edited by Mak the Night
Brain Damage
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All part of the fun, as others have said.  With my f/11.8 MCT I have 40, 32, 18, 12, 9, 7, and 6.

I too have generated a table of magnifications, and it includes the AFOV and TFOV of each EP.  It's very handy taped to the EP box when you're observing.

I'm aiming for an 8SE f/10 in the future and I shall get a good WA 24mm EP for that.  And because it can take a focal reducer to get the mag down and angles up, there'll be an additional table!

Calculations, theory, observation - fun all the way!



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