Understanding eyepieces

[tim26792]

Morning all,

As a novice I have been trying to learn about eyepieces and the maths that you need to know to select your collection.

After reading the excellent primer on here I think I have a basic understanding, could someone check my maths!!

Chosen Telescope Skywatcher 200p f/l 1200

Based on the above telescope the highest mag would be 400x potential. I understand that mostly 400x will not be usable but for the sake of my working out I will stick with that.

HIGH POWER

3mm eyepiece = 400x

4mm eyepiece = 300x

5mm eyepiece = 240x

LOW POWER

EXIT PUPIL

30mm eyepiece = 40x 5mm

42mm eyepiece = 28x 7mm

48mm eyepiece = 25x 8mm

Thats about the extent of my understanding for now is this on the right lines?

Thanks

Tim

[Andrew*]

I'm glad you found my primer useful!

You've got the idea, but for your maximum power eyepiece I would stick to a 5mm or 6mm eyepiece at first - being able to use 400x is very rare, and would not be worth the expense of a 3mm eyepiece. And for low power, 35-40mm would be a safer bet.

Good luck finding the right eyepieces

Andrew

[southernman]

Based on what I know your maths is correct. Whether the low mags are usable with your scope is another matter. No doubt others with more knowledge will advise on that.

[johninderby]

The Televue eyepiece calculator will show you which eyepieces would work in your scope. While of course it is limited to Televue brand eyepieces it's still very useful info.

http://www.televue.com/engine/TV3_page.asp?NoBack=True&id=89&plain=TRUE&focal_length=1200&fl_unit=mm&aperture=200&a_unit=mm&fratio=&_2in=yes&Ethos=TRUE&Nagler=TRUE&Panoptic=TRUE&Delos=TRUE&Radian=TRUE&Pl%F6ssl=TRUE&sortby=SortFocalLength&title=SW+200P+Eyepieces&SUBMIT1=CALCULATE

John

[tim26792]

Thanks for th link looks very useful

[John]

Don't forget medium power - it's the most used type of eyepiece. Thats why Tele Vue (who know thir stuff !) chose 13mm as their 1st Nagler and Ethos released - it's a really useful "workhorse" focal length in many scopes (including yours)

I agree that, at the top end, 5mm would be the shortest focal length you would use regularly. At the other end of the scale I reckon 32mm is an optimal low power - 42mm and 50m just don't work well in F/6 scopes.

[MorningMajor]

John beat me to it, but I agree, don't forget your medium powers, The 2 EP's I use the most are 15mm and 30mm and along with a 2 x Barlow I can avoid endless eyepiece swapping

[twotter]

13mm really does suit my 200P dob

[tim26792]

Thanks for all the advise. Just looked at the tele vue eyepieces I assume they must be one of the best looking at thier prices.

[stick]

i agree with the others mate

i would suggest going for a 5 or 6mm for your high power, say maybe a 15 or 18 for you medium and then maybe a 25 or 32 for your low.

If you try and push the scope too far, the image will likely just be fuzzy (even if it is bigger !)

you could always Barlow the EPs to spread your range a bit but bear in mind this will also introduce more distortion

The BST Explorer ED EPS are good and reasonable value (about 40 quid each...ok not top end but better than most supplied EPs. You can always upgrade later)

best of luck

:icon_salut:

[tim26792]

Thanks