Opinions and suggestions on adding the next eyepiece in a collection

[04Stefan07]

As my signature states below I have a 130 SLT and Skywatcher Refractor.

- 130 SLT (apeature: 130mm, fl: 650mm)
- Skywatcher (apeature: 90mm, fl: 900mm)

I currently have a 25mm that came with the Skywatcher and a Celestron 9mm X-Cel LX (which I bought after).

A few questions on adding another one or two more eyepiece based on my telescopes.

1) What would be the lowest power I can go to still get use out of the eyepiece and scope?
2) Which other eyepieces (in mm) would be appropriate to add to my collection? I was thinking maybe a 15mm and replace my 25mm with a better one.

Example: 9mm, 15mm, 20mm, 25mm

I use 1.25" and mainly look at the Moon, Jupiter, Saturn, bright stars and eventually nebulae.

I have another thread asking which brand to get but this thread is asking what is a good range of eyepieces to have.

Thanks

[barx1963]

Stefan
There are a few aspects to your question. Firstly I will limit my answer to the 130mm scope as I have some experience with scopes of this size. There is an oft quoted and oft misunderstood idea of "maximum practical power" of 50x per inch of aperture which in a 5" scope gives a theoretical maximum poer of 250x which would equate to 650/250 = 2.6 so in theory a 2.5mm EP can be used. This is purely theoretical and will require perfect conditions. In my experience the maximum is probably closer to 10x or 15x per inch or 50x to 75x in this scope. This power will give reasonable views in your scope regardless of seeing as long as the sky is clear. If it is still, then pump it up. My old 5" was a FL of 900, and I found a 10mm EP giving 90x was next to useless.
So that gives us some clues. Using your 25mm gives 650/25 = 26x so that EPO is fine. The 9mm you have gives 650/9 = 72x so that will be OK for good night. To get a mid range say a 12mm EP gives 650/12 = 54x.
So I would look for some in the 12 to 15mm range.
If budget is not a serious object the trusty 13mm T6 Nagler is a cracker of an EP, but at $339 is a serious investment. For an economical option the 15mm GSO/ Bintel superview ($49 at Bintel) gives 68deg AFOV and 15mm of eye relief.

Malcolm

[raymo]

Hi Stefan, If you want some wow factor on a budget, I suggest the 30mm
80 degree wide angle 2" eyepiece that sells for about $80. I think
Andrews still sells it. They also do a 15mm version. I have one I use in my 10" dob, so I used it in my 130SLT. The 30mm has a screw on 1.5x
barlow, so you would get 22x-33x-44x and 66x from the two eyepieces.
I don't know if your 90mm Skywatcher has a 2" focuser, but if it has,bonus. Incidentally these eyepieces have been reviewed by several
magazines etc: and have been rated as great value for money.
raymo

[04Stefan07]

Thanks guys! Still learning my way around eyepieces, never knew it was such a detailed topic.

[Raj]

Hey guys,

I just bought a 2" 30mm 80deg Amdrews EP second hand, didn't see that it has a screw on bit...hmm, will have another look. I see Andrews sells it for $169. Am I thinking of something different? Excited about using it, have to wait until next weekend though :)

Thanks

Raj

[raymo]

Hi Raj. Being as you bought it second hand, maybe the barlow is missing.
The barrel is chromed of course, and internally threaded. The barlow
section is black and approx. 25mm long and screws into the end of the barrel like a filter would. It has the same thread as
2"filters, so will fit any 2"eyepiece . It came with the barlow section in
place, so a newbie could be forgiven for thinking it was part of the
eyepiece. Hope this helps. Maybe it is also sold without the barlow.
raymo

[Raj]

Hey Raymo,

Thanks for that, just had look, I am able to unscrew the metal barrel until all that is left is the lens with the hatched grip, no attachment otherwise, so I didn't get the Barlow it seems, or this version didn't come with one. Oh well, going to buy a Barlow anyway, so that's ok.

Thanks again.

Cheers,

Raj