Sorry for another “what eyepieces should I buy” thread. I am patiently waiting on an Apertura AD12 and would like some guidance on eyepieces for my Bortle 4 skies. I’ve heard the 30mm that comes with the scope is half decent but the 9mm super plossl is pretty bad. For starters, I’m looking at 2 eyepieces that fall into the best bang for your buck category in the mid price range. APM seems to be the current brand that meets this criteria. I am mostly interested in DSO viewing but it will be nice to look at planets or the moon if they are in the sky as well. I’m leaning towards a 13mm XWA 100 degree that I can use with a 2x Barlow for planetary viewing. Then I’m also looking at either a 30mm UFF or a 20mm XWA.
A couple questions...
1. At f/5, I will be dealing with some coma on the 100 degree FOV eyepieces. Is this dependent on eyepiece focal length (ie. the 20mm would show more coma than the 13mm)? Would it be better to consider eyepieces with less FOV?
2. The 30mm UFF and 20mm XWA have nearly identical FOV. Would you actually consider them replacements or would they still complement each other once I start expanding the eyepiece collection more?
Any other comments would be appreciated. Thanks!
1) No, coma is dependent on apparent field. It appears the same in all focal lengths of eyepieces with the same apparent fields.
It is half as wide in a 10mm 100° eyepiece in a linear sense than it is in a 20mm 100° eyepiece, but because the comatic star has a size, it is magnified 2x in the 10mm, and 2 x 1/2 = 1.
To to avoid coma, stick to narrow apparent fields. In a sense, coma correctors made 100° eyepieces possible.
2) The 30mm UFF is a bit better corrected at the edge than the 20mm XWA, coma excepted. However, the 2 might complement one another due to the 50% difference in magnification, but only in a long focal length scope.
I see no reason to use eyepieces closer together than 45-60x at low power in a 12", and one would yield 51x and the other 76x--too close together to be viable in that scope.
I think the next logical step from a 30mm is a 15mm +/- 2mm. There are lots of users of similar scopes here on CN who flip from a 30mm to a 13mm as the next eyepiece.
Depending on your viewing preferences, you could jump 50x between eyepieces to 70x between eyepieces and have a very nice set.
One setup I am recently finding favor with is to have a favorite magnification, then have the magnifications on either side slowly increase in steps from there.
To illustrate: a favorite magnification of 160x, with the first steps on either side to 120x and 200x, then 70x and 250x and then 330x, 430x, etc.
If you favor planet and lunar viewing, you might want higher magnifications close together to hold the magnification limit to what the seeing allows.
In that case, you might favor constant steps in magnification from the bottom up, which puts the % changes to get smaller as magnifications increase, e.g. 50/100/150/200/250/300x etc.
That will make the focal lengths appear to get closer together at high power as the % increase diminishes.
Were I buying eyepieces for a 12" to start with, I'd probably start at 22mm instead of 30mm, but I have no problem with the 30mm.
I might then move to 17.5mm, 10mm, 6mm and stop there until I knew where I wanted to go.
I'd fill in the gaps as I learned what kind of viewing I favored and figured out which gap needed filling.
Your preferences might change, too, leading to a different line of eyepieces.
It took me a long time to realize my favorite range of eyepieces was 8-11mm with my 12.5".