Few questions about my first telescope purchase

[fooser82]

1.) I'm currently considering either a 5" or 7" mak-cass, it seems that as long as I use a 5mm or 9mm eyepiece respectively I can get the same magnification, so why go with the more expensive 7"?  The only difference (it seems) should be light, but as I'm interested in planetary viewing, and the planets are very bright, this shouldn't matter right?  Are there other benefits to a larger scope for planetary observation?

2.) Eye pieces with large field of view (70-100') can have the same magnification as smaller ones (40-60'), why not get the large fov?  Is it just cost or are there other pros and cons?

3.) I'm confused about the purpose of a barlow lense, it seems like I can get the same magnification with just a single eyepiece, which is cheaper than an eyepiece + barlow combo.  I do see the benefit in having two magnification levels for every future eyepiece, which is cool, but is there any other reason?

[CodnorPaul]

1 - I would always advise aperture, you would I believe get better resolution so see finer details when you increase the mag

2 - if you are only interested in planetary then there is not much benefit, as long as the eye relief is good in the small FOV EPs - I find plossls really uncomfortable to use at high mags

3 - A barlow + EP combo should be cheaper than 2 x EPs at different mags, and then you use the barlow in everything, but are you comparing apples and pears here?  You need to make sure you are comparing similar/the same quality EPs to make that comparison.  In an F15 Mak you probably don't need a barlow - my F10 scope only ever uses 10mm as the viewing conditions don't allow for more, and that gives me 200x mag, normally I find viewing at 12mm best.  5mm will be far too much IMHO.

[ronin]

1) Doubt that a 5mm will get a great deal of use, especially in a Mak. Concerning size, light gather is relevant, try Uranus with a 5" and then with a 7". The 7" wins.

2) Cost, a good 60 degree here is £50, an 82 degree £130, the 100 degrees are around £200. One thing there is difference between the field being visible and visible and sharp. How often do you vies a planet with it at the edge only ? Bet it is always in the middle as much as possible.

3) Usually the addition of a barlow will degrade the image a bit, so if planetary is your thing you very likely want the object to be a sharp as possible to draw the detail out, so a 12mm and barlow may show less detail then a 6mm, in which case you lose.

[fooser82]

Thanks very helpful!  Still a tough decision about shelling out the extra $1k for the 7" upgrade, especially as a beginner... but don't want to kick myself for not doing so either.  Ugh.

[cantab]

1) The larger aperture gives more resolution and thus allows higher powers to be useful, when seeing permits.

2) Besides cost, wide field eyepieces have more elements in them. Some observers feel that simpler eyepieces with less glass are capable of better views, but I expect only an expert putting in a good length of time at the eyepiece will be able to tell the difference.

3) Simple eyepiece designs like Plossls and orthos generally have short eye relief at short focal lengths. A 2x Barlow and a 10mm Plossl will have better eye relief than a 5 mm Plossl, making for more comfortable observing. Indeed many if not all eyepieces with short focal lengths and long eye relief have a Barlow-like lens as one of their elements.