Eyepiece advice for fast Newts

[ColHut]

I saw this posted here and wondered what users thought.
(http://www.astromart.com/articles/article.asp?article_id=50)

Let’s say you have an excellent fast Newtonian, but you’re stuck with eyepieces that don’t work well with it. You can’t afford a Paracor and all-new premium wide-angle eyepieces. What to do? I’d suggest getting a high quality Barlow lens that multiplies about 1.5-1.8X. This will both increase the power your present eyepieces deliver, but it’ll also radically reduce the astigmatism you see near the edge of their fields, because your eyepieces will be working with a shallower light cone. And, now that you’re effectively using a longer-EFL telescope, you can ad something like a 35- to 40-mm wide angle eyepiece to get back the lowest power you lost when you added the Barlow.

regards

[bmitchell82]

definately this could work, but your adding in alot of stuff and your AFOV will not be the same, i thought about this idea a fair bit when playing around with Starry nights pro 6, i entered all my eye piece combinations in with all my barlows and what was surprizing that you got the best FOV from the eye piece alone and plus you would have to have a good formula to figure out what eye piece you would need to get back to your original magnification :D.

If your seriously into visual and you have a fast newt aka <f5 then bite the bullet and buy a paracorr. they are tuned for visual use with adjustable top to get the distance from the lense to eyepice correct.!

Just like Photography, we need to get a MPCC or else your photos look like your heading into warp travel!

[Wavytone]

Best focal ratio for amateur-sized Newtonians is f/7.

Seems to me a lot of people are buying faster ones without realising the effects of coma, astigmatism and field curvature will severely limit the useful field of view and, with what you'll spend on extra bits of (expensive) glass to try to alleviate these, they'd be better off if they had opted for a slightly longer scope in the first place.

If compact tube length really matters then buy an SCT.

[Satchmo]

Gotta love those old Patrick Moore books :)

[Wavytone]

Mark, I'm not repeating this from Patrick Moore (gawd)...

I suggest this because:

1. an f/7 scope permits a range of magnification from highest to lowest of 10:1 utilising entirely reasonable eyepieces.

With longer focal ratio scopes, notably from f/10 upwards, there is not much point in eyepieces under 8mm, whereas achieving a maximum 6mm exit pupil requires huge eyepieces.

Conversely, scopes f/5 or less can't really utilise long focal length eyepieces because the eye pupil would be too great - as you may recall I had an extreme example of a f/3.7 Newtonian in which anything longer than 15mm was pointless.

2. On the occasions you want high power an f/7 can deliver, with eyepieces in the range 7mm down to 4 or so. Plenty of good choices here and they don't have to be expensive, either.

3. When you want wide fields an f/7 can fill a 2" barrel eyepiece with a glorious field of view edge to edge.

4. The consequences of having a very large secondary obstruction (30-33%) vs a small one...

5. Aberrations. I suggest that you of all people know full well the mathematics of coma, astigmatism and field curvature of short focal ratio Newtonians. At f/4 the diffraction-limited field of view is just a few minutes of arc across.

[BC]

Yeah, I would have loved an f/7 10" when I started, but "a lot of people are buying f/5" because that's what is in the shops for a remarkably good price. I'm not aware of f/7 10" for under $1k, but maybe I didn't look in the right places. I think most of us aren't up to building our own from scratch; not at the beginning anyway.

Bruce

[Wavytone]

Absolutely - you can't buy an f/7 Newtonian in any aperture, even if you want to - which staggers me.

Manufacturers are driven by lowest-cost - and the shorter the tube the cheaper it is - oblivious of the consequences - knowing that most don't know why they SHOULDN"T buy short focal ratio Newtonians.

Which leaves making one as the only viable alternative.

BTW here's the maths http://www.telescope-optics.net/newtonian_off_axis_aberrations.htm


and note: "This means that one could see it (coma) at the edge of a 40° AFOV eyepiece at ƒ/4, regardless of its focal length"

This suggests widefield eyepieces (65 degrees and more) in an f/4 newtonian are rather pointless. You can struggle with Parracors and the like, but it would be a lot better to start with an f/7 mirror.

[Jim McAloon]

I think there's a bit more to it than that, even granting the optical reasons why f/7 is preferable (and that after all was what Hartung recommended).

An f/5 tube is not only cheaper to manufacture, it is also - in apertures of 10 and 12 inches - far more portable than f/7. A 12 inch f/5 isn't too hard to move in a car. The same aperture at f/7 would be nearly two feet longer, and a very different proposition. This is a real consideration if you do not have a permanent observatory. Thus, f/5 is likely to have a bigger market than f/7.

As in most things, there are tradeoffs. The coma in my GSO 12 inch f/5 is not oppressive. It's an acceptable price to pay for the lightgathering power. And, incidentally, for paying about one fifth of what one would have to find for a 12 inch Schmidt-Cassegrain.

jim

[ColHut]

Thanks all for your comments. Bang for buck is certainly what got me an F5 newt. I knew that eyepieces would have to be in the 2.5-25~32mm range and best 10-20mm. I always figured that higher magnification wide angle eyepieces would be an issue with coma, from the short ratio and eyepiece astigmatism was likely to be bad in inexpensive eyepiece. One day a parracor of some sort and some really good wideangle eyepieces! I was a little suprised that planetary eyepieces could struggle at high focal ratios, but using a 1.5 barlow would enable the use of orthos with hopefully only limited light loss.

regards