Quote:
Originally Posted by morls
I'm using a 200mm dob, f.l. 1200mm, so it's an f6....
I'm interested in planets and deep sky, just beginning to find my way around really. I like the idea of having a range of magnifications, especially down from 20mm...
If slowing the system to f12 helps with planetary observation that's great....does this mean I could get away with using, for want of a better term, "less-better-corrected" eyepieces to get good results at high magnification? I'm asking this because I've read that faster scopes require, in the shorter focal lengths, more expensive eyepieces that compensate for the faults inherent in fast systems...
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Faster 'scopes generally require highly corrected eyepieces for all focal lengths (not just short) to ensure astigmatism free views off axis. A Barlow will reduce eyepiece astigmatism and improve off-axis sharpness, (unless it's a really poorly corrected eyepiece and/or Barlow). If you want to go down the Barlow or Powemate path go the 2" to give you maximum flexibility for imaging as a 1.25" system will vignette with a typical DSLR camera.
If I may go off topic here a little and suggest an eyepiece path for you that is not too expensive but will offer you outstanding quality.
Tele Vue 25mm Plossl good low-med power to complement your 42mm wide field. You will get 48x mag. at 4.2mm exit pupil.
Tele Vue 20mm Plossl for good deep sky viewing at 60x.
Tele Vue 1.25" 2x Barlow to give 12.5mm (96x) and 10mm (120x).
Tele Vue 1.25" 3x Barlow to give 8.33mm (144x) and 6.67mm (180x).
You could later add a 15mm Plossl to give 80x, 160x and 240x and you would have a nice spread of powers and an equally nice sapread of true fields. Note you will likely view the major planets from 120x to 180x most of the time and over 200x for nights of above average seeing. Rare nights of exceptional seeing will allow you to go to say 300x but to do so you would have to add either the 11mm or 8mm (or both) Tele Vue Plossls to your collection and use them with Barlows. Note using longer focal length eyepieces with Barlows or Powermates allows for more comfortable viewing than using short focal length eyepieces as the eye lens is a lot larger and more comfortable to look through (at least for Plossls) than on shorther focal length eyepieces.
Note the above is for a 50deg apparent field of view (AFOV) set (all 1.25"). I would also add the 32mm Plossl for maximum true field for a better corrected view than say the 42mm. This would also have the advantage of allowing 1.25" filters across the board.
Some may argue that 50deg AFOV is not enough for an untracked DOB system and prefer the wider AFOV 68deg and over. However, these are much more expensive if you want to maintain abberation free views. If you want to go down this path then may I suggest a 19mm Panoptic and 2.5x Powermate giving 63x and 158x as a start and save up for other eyepieces that would suit your needs. I would recommend the Powermate over the Barlow if choosing the Panoptics to maintain eye relief and ensure no vignetting.