Dear all,
Would appreciate your views on choosing Pentax XL 14mm, 21mm or 28mm, for 1800mm focul length (f/12) scope...not able to decide which one is the best for deep sky - nebulas, clusters.
Given that my scope's long focul length doesn't allow a big TFOV with most of the eypieces, even Pentax with 65* AFOV (except 28mm, which is 55*) gives the following TFOVs with my scope:
1. With XL14 (128x mag) at = 0.51*
2. With XL21 (86x mag) = 0.76*
3. With XL28 (64x mag) = 0.86*
My view is: since I have a good 40mm eyepiece (43* AFOV) which, with 2xbarlow can give 90 times magnification - I could use that a general deepsky eyepiece. In that case I can get the XL14mm, which would give higher magnification, although TFOV will be lower. Is this assessment correct in your view or XL21 or 28 a better option, or are there other eyepieces which are still better.
Secondly, has anyone used a focul reducer? What are the pros & cons of using that for visual observation purpose? I think by reducing the focul length to say, f/6, I can get a bigger TFOV with all eyepieces, of course with additional lens there will be loss of light.
Thanks in advance for your views.
Cheers.
Need help in choosing eyepiece
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harlequin2902
- Posts: 744
- Joined: Tue Sep 30, 2003 11:04 am
- Location: Singapore, Sengkang
The XL14 (giving you 128x magnification) is just too much magnification for comfortable observing of deep sky objects in general. For open clusters and the larger nebulae, we usually observe them at 60-80x magnification. And if we really want to have a closer look, all we need to do is to slip in a barlow in between.
I see that you already have a 26mm Super Plossl, but if you want a good eyepiece to fill the gap between what you currently have, given the XL's you listed - I think you can go for the XL21 or XL28. With their wide AFOV, either one should do ok in framing up most open clusters nicely within the FOV. And if you want to go higher magnification to furthur resolve globular clusters or the smaller and more compact star clusters (in places like Mersing), a barlow will allow you to achieve that with the same eyepiece.
Try not to observe using barlowed 1.25" 40mm eyepieces. 40mm eyepieces already have very generous eye relief, and if you barlow them, you'll increase the effective eye relief by about 4 or 5mm and it becomes increasingly difficult to keep your eye in the "sweet spot". And because you now have to observe from a higher spot above the eyepiece, not only will that feeling of detachment make the view less than desirable, you will be vulnerable to all the stray light in the environment.
I see that you already have a 26mm Super Plossl, but if you want a good eyepiece to fill the gap between what you currently have, given the XL's you listed - I think you can go for the XL21 or XL28. With their wide AFOV, either one should do ok in framing up most open clusters nicely within the FOV. And if you want to go higher magnification to furthur resolve globular clusters or the smaller and more compact star clusters (in places like Mersing), a barlow will allow you to achieve that with the same eyepiece.
Try not to observe using barlowed 1.25" 40mm eyepieces. 40mm eyepieces already have very generous eye relief, and if you barlow them, you'll increase the effective eye relief by about 4 or 5mm and it becomes increasingly difficult to keep your eye in the "sweet spot". And because you now have to observe from a higher spot above the eyepiece, not only will that feeling of detachment make the view less than desirable, you will be vulnerable to all the stray light in the environment.
Samuel Ng