I have an 8-inch Dobsonian. What eyepieces and filters would you recommend for viewing nebulae and star clusters? (I have a K25mm, K12mm and 7.5mm Plössl.)
Steve says: “Your telescope has a nominal focal length of 1,000mm giving it a relatively fast focal ratio of f/5, meaning it gathers a good deal of light.
Of the three eyepieces you mention you have in your collection, the K25mm and K12mm have the simple three-element design of a Kellner eyepiece which has a relatively narrow apparent field of view, although this may not be an issue for you when making lunar and planetary observations. However, Kellners work best in telescopes with a focal ratio of f/6 to f/8 or greater.
Plössl eyepieces are excellent for general use although their eye relief will diminish as the eyepiece’s focal length reduces. This design is most suitable for observing nebulae and star clusters. A 15mm and 25mm Plössl eyepiece would be good additions to your current line-up.
For lunar, planetary and double star observations, high magnification and reasonable eye relief (to make viewing as comfortable as possible) are prerequisites.
An orthoscopic design is an excellent choice in this regard, giving slightly better eye relief than a Plössl with high contrast views. 4mm and 9mm versions would be a good match for your telescope.
A filter would also be a worthwhile purchase. They screw into the internal threads within the barrel of your eyepieces and help enhance the contrast of nebulae against the background.
Popular choices are Ultra High Contrast (UHC) and OIII filters from Lumicon, Astronomik or Baader.”
Email Steve your astronomy queries to email@example.com and they could be answered in a future issue of BBC Sky at Night Magazine.