The earlier post on this site was a used-sale listing for a customised accessory kit: a metal Celestron eyepiece case holding Celestron 4 mm, 6 mm and 15 mm Plössls, an Orion 7 to 21 mm zoom eyepiece, an Orion 2x “Shorty” Barlow, an Orion 12.5 mm illuminated reticle Plössl for guiding, and seven Celestron filters. This rewrite explains what each type of accessory does, using Celestron’s own Eyepiece and Filter Kit as a reference, and corrects the filter names in the original listing.
Correction: filter numbers
The listing named the filters as “moon”, “Red no 25”, “Blue 80A”, “Orange No 25”, “Yellow no 12” and “Green no 58A”. Two of those cannot both be right: a filter number identifies one specific colour, so red and orange cannot both be #25. In Celestron’s Eyepiece and Filter Kit, #25 is red and #21 is orange. The listing’s “Orange No 25” is almost certainly a #21. Celestron’s kit lists its green filter as #58. Check the number printed on each filter’s rim to be sure.
What Celestron’s kit contains
For reference, Celestron’s 1.25-inch Eyepiece and Filter Kit includes:
- Five Plössl eyepieces: 32 mm, 17 mm, 13 mm, 8 mm and 6 mm
- A 2x Barlow lens
- Six coloured filters: #80A blue, #58 green, #56 light green, #25 red, #21 orange and #12 yellow
- A Moon filter
- A foam-lined metal carry case
Celestron says each of its kit eyepieces is a four-element design with a 52° apparent field of view, usable in any telescope with a 1.25-inch focuser.
Plössl eyepieces
Plössls are the classic all-round eyepiece: sharp, simple and affordable. Short focal lengths such as 4 mm and 6 mm give high magnification. Magnification is the telescope’s focal length divided by the eyepiece’s. For instance, on a 1500 mm telescope such as the NexStar 6SE, a 6 mm gives 250x and a 15 mm gives 100x. Celestron lists the 6SE’s highest useful magnification as 354x, so very short eyepieces quickly reach the limit of what the air and the optics allow.
Zoom eyepieces
A zoom eyepiece, like the 7 to 21 mm in the original kit, changes focal length when you twist it. On a 1500 mm telescope, a 7 to 21 mm zoom covers roughly 71x to 214x. Zooms are convenient for finding the best magnification on a given night without swapping eyepieces.
The 2x Barlow lens
A Barlow lens doubles the magnification of any eyepiece placed in it. Celestron points out that its five kit eyepieces plus the Barlow give ten different magnifications, equivalent to focal lengths from 32 mm down to 3 mm. “Shorty” Barlows are compact versions that fit better in some focusers and diagonals.
Illuminated reticle eyepieces
An illuminated reticle eyepiece has a lit crosshair or pattern in the field. The original listing describes the 12.5 mm reticle Plössl as being for guiding. In astrophotography, guiding means keeping a star precisely on the crosshair during a long exposure, so the telescope follows the sky accurately. Reticles are also useful for precise centring when aligning a GoTo mount.
What the coloured filters do
Celestron describes its coloured filters as planetary filters, and gives two examples: a blue filter to see what it reveals on Jupiter, and a red or orange filter to bring out the polar ice caps on Mars. In general, a coloured filter darkens features of the opposite colour and lightens features of a similar colour, which can raise contrast.
- #80A blue: Jupiter’s belts and features
- #25 red and #21 orange: Mars
- #12 yellow, #56 light green and #58 green: subtle contrast changes on planets and the Moon
- Moon filter: a neutral-density filter that, in Celestron’s words, dims the brightness and increases contrast for lunar observing
Filter effects are subtle. Experiment with each on the same object to see what helps your eyes and telescope.
Using the kit for photography
The original seller said the kit was assembled “to assist photography”. A zoom eyepiece and a phone adapter make it easy to frame the Moon, and an illuminated reticle helps with guiding longer exposures. Filters can also reduce glare on the Moon and bright planets in photos.
Caring for your kit
- Keep eyepieces and filters capped and in their foam slots.
- Blow off dust before wiping glass.
- Use lens tissue or microfibre with a small amount of lens cleaner.
The bottom line
An eyepiece and filter kit is one of the easiest ways to get more from a telescope. Learn what each piece does, keep within your telescope’s useful magnification, and check filter numbers carefully: the listing’s “Orange No 25” is a #21.
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