Addendum

Optical Filters

updated: 2024-11-07

Broadband vs. Narrowband Filters

  • Broadband filters effectively capture galaxies, reflection nebulae, dark nebulae, and natural star colors. 
  • Narrowband filters are best used on emission nebulae and supernova remnants.

Filter Impact on the Backfocus

The back focus of a telescope changes when a filter is used due to the thickness of the filter. A general rule of thumb says that the back focus must be extended by about a third of the filter thickness. With a 2 mm thick filter, this means a change in back focus of around 0.67 mm. The effect on the optical length due to the use of filters is shown in the table below (column: Optical Length)


F: Filter 

FilterShort

FilterName

FilterThickness

OpticalLength

-

-



Asol

Astrozap Solar

0

0

Bsol

Baader AstroSolar

0

0

CS

ClearSky

1,5

0,45

N

NONE

0

0

OIII

OIII

1,5

0,45

OLeN

Optolong 2" L-eNhance

2

0,6

UHC2

Omegon UHC 2"

2

0,6

UV

UV+IR

1,5

0,45

Table from StarLustDB: qryFilterSelect

Filters in use:


Articles:

10 Light Pollution Filters for Astrophotography by Trevor Jones

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