Which filter

Which Nebular filter for DSOs and Astrophotography

Date: 09/05/2010

We have several kinds of filters in stock.

Firstly, they typically come in 3 sizes : 1.25", 2", and SCT. The first 2 sizes fit 1.25" and 2" eyepieces or barlows, and the SCT format fits the rear cell of SCT telescopes. Here's a matrix to help you decide which size to get :

	1.25"	2"	SCT
Telescope has SCT Rear cell	ü	ü	ü
Telescope takes 2" eyepieces	ü	ü	û
Telescope takes only 1.25"	ü	û	û
Prime Focus photography	Adapter needed	Adapter needed	Simple
Eyepiece projection	Simple	Simple	Simple
Typical price range	$100-S$200	$120-S$400	S$130-S$400

There are also basically 4 types of Nebular filters. They are Light Pollution Reduction (LPRs) (also known as Broadband), Narrowband (also known as Ultra High Contrast - UHC), Oxygen III, and Hydrogen-beta. These types are all named based on which frequencies of light they let through.

	LPR	Narrowband	OIII	H-ß
Filters out stray photons from street lamps, and dim skyglow	ü	ü	ü	ü
Light pollution level	mild	moderate	extreme	minimal
Transmission	>90%	>90%
Filter passes	450-530, >650nm	480-505nm*	496-501nm*	486.1nm*
Contrast	low	moderate	highest possible	specific
Stars and Star clusters, spiral galaxies (M33, M101)	ü photos only	û	û	û
Diffuse Nebula : Crab (M1), Lagoon (M8), Great Orion (M42) Swan	ü	ü	ü
Reflection Nebula : Pleiades, Trifid	ü
DSO spread over wide area : Veil, Rosette, N. American. Emission nebula : Lagoon, Omega.	ü	ü	ü
Planetary nebula : Dumbell, Ring	ü	ü	ü
Faint planetary nebulas : NGC7293, Bell 33, Jones 1			ü
Horsehead, California, Cocoon nebula				ü
For visual use	ü	ü	ü	ü
For astrophotography	ü*	û	û	û

Wavelengths of interest :
560nm = yellowish light of sodium and mercury vapor street lights and airglow.
500nm = ionized oxygen and atomic hydrogen
486.1nm = atomic hydrogen-beta, blue-green
656.3nm = hydrogen alpha, red color

* Reduces sky fogging, exposure possible to increase by 3x

Lastly, it can be confusing to figure out how to compare filters sold by the various manufacturers so here are the names used by them :

	LPR	Narrowband	OIII	H-ß
Orion	SkyGlow Broadband	UltraBlock Narrowband	OIII	H-beta
Lumicon	Deep Sky	UHC	OIII	H-beta
Astronomik	CLS	UHC, UHC-E	OIII	H-beta
Thousand Oaks	Broadband	Narrowband	OIII	H-beta
Meade	Broadband	Narrowband	OIII
Celestron	UHC/LPR	UHC/LPR	OIII
Hutech	LPS
Televue		Nebustar	OIII

Hope this helps !!

Joo Beng

Which colour filter do I need?

Date: 09/05/2010

Here's a table summarising the different objects that can benefit from different colour filters:

	Blue	Red/Orange	Green	Yellow	Violet
Moon	ü lunar features	ü lunar features	ü lunar features	ü lunar features
Mercury		ü for daylight and twilight observations			ü faint features and details
Venus	ü dark dusky areas of upper clouds	ü for daylight obs, terminator views	ü for daylight obs, cloud patterns		ü darker areas and upper clouds
Mars	ü surface features, clouds, polar caps	ü sharpens boundaries of yellow dust clouds, ice caps. surface features	ü contrast for polar caps, dust storms, clouds	ü surface features, dust clouds	ü detects haze or clouds over polar caps
Jupiter	ü contrast in cloud belts, Great Red Spot		ü Great Red Spot, blue-red low contrast in belts	ü orange-red features of belts, polar regions
Saturn	ü low-contrast details, belts, zones	ü bluer clouds, polar regions	ü contrast in white objects in atmosphere	ü orange-red features of belts, zones	ü enhances ring structure
Comets	ü Enhances tails. Combine with orange for sodium D line.	ü Enhances dust tails		ü highlights yellow dust tails and heads

Hope this helps !!

Joo Beng