Halos around bright stars and Astronomik "No-Reflection" filters
In recent months there has been a lot of discussion about the presence of problematic halos around bright stars when using various filters during imaging. Some people using Astronomik filters have also been involved in these discussions.
Our emission line filters H-alpha, OIII, H-beta and SII for CCD astrophotography can be affected by halos if they were from batches produced between January 2007 and June 2008.
If you see halos around bright stars in your images, there are a number of possible reasons for this and the filter is only one possible contributor.
In recent years very fast optical systems have become popular for imaging. The energy in a filter induced halo grows exponentially as the f-ratio decreases. Additional to this, the smaller the FWHM band pass of the filter, the stronger the halo. The main reason for seeing these halos is the use of less than optimal coatings on optical surfaces in your imaging train. Usually every surface gets an anti-reflection coating, but you can do this with low cost or with high cost coating methods. Any cheaper and/or inferior coating on an optical surface, particularly close to the imaging plane, will likely cause strong halos. These effects become more likely and more prominent when you use any secondary optic like a field-flattener, reducer or coma corrector. When imaging with SLR lenses you might discover halos with one lens and no halos with another, even when they are from the same manufacturer.
What has Astronomik done to solve this?
Over the past year or more Astronomik has put significant effort into researching this topic. Our goal was to reduce halos caused by any of our filters to a level were it would not noticeably affect imaging. By making a number of modifications in the coating of our filters we were finally able to reach this goal. As a result of this effort we are confident that images taken with any Astronomik filter currently shipping will show no halos.
While the halo effect was most noticeable in narrow band filters, during our filter development and improvement process we also improved our LRGB filter range. There were very few customers reporting problems with our LRGB filters and most of these were imaging at a very high level. Never the less, Astronomik saw some potential here and have taken the opportunity and made our popular LRGB series of filters halo free too!
Here you will see a comparison image with an old and a new filter: