Astronomik Facts
Some comments and answers on often asked question about our filters

1. Type of filter
   Astronomik filters are dichriotic interference filters, that are produced by a new method. Modern disposable inserts for lathes or mills are coated with a comparable technique. The coating guarantees a strong protection of the glass below.
   

2. Cleaning
   Cleaning of the filter is very easy: Unless the hands are not polluted with silicon-carbide from cutting a mirror, there is hardly a chance to scratch the filter. During the production the filters are washed in a light soapsuds and cleaned afterwards with a cotton cloth. We have made good experience also with micro-fibre towels.
   

3. Where to use it?
   The UHC- filter will bring the best increase of quality of objects on the outskirts of the city ( skies with meagre Milky Way- visibility) with 4-6" aperture and with a focal ratio of 1:5. Of course, the range of application is much wider. I for myself have seen with an 2" instrument or under a 4mag sky objects, that would be impossible to be seen without filters. Despite that fact, the filter will do a good job as well at a 20" under optimal conditions. Downwards the price relation between filter and telescope is more the knockout- criterion. :-)
   

4. Use with a zenith mirror
   The Problem behind this question is the following: If you use an optical component with mediocre quality right before the focal point, the losses in the quality of display are little, because only a tiny part of the component is used for image forming. Example: At an instrument with F/10 and a focal distance of 20mm, only an 2mm area is necessary to display ONE POINT. If you use a zenith mirror or a binocular viewer, the distance gets bigger, perhaps 150mm and now a 15mm- area is necessary. With the Astronomik filters the area you need to display one point should not be larger than 25mm; there is nothing against the use in front of a zenith mirror or a binocular viewer.
   

5. Range of Quality
   You have to separate two criteria: the optical quality and on the other hand the band pass curve:
   a) Each Astronomik filter is tested of it´s optical quality on the optical bench. Only filters that fullfill our quality-standard are shipped. Til today there are no reclamations due to a lack of optical quality (there were two or three reclamations, because the customers had hoped to observe with a little refractor and UHC- objects for a 12" scope).
   b) The band pass curves vary a bit from one charge to another. Let´s take the OIII- line for example: Here, the measured filters had a transmission of 97 and 99,5 %. The variation within one charge are so minimal, that we can not establish them. Due to this reason there will be no individual separate test protocols, because the expense would be related to the use much to high and the filters would be certainly more expensive.
   

6. Differences to other products
   Concerning our filters we took care of some certain points, that we noticed at other products:
   
   1) the optical quality of the filters (also view 4a)
   
   2) The transmission curve is optimised for apertures from 1:4 on. The Fwhm is a little bit bigger, but the light from the edges of the optic will reach the observers eyes.
   
   3) The durability of the filters
   
   4) The filter glass is very thin, so that unavoidable intern reflexes are reduced.
   
   5) All Astronomik Filters are made from the same polished optical glass and have the same thinkness. This is very important if you do CCD imaging because you have no shifting of the focal plane due to differences in the optical length of the various filters. With Astronomik filters you don´t have to refocus each image.
   
   The quotes above count normally also for our OIII, RGB and IR cutoff filter, as well as for projects in future. We will make no compromises concerning the quality.