Ronchi star testing filter update.
I have been notified by the parcel service that my Baader SC filter can be picked up. Do other countries have such a sophisticated parcels/package services? When you buy something online you give your email address and telephone number to the vendor. These are then used to automatically contact you by SMS or email when the items is waiting at a local package/parcel outlet or post office. Far more reliable and user friendly than waiting in all day for the post person and then not get the package. Or they didn't bother to ring the doorbell and took it back to the sorting office. Almost every village has a collection center so they are usually within easy reach and particular outlet options are suggested depending on one's home address and can be specified when ordering. A further option is to have the item delivered directly to one's place of work.
The absence of a green filter made my results of Ronchi testing achromats somewhat difficult. The multi-coloured images showed how each colour is brought to a different focus. This tended to soften the Ronchi bands.
After much reading online I placed an order for a Baader Solar Continuum filter. This transmits only a narrow band in the green. Hopefully, and according to reviews, it aids the testing of achromats. It also helps to bring out the Solar surface detail provided an IR filter is also used in tandem to stop reported leakage in that area of the spectrum.
I must add the usual safety note here that the Solar Continuum filter is NOT a safe, solar heat rejection filter on its own. It still passes more than enough heat and light to instantly blind any user who has not fitted a full aperture safety solar filter as well. These solar safety filters are readily available in film or glass form. The very popular Baader AstroSolar Safety Film filter reflects away almost all of the heat and light before it enters the telescope. This is vitally important to avoid blindness or sharply focused heat which can rapidly cause a fire or damage the instrument's optics through rapid overheating. There is still enough light to safely see the Sun's image visually in white light and to safely take pictures.
Only when one looks directly through the filter can the strong green cast be seen. I held it up to my compact camera and took a snap of its case to show its effect. Note how the printed spectrum on the packaging is now all but invisible.
This will be dependent on having a driven mounting which can easily manage the 7" f/12 refractor. The Fullerscopes MkIV proved inadequate and the VFO drive box died.
Click on any image for an enlargement.