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What's the actual size of your 2" filters in "mm" with and without frame/ring? What step down adapter is suggested from a 52 mm to "-- mm"?
Question by: Waqas Ahmad on Oct 10, 2016 7:54:00 PM | 1 Answer(s)
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Baader Semi-Apo Filter
Combination of "Neodymium Moon & Skyglow" and "Fringe Killer" Filters
Time and time again we were made aware of customers who were combining our Neodymium Moon and Skyglow filter with our Fringe Killers to deliver amazingly colour-corrected images to achromatic refractors. This combination however did produce refelctions.
After analysing these reports we developed our Baader Semi-APO filter with IR blocker, a reflection-free combination of the Moon & Skyglow with a stronger coating of the Fringe killer. This result was a virtually colour-neutral image without the blue and red halos (violet fringing) and an increase to image contrast.
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Information about Baader Filters and Filtercells
About Baader Filters The variety of uses for filters in amateur astronomy has considerably increased during the last decade, enabled by both more accurately manufactured optical accessories, and, above all, by the “digital revolution“. In the old days, colour filters for visual planetary observations were not screwed in the front part of the eyepiece, but were simply placed between the eyepiece and the eye. Plane-parallelism of these filter glasses was not important, because they were not in the optical path of the telescope. Today, filters are placed in the optical path of the telescope, even well in front of the focal plane. This definitely requires some degree of plane parallelism and accurate production of the filter glasses. Every single cell mounted filter delivered to our customers is cut as a round or square disc in the according size (1¼", 31mm, 36mm, 2", 50,4mm, 50x50mm, 65x65mm), and then is polished plane to a quarter wavelength on both sides on a computer numerically controlled polisher. After that, the polished blanks are submitted to the costly coating procedures. This sequence is also used for all unmounted filters. We deliberately avoid cutting filters from larger sheets, because the coating layers can be damaged at the edges and suffer from microscopic...
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Off-standard single filter sizes from Baader - why not:
Quite often we receive requests for a single filter in an off-standard size. In all cases we are sorry that we must answer as follows:
Sorry (we know it would be so very much cheaper in production - and we would be so much more flexible to fill special requests) - but we have decided long ago to not cut or saw our filters from large plates because this would leave the coating stack open and mutilated (with microscopic cracks) all around, prone to aging and peeling.
Many times we had the chance to inspect our competitors filters after several years of use (due to our 30+years of servicing SBIG-CCD-cameras/and filter wheels) and we realized already 15 years ago how moisture and heat stress can deteriorate even most modern hard coatings, slowly peeling off from the carrying substrate over time, unless the coating stack is sealed all around the filter stack.
As a consequence - in order to create our filters to remain impermeable - we only offer all filter substrates already cut to final shape and run each substrate on an double deck auto-polisher to achieve perfect optical flatness and freedom from cone errors.
Then we do individually coat these substrates in 500 pc per run as minimum to fill a complete coating chamber, in a way that the coating stack (many times 50+ layers) applied onto each filter won't reach to the very edge of the round or square substrate, so that the coating stack remains completely sealed from all sides. In this way we can ensure that our filters will not age at all.The sad effect is that we cannot offer other sizes unless the inquired production quantity were in the range of 250 to 500 pc (depending on size) and the tooling rings or square holders will be paid for, which serve to precisely center each individual filter substrate within the rotating calotte inside the coating chamber. Sorry - as explained above - we just will not coat onto large plates and cut any shape from them, also because such large plates cannot be polished optically flat in the same way we do it.
For your most urgent need and for single piece solutions we can only recommend to order the next larger size of our respective filter and have that cut to shape by an ophtalmologist locally. We can supply the round filters without the metal cell in such cases; square filters come without cell in any case.
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Unmounted Filters – which side should face the telescope?
Question in Detail:
I just bought LRGB 36mm unmounted filters. I have question: which side of filter should be placed towards telescope? Is it better way of distinguish than "more shiny surface towards telescope"?
Answer:
Always put the more reflective side towards the telescope side. To guide you we already put a small arrow on the filter rim, on those filters were the position matters. This arrow indicates which face of the filter should be directed towards the sky (telescope-sided). All cell-mounted filters are already oriented in a way that the most appropriate filter face is facing the sky when the filter would be mounted directly onto the front end of the nosepiece of a camera.
If you mount your filter the other way, any reflected light would have a short way to the camera sensor, resulting in a higher risk of getting some kind of back-reflections inside the camera field. Many sensors have highly reflective areas near to the light sensitive area, also the area with the bonding contacts is sometimes highly reflective.But: this is true only for instruments without optical elements near to the focal plane. If you have f.e. a coma corrector, field flattener, focal reducer, focal extender (to a lower degree due to concave surface), or in extreme cases a whole lens group for more complex field corrections a few centimeters in front of the filter it could be useful to flip the filter against the rule from above (thus having the arrow pointing away from the telescope). Cause in such cases the likelihood of reflections from the sensor could be lesser then fort- and back- reflections from such glass-surfaces. If in doubt, it helps to make some test images from a star field with bright stars, using the filter in both ways for comparison.
Should you really have some reflections with both positions it can be more effective to add a spacer between filter and camera, eventually shifting the reflection out of the image field. With focal correctors having curved surfaces changing the filter-lens distance could help also.
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