Astro T-2 System

If possible please try to unscrew the original clamp counterclockwise and see what diameter the thread is. A ruler or something should be quite
good for it to see if it is 54mm. Presumably it is a M54x0.75 thread as Explore Schientific/Besser/Omegon is doing and by that the ClickLock
#2956253 should be fine.

On all these new Ultra-Narrowband-Filters we have done our utmost in the selection of the costliest coating materials, to come up with a filter design that suppresses Halos to the max - by optical design. Anyone can create the similar transmission characteristics with much less expensive "rare-earth" materials. But inevitably such a manufacturer then does end up to accept less effective off-band-blocking and more stray-light to go into halos around stars.

However - there is no standard-ideal configuration of optical surfaces placement within the optical train. If there is a flattener lens involved "anything" can happen. We have seen Halos, produced by flawed coatings of the camera window or produced by reflections stemming from retro-reflection due to the radius of reducer/corrector lenses in the optical train but NOT of the filter coatings (easily visible when rotating the filter itself). We have seen Halos go away when changing the filter distance to the chip plane asf. And if the distances of the filter position between chip plane and reducer of flattener lenses work for 656.3 - it does NOT mean that the same optical distance will work equally well at 501 nm.

We "think" we have done all we can. You will need to judge when putting our filters to the test in your optical train (with the willingness to do some testing and searching for the optimum placement of the filter within the given space).

And when the result does leave something to be desired (and each and every O III-filter on this earth will show more residual halos around very bright objects than an H-alpha-filter), then you possibly could arrange "right of return" with your dealer.

For the SkyWatcher EvoStar 72 ED you will need the #2956254 M54a x 1-ClickLock (Skywatcher / Orion) clamp

We cannot say if there will be more constrast with this filter than with the competiting filter because we have not tested their filter. Generally our visual OIII filter can be used with DSLR but even an modified DSLR is not a good camera for narrowband (OIII) photos. Only every 4th pixel is sensitive for this wavelength. You loose 3/4 of the resolution of the sensor wich makes the photo weak and unsharp.

The BCF filter will work for this camera