The Baader M68 is a system that fulfils many wishes in an elegant way and with high stability. All distances can be set precisely, and with the Baader M68-Tilter (#2458170 , € 255) an ingenious extension has been added to the system.
But before talking about the tilter, let me tell you how I came to this system. When I was testing the prototype of the Baader FCCT (Filter Changer Camera Tilter) for RASA 8" and QHY cameras (various versions available) for the RASA 8, I wished that the connection and collimation possibilities for the C14 were just as sophisticated. I have acquired enough experience so that I can "read" the image of the C14 so well that I know whether I am really seeing a tilt or just a problem with the distance. Of course, this led to the desire to use a tilter to adjust the field perfectly to the camera sensor.
I mentioned this idea in passing once during a conversation with the Baader company, and we discussed what functions would have to be incorporated so that imaging with the C14 EdgeHD could be optimised as easily and smoothly as with the RASA 8 and the FCCT. After all, we are talking about a large Schmidt-Cassegrain telescope, and that should be capable of 36.3MP full-frame images – or even more?
The fact that I kept coming close to ma desired image quality, but couldn't reach what I would have liked, may also be due to the fact that I use five different cameras on the C14, and each one is a little bit different, with small subtle differences in sensor position and distance (back focus). It has been clear to me for some time that the backfocus on the EdgeHD has to be met with a tolerance of 0.2mm! And the images get even better when the error is smaller.
Well, and now there is this little "M68 filter wonder", and the handling is very good! But that doesn't change the fact that the optics really have to be very well collimated! Because, of course, it is not possible for a tilter to solve every problem. But if everything fits, then even 61 megapixels can be used on the C14 EHD, with round stars from one corner of the image field to the other.
And how do you get the most out of a 61 megapixels camera attached to the C14EdgeHD? My first tip for proper preparation is to buy a Celestron EdgeHD directly from Baader, because they know a significant tad more about it - I can't write why now, but I know it.
When preparing the optics and adjusting the accessories to match the backfocus, the distance for the M68 system with the tilter unit has to be calculated 0.2 - 0.3mm shorter than the ideal working distance of 146.05mm of C9.25, C11 and C14 EHD telescopes! In the white paper on the EdgeHD series you can see what shape the stars have over the field of a full-frame camera when the distances are correct.
The camera adapter with the M68 tilter is then attached to the EdgeHD and the star must be focussed with an accuracy of 99.9%. Only then the optics are in "best shape" and photography with up to 61.1MP is feasible.
Then I attach the Nikon D810A and place the tilter into the adaptation in such a way that even if I change the camera, the tilter remains in the system just as it was when I made the adjustment!
When everything is really in place, I collimate the EdgeHD with the help of that star so that the secondary mirror is perfectly orientated to the main mirror. Important: Do not be satisfied with 99% perfection, but gor for 99.9%! This can be done at any moon phase with really GOOD seeing. Then it is the time for the tilter.
What must be done and be taken care of?
I use a sturdy dew cap instead of a flexible one that fits exactly on the two 3" rails on the C14. Thus it is not "crooked" - I checked that! To see the field tilt better, I use a Bathinov mask. I put this on the dew cap (flat and not sticking out) and adjust the focus, then I take a picture of about 30 seconds so that I can check the field. (The tracking must be TOP, because otherwise you will try to adjust for this error, too!)
Now I can focus on the worst corner of the image where the Bahtinov mask shows the greatest deviation. I move the star to the corner of the image and change the focus NOT AT ALL! For this, it is ideal if I have previously aligned the M68 Tilter with the axes of the sensor (as far as possible), because then I can find the right adjustment screw with simple trial and error - i.e. by turning a screw and seeing what happens!
If the image changes in such a way that the middle line of the Bathinov pattern moves towards or away from the centre, then it is the right screw. If nothing happens, then the selected screw is not effective in that corner of the image. Yes, this method requires some time, and I had to work out the practice first.
Tip: As a further aid, the Bathinov mask can be rotated by 90° to see the deviation clearly, if necessary.
Write everything down, note every "turn" of a screw and don't forget: If you can assign a screw to a corner of the picture, make a sketch of it. This sketch shows the position of the M68 tilter in the M68 system. Give each screw a NUMBER and then enter this number in a second sketch, which should represent the sensor. Then you know, screw 1 changes this,... Then you have completed this exercise!
After every second screw that was used for the adjustment, the following applies: Place the star exactly in the centre again and focus! Because the closer you get to perfection, the smaller the deviation of the centre line becomes! Therefore, the focus must be perfect, then you will always recognise the one corner of the picture that still needs to be corrected. This sequence is interrupted by test shots, so expose for 30 seconds and look for the worst picture corner! The whole thing ends at the point where all corners of the image are at 99% perfection.
If the stars in all the image corners are slightly elongated (egg-shaped towards the outside!) because we used a 0.2-0.3mm shorter backfocus to allow for tilting, then the distance must now be lengthened by 0.2 - 0.3mm. For this purpose, Baader M68 offers M68 Fine-Adjustment rings made of aluminium (0,3 / 0,5 / 1 mm). These are inserted quickly. Then: Focus again and check the field, but this time without the Bathinov mask! In case there is a deviation, simply adjust again, the sketches should make this easier!
This way, the whole system can be adjusted; if the M68 tilter is removed together with the camera, the adjustment is retained even when the camera is replaced with another one. With the tilter, the M68 system now not only offers the possibility to mount even heavy accessories stable and without vignetting, but is extended by the possibility of simple and stable adjustment.
About the author
Christoph Kaltseis is not only an Adobe Photoshop specialist and as Nikon Professional touring for Nikon, but also an experienced astrophotographer. He is one of the founders of the Central European DeepSky Imaging Conference (www.cedic.at), which is held every two years in Linz since 2009.
In addition to his various projects, Christoph has developed an innovative image sharpening process called APF-R (Absolute Point of Focus)in recent years. The procedure is not always the same, but is adapted to the combination of lens and camera. Therefore, a flexible method was necessary to achieve the desired results.
In his career as an astrophotographer Christoph has also created several APODs (NASA Astronomy Picture of the Day), e.g. the APF-R-processed image of the M33 Galaxy or the Heart of the Orion Nebula (M42).View all posts from