Astrophotography involves replacing an eyepiece with a camera. While this seems simple in theory, for the high resolution and large fields we try to achieve with photography, the position of the camera’s sensor at the focal plane is critical.
Even with perfect optics, the image we capture can be ruined if the camera is attached with a minor tilt.
Here, we’ll take a look at the Baader M68 tilting device that can be very useful when the user needs to correct the camera’s tilt at the focal plane.
Having the sensor at the correct position means it is perfectly focused. That means ensuring that the entire sensor coincides with the focal plane of the telescope. That is, no corner of the sensor should be closer to, or farther from, the telescope’s focal plane.
What is Tilt?
Having the sensor at the correct position means it is perfectly focused over the entire sensor surface. That means ensuring that the entire sensor coincides with the focal plane of the telescope.
A tilted sensor will produce images with round, focused stars in the center, while stars in one or more of the corners are out of focus or deformed.
If stars in the center of the frame are also deformed, you need to look at other factors such as tracking or collimation.
The larger the sensor, the larger this effect because a slight angular tilt translates into a large distance at the edge of a larger sensor. Also, smaller pixels are able to resolve tiny details, so with higher image scale the defects will be more apparent.
The focal ratio also plays a role. Faster telescopes (lower f-ratio) have a narrow critical focus zone (very small tolerance) and will be much more sensitive to any misalignments due to tilt.
What Causes Tilt?
Tilt can be caused by a number of issues, and it isn’t always easy to spot. In many cases, it is one of the many adapters or connectors between the telescope and the camera. It could be that the focuser is not perfectly square, even if off by a very small angle. Or the sensor itself may not be perfectly parallel with the camera connection threads.
My first impression of the Baader Tiler from photos was that it didn’t look very sturdy, and I was concerned about hanging my full frame camera and 7-position 50mm filter wheel off what looked like a tiny part. Once I had it in hand, though, it felt quite solid. It worked well with my CMOS camera; I didn’t test it with a very large CCD setup with a heavy 50×50 filter wheel.
The Tilter is made of two separate rings, one with male M68 threads and the other side with female M68 threads. The angle at which they are joined together can be altered, thus adjusting and correcting the tilt angle.
There are three stainless steel pins on the Tilter that ensure its two parts don’t come apart, even if you accidently loosen the adjustment screws too far.
The all-CNC aluminum body is relatively thick walled, while the optical length (back focus length it takes up in the imaging train) is only about 10mm. The movable part can extend about 0.75mm from its fully retracted state. The Tilter weighs 85 grams (3 ounces), light enough to not cause any additional stress on most focusers.
The clear aperture is quite large at about 63mm (2.5 inches), and there are male and female M68 threads on the sides. For smaller popular threads, like M54, M48, etc., the user will have to provide step down rings, as Baader doesn’t offer any other sizes (at the time of writing).
The Baader tilting unit is very practical. Unlike many common tilters that require the user to remove the camera to be adjusted, the Baader unit can be adjusted in place without the need to disassemble the imaging train. This makes the tedious task of tilt correction much quicker.
There are six adjustment screws located at three locations 120 degrees apart. Each pair acts as a push-pull mechanism. You loosen one screw and tighten the other to move each side up or down.
The Tilter can correct for up to one degree of tilt, but that’s really more than you should have. If you need more adjustment than that, there is a major misalignment in your imaging train that needs to be addressed. The main job for the Tilter is to provide fine adjustments.
I measured one full rotation of each of the screws, which extended a side by 0.53mm. That means a small but achievable adjustment of 10 degrees of screw rotation will result in a 15-micron extension. That should be fine enough for most cases.
While the push-pull mechanism works well, it may take a bit of getting used to, getting the pressure of each pair of adjustment screws just right so there is neither too little or too much pressure between them.
The Baader Tilter is slim and light enough to fit in many imaging trains, and solid enough to securely hold the majority of popular cameras. The M68 threads provides a clear aperture large that it won’t obstruct most of the imaging sensors available.
Overall, I found this solution offered by Baader very practical. The most important feature is that the adjustment screws are conveniently located on the sides of device. With dozens of fine adjustments needed to perfectly correct tilt, the Baader M68 tilter can make that task less daunting!
Product Link: www.baader-planetarium.com