Although I got my C9.25 mainly for planets, the fact that my tiny iOptron ZEQ25 mount seems to handle it for longer exposures made me look into using it for DSOs as well. The problem of course is that it is quite slow at f/10, has a very demanding 2350mm focal length and has quite some coma on an APC-S sensor. Supposedly all these problems can be abated with the Celestron f/6.3 reducer/corrector (1480mm focal length, 2.5x less exposure, less coma), which is also relatively inexpensive as far as reducers go. One issue I found before trying the reducer is that there is not much info on using these photographically. Even the included Celestron manual doesn’t mention anything about proper distance from the sensor, how much correction it does (it vaguely says that it improves but does not eliminate) and what about things like vignetting? So I did some experimenting with my Canon DSLR and wrote down my observations for myself and whoever is planning to use one.
Reducer – Sensor distance
Some people say 110mm is the “right” sensor distance, others say 105mm, so which is it? Also, where do you measure it from (A or B in the pic)? Some people say from the glass, but the glass extends to the end of the male thread (point B), so it goes “inside” your female-threaded adaptor (A-B is about 1cm). Well, there is only one way to find out, let’s measure it and see at what distance it performs as a 0.63x reducer!
So, I tried a few distances and it seems that the often quoted 110mm or 105mm can give you more than 0.63x reduction, even if you count from point A. Let’s see some examples of distances and the reductions. I am always measuring from point A (since it is easily visible when you have coupled the reducer with an adapter) to the sensor of the Canon DSLR. You can measure up to the rim (EOS mount) of the camera and add the flange distance of 44mm (or 43.5mm in my Full Spectrum modified 600D). Be careful, you should also be able to measure just to the T-ring and add the 55mm T-mount flange distance, but for some reason many T-rings are 10mm instead of 11mm, making the distance 54mm to the Canon sensor.
|Distance to sensor (mm)
Or, in a chart with a nicely fitting quadratic curve:
As we can see, the distance that actually gives us the nominal 0.63x reduction is around 100-101mm from the start of the reducer thread to the sensor. So how does the field of view with and without the reducer compare at around this distance? Here is an M82 non-reducer frame superimposed (the highlighted rectangle) on a reducer frame:
So, how corrected is the image with the reducer near the nominal distance? We shouldn’t expect Edge HD quality flat field from a cheap reducer of course, but is there really an improvement? Let’s look at some corner crops from my test photos with the Canon 600D (APC-S size sensor):
The stars are little comets at the edges of the APS-C sensor without the reducer, which is of course not great as the C9.25 doesn’t give you a wide field anyway, so you are not going to do much cropping. With the f/6.3 reducer there is a marked improvement at the edges. Still not round stars, a bit elongated, but definitely less coma. However, you have to realize that these would not be the stars that would be at the edge of the non-reduced field, the reducer brings a whole extra 60% of field of view, so another interesting comparison is to see how the stars that would be at the edge of your sensor before the reducer turn out after the reducer. From the third set of crops you can see there is almost no coma left at what is no longer the edge of the reduced field. Not bad.
So, how do you achieve the exact spacing you need? There are two ways you can go about this. The easiest is to get a short 2″ SCT visual back like the TeleVue ACS-0004, then with a 2″ T-adapter you can adjust the exact spacing (make sure the T-adapter has a thread for 2″ filters as well – e.g. this is a nice inexpensive zero-profile one). Cheaper visual backs can be found on ebay (e.g. this 50mm-long one with compression rings). You can see what I am talking about:
The other method would be to get an SCT to T adapter and then t-mount extensions of the desired length. It is a bit trickier to add 2″ filters with this method, but you can do it using step-up and step-down rings. I couldn’t find 42-48mm step-up/down rings to couple the 2″ (48mm) filter directly to the T (42mm) thead, but I did combine 42-52mm rings with 52-48mm rings:
I have tested both methods for vignetting and they produce the same result (see below).
Another important question is how well can a reduced field illuminate the sensor? Well, some light frames could “enlighten” us:
Well, while we have an average darkening from center to the edge of 8% without the reducer, adding the reducer increases that to about 40%. It is a significant increase, but something like that was expected. In fact, for a reduction as much as 0.63x I would not say it is bad.
There is also a Meade version of the f/6.3 reducer. They are supposed to be exactly the same, however for at least a while somewhere between 2005-2010 Meade was selling a reducer with a shorter focal distance, which should be fine for visual and with a CCD camera, but it might give you some trouble with the spacing if you use a DSLR camera. So for second-hand made in China Meade reducers (the older made in Japan never had the issue) have that in mind.
I am pretty pleased with the reducer, it doesn’t eliminate all the SCT disadvantages making it into a fast/flat astrograph, but it does help in all areas that need help. Given its very reasonable price, I’d say it is one of the best investments for your SCT.