Astrophotography can be a rewarding hobby, but at the same time can be frustrating. The frustration comes from trying to get those near perfect celestial photographs without getting the annoyance of star streaks know as field rotation. There are a variety of methodologies for avoiding field rotations, namely using German equatorial mounted scopes or wedges attached to fork mounted scopes. Fork mounted scopes are also known as altitude-
2.0 Addressing the Main Pros and Cons of Field De-
Whether polar alignment is more accurate than altaz using a de-
The addition of a de-
We can, off course, make compensations for these de-
1. To compensate for the de-
The balancing is accomplished by sliding the weight system up and down the SCT and also moving the set of weights along the little rod they are attached to on both axis as show in figures 1a and 1b, performing the vertical balance first then the horizontal balance last. Once balancing in both axes is achieved, you should be able to completely disengage the SCT and be able to rotate the SCT freely up and down without it pivoting straight down on its own (in other words, it should statically remain in any position without moving). This of course, reduces the stress on the tracking gears and makes for more accurate pointing and tracking.(Details can be seen at http://starizona.com/acb/basics/using_balancing_fork.aspx).
Counter Balance Figure (courtesy of Starizona)
Figure 1a Figure 1b
In terms of the extended Visual back that may cause the scope to not clear the fork, it is true that the upper 20% of the sky is lost, which some consider the most interesting portion; but remember, the earth rotates, so eventually a good portion of the upper 20% of the sky will eventually make its way into the allowable view of the scope as long as darkness presides.
1. As for the visual back extension causing increase vignette, I’ve never seen this with the extensions I have used, although it does become more apparent when one uses focal reducers (whether extensions are used or not).
2. On the issue that a guide scope and PEC cannot be used, differential flexure (sagging, flexing, and shifting of the guide scope in its rings) is a problem with guide scopes, so I basically stay away from guide scopes whether I used a der-
Other cons may exist for using a de-
Figure 2 depicts the visual back setup. It includes a Canon EOS20Da DSLR as the main camera, Taurus Tracker III as the OAG, the Meade De-
4.0 A picture is worth a thousand stars
In showing the accuracy of the Meade De-
With the aid of tracking, the de-
Now, let’s look at exposure times for other celestial objects. The crab nebula (Figure 4) was taken at a 15 minute exposure (no stacking). The image only was contrast enhanced using PhotoShop. Notice that no field rotation is evident.