Star Testing the AstroMaster 130

The out-of-the-box AstroMaster 130 telescope is a beginner level instrument. The 130mm f5 scope does not have enough focal length for serious planetary observations, but with a 2x barlow the resultant focal length can be increased to 1300mm. The aperture is big enough to visually observe some Deep Space objects, however this requires good collimation due to the small f-ratio.

After doing some visual observations, I wanted to try photography with the scope. First of all, at that time I only had a Canon A530 in use, so I was limited to afocal photography. The camera had CHDK installed in it to allow for any significant exposure lengths. The camera turned out to be a tricky one to focus and adapt to the scope. The main problem being the moving lens of the camera. Each time the camera powered down everything had to be started from the beginning. Anyway I managed to get some test photos done. When the focusing was at least close to successful the star images contained mysterious aberrations.
The image on the left is a 100% crop of the full image containing the strange cross like appearance of the stars. At first I thought that the aberrations were due to scope vibrations during the exposure. I ruled this out as the aberrations were to be found in each of the 10 exposures.
After taking a few more exposures of different targets it was quite clear that something was wrong. This aberration was not visible in visual observations so this was my first lesson in how unforgiving photography is of problems in the optical train of the telescope. The next step was to plan the star testing of the AstroMaster in order to find out was could possibly be the source of these aberrations.
The first problem I ran in to was, how to get enough magnification for star testing. I didn't have a good barlow at the time and secondly I did not want add any additional glass in the optical path in order to avoid aberrations induced by the additional elements. After some pondering I decided to go with a webcam at prime focus. Due to the small size of the chip on the webcam I could get enough apparent magnification to make some analysis of the problems. Another good thing with using a webcam at primefocus is that no additional optical components are introduced in to the lightpath (if the IR filter is removed from the webcam). The next couple of days were spent waiting for somewhat good weather to carry out the tests.
Finally the skies cleared and I was able to get started with the testing. The first test session was carried out with the scope in the same condition as while taking the initial photos. I used two stars for reference, Polaris and Vega. Making sure that the scope was at ambient temperature I began the tests. First I aligned the scope to make the tracking of my targets a little easier. The alignment was not spot on, which was soon apparent when even Polaris wanted to stray out of the small field of view of the webcam. Imaging was done using a video capture utility called SharpCap.

During the first test test session it was clear that there were some obstructions causing additional diffractions in the image and astigmatism caused most likely by pinched optics. The additional difrraction pattern can be easily seen on the left side of the inside focus images (to the left of in-focus image). The in-focus images display the astigmatism. The in-focus image should be a round point like star, but instead looks like a diamond. The same obstruction that causes the diffraction spike on the inside focus images can be seen causing a dent in the outside focus images. The first problem I decided to tackle was the obstruction in the lightpath causing the additional diffraction. This turned out to be a small retainer and screw apparently intended to secure the secondary in its holder. This turned out be an unnecessary precaution due to the the way the secondary was mounted. The problem with the retainer was that the screw extended in to the lightpath and the retainer itself was bent over the secondary mirror.
I removed the retainer from the secondary and conducted a new test. The effect was clear. Without the additional diffraction pattern the astigmatism became clearer.

The above image displays the situation after the removal of the retainer. No more additional diffractions caused by objects in the lightpath. However the astigmatism is now more clearly seen. The asymmetric star image can easily be seen to rotate 90 degrees CW going from inside focus to outside focus (best witnessed between the first and third image in the bottom row). The cause of the astigmatism was still a mystery at this stage. I was hoping that it was not due to a misfigured main mirror. I decided to examine the secondary first as my friends told me that they had had similar problems with their secondaries in several different scopes.
Again the poor AstroMaster was in pieces in my study. Trying to get to the source of the problem turned out to be a little trickier than I thought. The secondary mirror is installed in to a tight deep depression and then glued all around the bottom edge. A wrong way of installing a secondary if I ever saw one. I also managed to break the original secondary while trying to remove it from the holder. I managed to get a new one, but decided go for a slightly smaller diameter, beacause I was already planning a modification to the spider vanes and secondary holder. The smaller secondary fit my plans better. Anyway after doing some modification to the original holder and insatlling the new secondary I was ready for new tests.

Immediately at the start of the tests it was apparent that the modifications were successful. The horrible infocus image is noe more starlike and the overall appearance of the diffraction pattern is much better. A few things can be deduced from these images: The mirror is slightly under-corrected and the collimation is a little off. Lso the in focus star image is slightly box-like, which attribute to the diffraction caused by the excessively thick spidervanes of the original configuration.
The main mirror shall stay as it is as the under-correction is not that bad, but the spidervanes will be redone.