GSO RC 8" (review May 2009)

Use

This OTA is designed to do one this in particular. It is an imaging telescope. To use this design for visual work is really a waste of time and you would be better served by other optical designs. Having said that though I did star test this OTA to determine whether there were any defects that would warrant a return to factory.

Aside from miscollimation, I observed the following with a star test:

- no visible astigmatism.
- no roughness in the figure.
- no apparent turned edges or circular zones.
- diffraction rings were even and sharp all the way from the outside to the shadow of the secondary.
- stars were as sharp as in my 4 inch Tak at focus.
- no coma was seen to the edge of a 24mm pan optic.

In short, these optics were impressive at first light. The airy disk was easily visible at focus with a nice even pattern surrounding it.

However, imaging through the scope revealed some minor problems that will need correcting to gain nice results. All the images were taken unguided, I am yet to receive guide rings I ordered for the vixen dove tail. It also seems I need to attend to a more thorough polar alignment, my alignment was conducted with the TSA which has a much shorter focal length. The stars in the image are not round for that very reason.

I am using a Takahashi flattener to flatten the field. I had initially thought that the flattener was contributing to the flaring seen in the images below, however subsequent imaging runs have shown that the problem stemmed from the focusor. In the images below stars on the right hand side of the image are even and round, yet the stars on the left of the image have some flaring. The result of flop in the focusor or it not being orthagonal to the optical alignment was the contributing factor. I ordered a Feather Touch focusor which did correct the problem. The Feather Touch can handle more weight and designed with imaging in mind. It does not move at all.

Of concern in the image below is the internal reflections. I had suspected that this was coming from inside the focusor. It was not baffled and I noted that there are some shiny surfaces in there that could be causing the reflections. Another possibility is that the baffles were not working as they should, however this line of thinking is inconsistent with the image above which presents no reflections whatsoever. Even with the new focusor this problem presents when a very bright star is just outside the field of view; an ongoing investigation into how to solve the problem is current. I will inform once I have corrected the problem.

The image of the moon below is the most impressive taken during the first imaging session. Each image was very sharp and well detailed. I was immediately excited by this development. I am very confident that the figure of the mirrors has been under taken very professionally and the resultant detail is proof of this notion.

Post Purchase Modifications

As mentioned above I have purchased a better focusor for the telescope. I bought a Feather Touch 2" focusor with 1.5" of travel. The flange adapter that is needed for it has a 90mm thread with a 1mm pitch. The resultant images are now spectacular. This link (click here) will take you to one of the images, which has nice round stars and a flat field. I have recommended to the manufacturer tha they replace the current stock focusor and use a much stiffer version.

Overall the telescope is a great purchase. The more expensive RC telescopes certainly produce wonderful images but this RC is even more attractive because I could afford to purchase it. The mirrors produce sharp images and are prefect for the budget minded. If you want to do narrow field DSO imaging, then this telescope is for you. Of course if you want to buy the more expensive RC versions then by all means go right ahead.