Toscano 8 inch RC Update 23rd August 2006

 

This is the first update following the initial period of use with the Toscano 8 inch RCT. I have managed to acquire three images to date using the scope, two at the native f9 and one using the AP 0.67 focal reducer.

 

 

First impressions.

When the scope first arrived I was impressed with the overall quality and finished. For the price it represented good value for money. The scope was supplied with a William Optics SCT focuser and a set of good quality tube rings. However, I replaced the focuser with a TCFS focuser.

 

 

Initial visual use.

After mounting the scope I set about checking the optical quality. A quick check showed that the scope needed some post travel collimation. This was achieved visually using the secondary adjustments only. Although I have owned SCT type scopes in the past, I was inexperienced at the time and had never really carried out a star test so had no real idea what to look for except through my experience with refractors. my first thoughts were that the out of focus image was not ideal. However, after some consulting with Ian King and a bit of research it was decided that the out of focus image was perfectly normal for an RC.

 

 

Getting the focal plane right.

After some initial use with my ST2000XM and filter wheel, I determined the correct spacing required and had some extension tubes made to get correct focus position.

 

 

Precise collimation.

The next job was to pin down the collimation to as good as I could get. For this purpose I purchased a Takahashi collimating microscope. However, this was dogged by a few problems that were not the fault of the scope, rather of the TCFS focuser that I had fitted. It quickly became apparent that the focuser was not on the optical axis as checked with a collimated laser. Unfortunately the model of scope that I had did not have any adjustment to the rear focuser attachment tilt / tip as with previous models. In an effort to get the optical axis correct I needed to adjust the spider centering and use some carefully placed sticky tape on the TCFS SCT mounting adaptor. Unfortunately this condition was not ideal and while it gave reasonably good collimation, it produced double spider diffraction spikes on bright stars due to the bend in the spider vanes. I decided that the only way to get it right was to climb a very steep learning curve involving the various steps and procedures needed to collimate these types of scopes using the Tak collimating scope, and what the various collimation images mean in terms of the information they give you and the adjustments required. Along the way I have a few recommendations to make regarding the scope and the methods that I use for collimating.

The first thing I decided was that the existing secondary adjustment screws on the scope needed replacing. They were of a Philips head type and this meant that the cross head screw driver was prone to slipping when the screws got close to being tight enough to hold collimation. I replaced the screws with some socket head 4 mm x 20mm screws - much better. After a bit of conversation with Ian King, the latest scopes use a push pull adjustment that is much better that the spring loaded system on my version.

The next job was to get the focuser on the optical axis with the spider mechanically centered so that I got rid of those double diffraction spikes. This was achieved my carefully measuring the position of the secondary using some engineers calipers and by placing some spacer washers behind the rear cell focuser mounting plate. I would hope that newer models of the scopes re-introduce the adjustable rear cell - a very useful feature that I feel is essential to getting the best collimation possible. However, it is simple to make a modification to the existing plate with the addition of some slightly longer screws to replace the existing ones, and the addition some threaded holes and screws for the plate. I intend to make this modification at a later date.

After all the learning and adjusting I have achieved what I feel is very good collimation. Along the way I have written a collimation article that sums up my experience and the methods I used during the collimation process.

Collimation article

 

Some images taken through the Tak collimating scope can be seen here

 

 

The collimation and field according to CCD Inspector.

I had also been trying out CCD Inspector by CCDWare. This is an excellent program and I feel that it provides a very good analytical tool, not only for image selection and processing, but also for helping to determine various optical errors and corrections that can be made. Initially I have been cautious about publishing data regarding my optical system using such a tool, but after a period of use that shows consistent, repeatable results, I believe that the data is valid.

The images below are links to larger versions. They show a series of data taken from my luminance sub exposures for the image of M56 with the scope operating at f9. The curvature and 3D images show good collimation, minimal tilt and tip for the image sensor, and a nice flat field for the ST2000XM image size. On the whole I think that they are a testament to the good quality optics and value for money of these scopes. While the curvature and 3D map were determined using an average of all the sub images, the individual data does not show a marked deviation from the average, therefore I would say that it is valid. As a point of interest reduction of various data shows that the scope is operating at a true f9.1 and focal length of 1820mm.

          

The next set of CCD Inspector data images show similar information except that this time an AP 0.67 focal reducer was added to the optical system. Again the curvature and 3D maps are averages of the data set. Using the focal reducer also gives a good flat field with a focal length of 1288mm at f6.44

          

 

 

Summary so far.

Overall my impressions of the scope are good and I am pleased with the results so far. It has been a bit of a difficult road getting the collimation sorted but mostly due to the TCFS focuser issues, although some modifications to the scope adjustment system would / will help. Of course the collimation is always going to be critical to these types of scopes but that is just inherent in the design and you have to accept that you will need to be meticulous and patient. Other areas of interest are that the scope has held collimation well, needing no further adjustment, the temperature variation in focus seems minimal, there does not seem to be any detectable flex at different pointing directions, and the finish is holding up OK. By the way I have not had any dew issues so far, even when using the scope and flexi-dew shield on muggy nights.