The Celestron 9.25 inch Schmidt-Cassegrain telescope

The foremost reviewer of telescopes in the USA, Ed Ting, has a website that gives short reviews of a large number of telescopes but he also now provides superb videos on YouTube.  In one he gives a list of his favourite telescopes.  His top scope was an  Dobsonian – my first scope was like this but is now little used as I am mostly using my scopes for astroimaging.  The first two ‘premium’ scopes that I bought many years ago were a Takahashi FS102 flourite doublet refractor – which was his number 3 – and a Celestron 9.25 Schmidt-Cassegrain having StarBright XLT coating – which was his number 2.  So I feel I am in good company.

The 9.25 has been said to be the ‘jewel in the crown’ of the Celestron range and also ‘the SCT for those who  do not like SCTs’.   There are two reasons why these quotes might have an element of truth.  Firstly as it uses an f/2.3 rather than an f/2 primary mirror as used in their other SCTs it might thus be easier to make to high standards.  Secondly this eases the curvature of the secondary mirror and so the combination increases the percentage of the field of view unaffected by coma.  The images given by this telescope could thus be actually slightly better.   The tube assembly currently costs ~£1,760 pounds.  It should be pointed out that there is now a ‘EdgeHD’ version that incorporates some lens elements within the baffle tube to provide a flat, coma free, field of view which would be an advantage for astroimagers using larger APS-C or even full frame sensor cameras.  This, however, costs £3,000.  The star field below was taken with my ‘standard’ 9.25 using a camera having a Micro 4/3 sized sensor. Below it is an image of the four corners showing that the stellar images are still quite reasonable.  Both types can be equipped with focal reducers; for the standard SCT this 4 element x0.6.3 design costs £125 but, for the EdgeHD, the 4 element x0.7 design costs £429. 

The downside of using an f/2.3 primary is that, in comparison to the other SCTs, it has a longer and relatively heavier tube assembly – not that much shorter than their 11 inch SCT.  Its size and weight puts it in a somewhat difficult category as regards the equatorial mounts that might be used with it.  It is often coupled to mid range mounts such as Ceestron’s ‘Advanced VX’ mount.  Whilst this should be fine for visual observing, astroimagers will find that it is better suited to couple with a heavier mount such as the SkyWatcher HEQ5 PRO.

Optimal position of the eyepiece and camera sensor

Celestron state that the ‘back focus’ of the telescope is 5.475 inches or 139 mm.   Using the 1.25 visual back that is supplied, I believe that the eyepiece field stop when focussed will be closer to the back of the telescope than this.  Using a 2 inch star diagonal will help somewhat. This means that to get to focus by adjusting the primary mirror the spacing between the primary and secondary mirrors will not be optimal. How much this affects the image quality is not obvious and I can find no real reference to this on the internet. One could use an short extender to get the exact distance, but in either case the eyepiece will be rather close to the back of the telescope tube making its use a little awkward.  Both problems can be solved by the use of a suitable spacer placed between the telescope back and star diagonal.  One device that could do this is the William Optics 2″ Rotolock Adaptor for SCT (at a cost of £99) which screws onto the telescope back plate and accepts a standard 2 inch star diagonal.

When used with a camera, extension pieces will be needed to achieve the optimal spacing.  I happen to have a Starlight Instruments ‘Crayford’ focuser to attach to my SCT.  I bought it to provide a fine focus adjustment so that final focus could be made without adjusting the primary mirror – and eliminate any image shift though the 9.25 SCT is actually quite good in this respect.  Using this with a 2 inch diagonal and eyepiece will make the field stop of the eyepiece considerably further away than optimum. (Though it nicely brings the eyepiece away from the telescope back.)  It does, however, provide almost perfect spacing when used with my CMOS astro-camera and, using it to get precise focus when imaging, is very helpful.

 A superb Lunar and planetary imager

The telescope aperture reacts with the atmospheric turbulence in an interesting way.  The ‘cells’ that make up the turbulent airflow have typical sizes of 50 to 100 mm but can reach up to 250 mm under superb conditions in the UK and even up to 400 mm at the Paranal ESO observatory in Chile.  If the telescope aperture is less than the cell size, then the image will move about but remain sharp – so lucky imaging can work well.  If, on the other hand, the telescope aperture is greater than this, the result will be a blurring of the image that lucky imaging cannot overcome.  Thus on a given night the best images may well be made with smaller aperture telescopes.  It is thus said that, in the UK, the 235 mm aperture Celestron 9.25 is probably the largest that can sensibly used on occasions when the seeing is good.

The nominal resolution of the 9.25 telescope is ~0.6 arc seconds. However, one effect of the central obstruction to slightly increase the effective resolution as the central Airy disk is narrower than that of an unobstructed aperture.  It turns out that the 9.25 inch is no worse than a 6 inch unobstructed aperture when imaging broader details on, say, the surface of a planet and even slightly better than an unobstructed aperture 9.26 inch telescope for resolving fine details.  It is thus not surprising that the very best lunar and planetary images have been made using SCTs up to 14 inches in aperture

The final consideration is the angular size subtended by the pixel size of  the webcam.  The Nyquist theorem states that to fully sample a surface this should be ~1/3rd of the hoped for resolution determined by the telescope aperture. (Not 1/2 as is correct for sampling a waveform.) For the 0.25 inch SCT, this is ~0.6 arc seconds.  We would thus like the pixel size to subtend ~0.2 arc seconds. A typical astronomical webcam will have a pixel size of ~3.5 microns.  Given the 2,350 mm focal length of the 9.25 SCT each pixel will subtend 0.3 arc seconds – quite a good match. Though my mono camera has a pixel size of 3.75 microns, my two colour webcams have pixel sizes of  ~4.6 microns and so the image will not be fully sampled if the seeing is very good.  One could then use a Barlow Lens to effectively increase the telescope’s focal length.  

This image of Jupiter was taken when Jupiter was higher in the sky than it has been for a while

This image of the Moon was taken on Sept 15th 2021 when the Moon was at a very low elevation but the seeing was extremely good.