Peter Drew

I think that rather haphazard articles like this from well respected sources are partially to blame for much of the confusion regarding contrast and other attributes of optical systems. My main point is item 6. This should read "25% diameter", 25% area would result from a huge central obstruction! To be meaningful, I think the apertures, focal lengths and magnifications should be equal, the differences should be more easily apparent followed by discussion to mitigate them. 🙂

The filtration in a solar telescope is so severe that only the Sun is bright enough to be seen at all. This means that your target is only half a degree across so a solar finder is a great help. 🙂

I think you are in the happy position of being practiced enough to be getting the best out of the equipment that you have. It is also very much down to the user, if all you needed to be a top imager was to buy the most expensive setup, then anyone could do it. The same applies to visual, some can see detail that seems invisible to others in the same telescope. 🙂

Yes, the tube radius is a consideration but I think this "extra" will be somewhat reduced by the amount that the third mirror is placed before the primary mirror. 🙂

How soon we forget! Not so long ago you would have been in raptures over "dreadful" images like these. Just underlines your current abilities if given the chance. 🙂

An interesting project. I built a 18" Cassegrain/Newtonian with a Nasmyth tertiary arrangement. I was a telescope designer and fabricator but not an optical worker. The optics were purchased and were of the relatively simpler Dall-Kirkham configuration. If you have the optical skill to make the Cassegrain secondary, then using the other optics that you already have is a good plan. 🙂

I think using the telescope will be quite straightforward but getting the best results from it will be quite a learning curve requiring a fair bit of patience. 🙂

Of course this can work, specially if you have well designed mounts. I was suggesting the optimum placement which is worth it if convenient. 🙂

I'm afraid this would compound the issue. 🙂

It depends a great deal as to what form of upgrade you are seeking and from what current telescope. A little more detail will be helpful. 🙂

+1

Some form of finder is necessary to aid GOTO set up, be it reflex or optical, Optical would be best I think but a well aligned reflex should be adequate. 🙂

Place the weights at the primary mirror end underneath the tube, The combined weight of the focuser and the finder is off axis, placing the weights as described will help cancel that. 🙂

My first recommendation would be to go for the 2.7m dome if the extra cost is affordable, an observatory is never too big, nearly always too small! I operate an almost 9m dome and we still bump into each other in the dark. You say that the main interest, certainly initially, is to share visual viewing with children and friends. Despite the genuine concerns expressed about stability, as an outreach specialist I can tell you that the children and friends will be thrilled by the views from a 11" SCT despite some shortcomings. The imaging dimension is a completely different consideration, both Olly and Steve are seasoned World class imagers and are naturally very sensitive about mounting requirements and telescope choice for imaging. An 11" SCT would be fine for lunar and planetary imaging but most imagers would recommend something of shorter focal length to stand a chance of reasonable success for DSO's. A small, short focus APO refractor mounted on the 11" might be a reasonable compromise. Good luck with whatever you decide to do. 🙂

I'm fortunate to have a 16" SCT with a SW 150ED piggybacked on it so I am able to easily get a comparison. The biggest difference to me is the brightness of Mars as seen in both telescopes at 240x, the ED image was crisp and contrasty, I would say "refractor like" 🙂 but significantly dimmer due to the smaller exit pupil, about near the bottom of the brightness level that is comfortable to my current eyesight. The image in the SCT, if anything, was too bright at 240x, unfortunately the seeing conditions didn't permit higher magnification to address this. I have a polarising filter somewhere which might help. I have tried all the common colour filters and for me, in the main, they produce a different colour Mars, I prefer the natural colour. An ADC filter has proved useful when Mars, Jupiter and Saturn are low but less so when Mars is above 30 degrees. In conclusion, I am happy to forego ultimate detail visibility in favour of a natural looking image.

The words clamp and mirror should never be in the same sentence! Make sure the mirror is free to turn before replacing the cell into the tube. 🙂

There are much more affordable options if you need that sort of aperture. 🙂

You will be lucky to achieve a textbook star image with a large 8" SCT aperture, the passage of light three times through the main tube there seems always to be something that upsets the theoretical image wheras smaller refractors with one light passage do better. I have access to 5",8",12" and16" SCT's and none give perfect star images in the seeing conditions in which they are used. They do, however, give excellent lunar and planetary images on a good night. 🙂

Should have ridden sidesaddle. 🙂

Make sure that the solar film when fitted is in a relaxed state, attempting to fit it tight like a drum skin will affect the optical quality, wrinkles will not be noticed. 🙂

115mm. O/D 🙂

No 2 for me, no contest!. 🙂

I don't think it's as bad as it looks. Provided that the issue is dealt with promptly by following the rice trick, at worst it may leave some form of light stain. This should not impact on the performance as much as the likely disturbance of the lenses if the objective is dismantled. Something to consider if planning to sell it in the future. 🙂

Although the question was not directed to me and I'm not an imager let alone an expert one, the 2nd image, to my eye, seems the better one. 🙂