Quote:
Originally Posted by ausastronomer
Hi Aidan,
I wasn't trying to discourage you from using the scope as a visual instrument, only to help you set your expectations at the right level. At low to medium power it will do a fair job and perform like a 12.4" scope because of the light loss due to the large central obstruction. At low and medium powers the image quality should still be reasonable. At high powers the images will definitely fall apart. I am not sure at what point this will happen. You should at least be able to run to about 150X with decent images as this is still only ~10X per inch of aperture. Hence my recommendation of using a 20mm to 25mm eyepiece. It may well go somewhat higher than this before it falls over. There's a lot going on here to affect the high power images and knowing where it breaks down is just going to be trial and error. As I mentioned in my previous post the large secondary obstruction will have a major affect on the MTF curves and it will also cause significant diffraction. In addition to this however there will be introduced spherical aberration because of the change in focus position. All cassegrain telescopes achieve focus by changing the intercept distance between the primary and secondary mirrors. In most cases the primary mirror moves relative to the secondary mirror and the telescope tube itself. This is a good thing for imaging because it creates a large back focus distance range for small amounts of mirror movement. Unfortunately it also introduces a problem which doesn't manifest when imaging, because the telescope is designed and optimised for this purpose. Cassegrain telescopes have specific curves on their secondary mirrors calculated for a given overall telescope focal length, a given intercept distance between the 2 mirrors and a given secondary amplification factor. In other words the secondary is ground to be correct for a specific overall focal length only, incorporating a known back focus distance and this is usually optimised based on the telescopes intended use, which in this case is imaging. As soon as you vary the distance between the primary and secondary mirrors the curves on the mirrors are no longer correct and spherical aberration is introduced into the system. The amount of spherical aberration introduced is dependent on many things including the focal length and ratio of the primary mirror, the secondary mirror amplification and the change in intercept distance between the two mirrors. As a rough guide for an average type F8 Cassegrain Telescope using an F2 to F3 primary mirror, if you change the back focus distance by 35mm you introduce a 1/4 wave of spherical aberration. If you change the back focus distance by 50mm you introduce a 1/3 wave of spherical aberration. When you use an eyepiece in this scope it will require somewhat more focuser out travel than is normally required when imaging, this causes introduced spherical aberration and the optical system will become over corrected. The same scenario applies to all cassegrain type telescopes notwithstanding that different cassegrain type designs use different curves on their primaries and secondaries, to achieve correction for spherical aberration. In the case of this telescope which is a Corrected Dall Kirkham it uses an ellipsoidal primary mirror with a spherical secondary mirror. Assuming the focus position between camera and eyepiece changed by 35mm the telescope system would be 1/4 wave over corrected and that's presuming you started with perfect optics. If you used a barlow which would generally require even more focuser out travel than just a native eyepiece the system would become further over corrected and the images would deteriorate even further due to the additional over correction. If the eyepiece in fact required additional focuser in travel compared to the focus position with the camera the system would become under corrected by an equivalent amount based on the change in back focus distance. With an imaging platform like this, the back focus distance is calculated and optimised to achieve focus on a CCD chip not on the focal plane of an eyepiece. Consequently you have a large secondary obstruction and a significantly over corrected optical system wreaking havoc on the high power image quality. These issues are a lot less of a concern at low and medium power.
Cheers,
John B
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Hi John
thanks for the response, the real reason why i was keen to get an eyepiece on this is just to take advantage of the aperture and get some decent planetary views as well as a couple of DSO's. but from what you say it wont really be possible. it was very much an after thought, i got a telescope to put a camera on the back, not really for visual but it is nice to let people look through it when they come down and see my observatory. there is nothing that will connect you to space more than seeing Saturn through an eyepiece. but if it is better to do it through a 10 - 12" dob then i will do that.
just one thing you mentioned that was new to me was around the focusing of cassegrains. you say that all cassegrains achieve focus by manipulating the distance between the primary and the secondary, i understand that is the case for SCTs but with a CDK the primary and secondary are fixed. if extra travel is needed there is no manipulation of the primary to secondary distance, you just move the eyepiece further back. so how would this impact the optical performance or am i fundamentally missing something here?