Quote:
Originally Posted by dulwich.hill
Hi John,
Thanks for your input. My understanding is that the central obstruction in the SCT is their weakness. The other weakness must be the combination of using the corrector lens and then two other mirrors. The image has to degrade more than a refractor or newt dob.
My preference would actually be for a nice 6" APO Triplet refractor, but my budget falls short by around $7000! The main requirement is that I want the telescope to easily fit in the boot so that I can take it camping with my kids and friends.
The SCT fits my needs as I can get what I think is big aperature in a small package. I currently own a 6" Achromat Refractor, it is great but I want more aperture and less chromatic aberration on planets. I looked through a C9.25 SCT at IISAC on the weekend at Jupiter. It was truly a beautiful site!
James
|
Hi James,
If you have any desire to image and you need portability then the SCT or ACF design will be the best option for you. However, don't be under any aspirations that it will perform as well as an equivalent aperture good quality newtonian. A good one will get close. They aren't all good however, I have seen several that were down right terrible.
As you correctly point out, the main downsides to a SCT as a visual planetary instrument is the large central obstruction, usually 35% to 38%. This reduces the light gathering area of the telescope by between 5% and 10% but most noticeably affects the MTF curves and contrast. There is an enormous amount of literature available on the internet about this. One of the best references on it is "Star Testing Astronomical Telescopes by Dick Suiter". Traces of the MTF curves show that at the lower spatial frequencies an obstruction <20% has little effect on image quality but an obstruction over 30% has a noticeable effect.
There is an excellent free program called aberrator you can download where you can use real planetary and lunar images and see the visual effects of a change in central obstruction on the image. You can also use this program to see the visual effects of other aberrations
http://www.softlookup.com/display.asp?id=2871
In addition to the effects of Central Obstruction the SCT design can be noticeably affected by cooling and thermal equilibrium issues, which affect the higher power views.
You then have the additional surfaces involved which reduce light throughput and through the effects of diffraction and other optical aberrations, reduce image quality. They are all only minor but they all add up. You have a corrector plate additional to a newtonian and you have a star diagonal additional to a newtonian.
All the above having been said, I am sure given your requirements a SCT or ACF design is likely the best option for you.
Cheers,
John B