Which scope? Is my math correct?

[Rex]

G'day everyone,
I have a HEQ5 pro and am researching scopes to put on it for astrophotography. I was thinking about a skywatcher ED80 or ED100, which I know do an ok job. But in my research I have found the GSO RC series of scopes.

I did a quick mathematical comparison using the area of the objective/ primary mirror to gauge light gathering power between a couple and was wondering if I have done this correctly.

For the ED series I basically just took the area of the objective, for the RC scopes I took the area of the primary mirror and then subtracted the area of the obstruction to get the usable area.

The results were surprising:
ED80: 5026 square mm
ED100: 7854 square mm
ED120: 11310 square mm
RC6 - 152mm primary, 76mm obstruction: 13609 square mm
RC8 - 200mm primary, 100mm obstruction: 23562 square mm

If I am correct that means the RC6 has similar light gathering power to an ED120.

Next question would an RC6 have the similar image quality as and ED scope.

Sorry to babble and thanks for your help in advance.

[RobF (Rob)]

Hi Rex

You might find it helpful also to play with a freeware application called CCDCalc. It lets you visualise what sort of FOV you can expect for certain scope/camera combinations. There really is no such thing as the best universal image scope. If you're starting out might be an idea to keep your focal length under 1 metre to make things a lot easier though. Keep in mind too that the "speed" at which you accumulate photons is proportional to the square of the focal ratio. e.g. an f/5 scope will pull down light twice as fast as a similar f/7 OTA.

There are all sorts of pros and cons to consider for which others here will hopefully share their experience.

[Poita (Peter)]

BTW, what targets are you interested in imaging?

[Rex]

G'day Rob thanks for the advice. I hadn't thought about the longer focal length being more difficult, I did realise that the shorter focal lengths are faster, however if you put a barlow on a faster scope to frame your object the way you want, would that be the same as having a longer focal length to begin with? Thanks for the link I will check it out. I was kind of hoping that whatever I get was going to last me a while and looking at the price, if my light gathering math is correct then the RC6 is much cheaper than an ED 80 and much much cheaper than an ED100. I currently have an Olympus DSLR camera and some lenses that I have been taking photo's with, and I recently got guiding working with a webcam. Although I havn't obtained any substantial data yet using the guiding because of the clouds. I can get my polar alignment pretty close as I can see the octans from my place and then I drift align to touch it up, so I was hoping the longer focal length wouldn't be too much of an issue??? But not really sure how much harder it is.


G'day Peter I like DSO's. With my current set up I am restricted to widefields but would love to take some Planetary Neb photo's, globs and galaxies are also a big interest of mine.

Any advice would be appreciated regarding the math in my first post and the optical quality of these scopes as a comparison.
Thanks again for your help

[rmuhlack (Richard)]

your calcs are correct, however i second the above comments regarding focal length. I've certainly found imaging with my 200mm f6 newt to be more challenging with regard to polar alignment and guiding, in comparison to my 200mm f4.

Another option you could consider with the HEQ5 is the 150mm (6") GSO imaging newtonians at Andrews. This was an option that I was initially considering earlier in the year - same aperture but shorter focal length than the RC6 and well within the load capacity of the HEQ5 mount.

Remember that you'll need to factor in a corrector too, regardless of what scope you go with - a coma corrector if it's a newt, or a field flattener if its a refractor or RC.

[Rex]

Hi Richard. I had originally planned on getting one of the gso 8" newts either f4 or f5. The only reason I was looking further was because I want to use an OAG instead of a guide scope and the newts probably won't come to focus. I was aware of the need for coma corrector or field flattener. Thanks for your help.
Thanks everyone for your help, you have definately given me some things to consider and think about.