difference between a 5 inch reflector and SCT

[Pitbull]

Hi all, apologies if this has been covered elsewhere but I am trying to work out what the difference is between a reflector, refractor and SCT for the same aperture size. I am looking to get into photography on a budget of course but cannot get my head around the difference in light gathering between the scopes.

I am considering spending my money initially on a HEQ5 mount then add the tube later as I already have a Skywatcher 130p and a Celestron 102 refractor on an AZ mount but I am trying to improve my chances of getting good photos and hopefully get into some deep sky photography when I have mastered the basics

Hope that isn't too confusing and any advice would be welcomed.

[Capricorn]

They gather the same amount of light. So in that respects no difference. The difference lies in the focal length, well usually.

The SCT will have a focal length of something like 50", 1250mm, the reflector will be closer to 25-30 inches, 650-750mm.

The image created at the focal plane by the SCT will be about twice the areas of the reflector and so half as bright. So the esposure time for the SCT will will be double that of the reflector.

For AP get a short focal length and then the tracking errors are reduced.

Also for AP get an equitorial mount.

[LukeSkywatcher]

I think Capricorn is right about the difference being in the focal length. Imaging wise.....long focal length (1000mm+) is good with planets and the Moon, short focal length (650mm+) is better with DSO's.

SCT's confuse me somewhat because the OTA is so short but the focal length is so long. I have read that you should double the exposure times if imaging with SCT's.

With AP though it is ALL about the mount. An EQ is ESSENTIAL.

I would imagine that your 130P will be very good on the EQ mount you plan on buying. No need to buy another tube.

[ollypenrice]

Light gathering is dead easy. It goes exclusively with the aperture of the objective and that is absolutely that, full stop. Double the aperture (Pi r squared) and you quadruple the light grasp and divide by four the exposure time on a modern camera. There are caveats based on stars being point sources but on amateur telescopes they are not really point sources and in the real amateur imaging world the distinction is academic.

Less obvious is what you do with the light you have grasped. Here F ratio is your guide. Ignoring the point source caveat (which is best ignored for what amateurs usually have in mind) F5 is four times faster than F10. Now getting a decent DS image is all about time. If you have four times as much data you will get a much better picture in the end.

So SCT, reflector or refractor at a fixed aperture for DS imaging?

SCT, no, too slow and not great in terms of off axis image quality, plus a load of other issures.

Reflector, maybe. Fast, cheap, but fickle to set up and prone to coma. Theory good, practice more difficult. The faster they are, the harder they get.

Refractor, maybe. It needs to be apochromatic and will get mightily more expensive, and slower, as aperture rises.

Me? Easy. A small fast apo refractor but others may disagree.

Olly

[LukeSkywatcher]

Me? Easy. A small fast apo refractor but others may disagree.

For imaging i agree.

[umadog]

If you're imaging you should be as concerned about the focal ratio as about the aperture. Basically, the image brightness is directly related to the angle subtended by the objective at the focal point. The magnification is directly related to the focal length.

In other words, if you have two 10" mirrors, one of which is f/4 and the other of which is f/8. The f/4 will produce a brighter but lower power view compared to the f/8. The faster scope will require a lower exposure time and when you're learning AP, that's a good thing. You can, of course, always increase the focal ratio with a barlow. A fast refractor is often thought of as being a good starter scope because it allows you take nice, short exposure, images. SCTs are not so good in this regard since their convex secondaries give them very slow focal ratios.

Angular resolution increases with aperture but beyond about 8" or 10" you will be seeing-limited on most nights.

[Pitbull]

Thanks all, sorry I didn't respond earlier, new year and all that

Some good advice and yes my ultimate aim is for better photography