aperture and contrast

[andrew63]

I have been observing some of the Messier galaxies over the last few nights and have been wondering.

I have a 100ED refractor and a lot is said how important aperture is - the more more the better. So contrast must also play a part in seeing the smudge against the background sky. With Saturn, there is almost too much contrast and you think you'd like to turn it down!

So this must play an important role in observing low contrast objects making up a little for the smaller light gathering capability. I not sure how you would equate this. Any thoughts?

Andrew

[brianb]

With Saturn, there is almost too much contrast and you think you'd like to turn it down!

The contrast features on Saturn's disc are very subtle, you need as much contrast as possible. If the planet is too bright (because of the dark background) observe in twilight or use a filter.

To get more contrast (visually) use a lower magnification. Of course this makes the image smaller too, which is why larger scopes give a more detailed view - when the unsteady atmosphere allows them to work to their capability.

[andrew63]

I guess I'm thinking if you are observing a faint low contrast galaxy say with a larger reflector or SCT would the greater contrast offered by a smaller lens refractor help to offset some of the advantage gain of a larger mirror - which is obstructed.

[brianb]

would the greater contrast offered by a smaller lens refractor help to offset some of the advantage gain of a larger mirror - which is obstructed.

At the relatively low magnifications used for DSO work the diffraction pattern modification caused by the central obstruction has no impact at all. The central obstruction reduces the light transmission but less than you'd think - a 33% by diameter CO reduces light by only 10% (square rule) which is the same as reducing the aperture by 5% ... and about that amount of light will be absorbed going through the relatively thick lens elements in a good refractor of any size.

[andrew63]

Thanks Brian, thats quite helpful as i may be thinking of getting something like a 6 SCT so i would benefit from most of the extra aperture for DSO.

[SiriusB]

I Agree with Brian that the central obstuction's effect is insignificant for most DSO's.(might possibly influence globular cluster resolution?? if large)

It's not the only factor that affects a scopes contrast though.