Question : Why do "fast" scopes get better signal ?

[Catanonia]

15 minutes on a F5.3 MN190 good seeing and no sign of the integrated flux on bodes, but 5 mins on a average seeing night with a F2.8 same apeture gives a definate reading of it.

Why is that ?

The maths doesn't work out unless I am missing how a low F ratio scope works and it's dependency on its light cone ?

Is a lower F ratio scope a steeper light cone and hence more concentrated ?

Forgive the stupid questions, just bugging me.

[Shibby]

Perhaps it's really the SNR that's better, due to the reduced noise associated with a 5min vs 15min sub..?

PS:Just seen your image, nice flux

[andrew s]

For a given aperture (which determines who many photons you capture per unit time) the faster scope has a shorter focal length and gives a smaller image scale. This means the photons per unit area is higher for an expended object or for the smaller linear size of the PSF (point spread function) of a star image. Hence more signal per pixel. However this also applies to the sky background so which shows the deeper images depends on this and other factors.

Andrew

[barkis]

For a given Objective diameter, Increase in focal length reduces brightness, Fainter stuff is easier to pick up in a fast lens/mirror,

A practical test might be a visual of M33.

Ron.

[Catanonia]

Thanks gents, simplier than I thought

[rfdesigner]

I've built a couple of spreadsheets to predict limiting magnitudes etc. and the thing that jumped out as being the over riding factor after you've got guiding, readout noise, dark current, RBI etc under control is skyglow.

I was shocked at just how much difference just a little bit more clarity would give. Yes fast scopes help you overcome your kit noise sources, but once you've overcome them, the key seemed to be arc-seconds/pixel, rather than focal ratio and of course skyglow.

Are you sure your skyglow was constant?

Derek

[Catanonia]

I've built a couple of spreadsheets to predict limiting magnitudes etc. and the thing that jumped out as being the over riding factor after you've got guiding, readout noise, dark current, RBI etc under control is skyglow.

I was shocked at just how much difference just a little bit more clarity would give. Yes fast scopes help you overcome your kit noise sources, but once you've overcome them, the key seemed to be arc-seconds/pixel, rather than focal ratio and of course skyglow.

Are you sure your skyglow was constant?

Derek

Skyglow with the F2.8 new scope was considerably worse than before and still showed massive improvements. With the F5.3 MN190 I never imaged with more than 1/4 moon in the sky as wasn't worth it.

The other night, I had to play with the new toy and so didn't care.

[rfdesigner]

same CCD?

Derek

[Catanonia]

same CCD?

Derek

Yes

[rfdesigner]

I was about to say "ah you have different image scales" but I've taken the time to compare the ED80 and your super newt and essentially they're the same instrument.. except the supernewt packs far more light into each pixel, but it'ts the same image scale.

So.. the supernewt is gathering 6.25 times the light.

That means than in two comparable images the supernewt will overcome the noise in your CCD 6.25x as easily... so a 1 minute 'supernewt' sub should equal a 6.25 minute ED80, assuming you have no dark current. If your dark current is starting to impact at 6.25 minutes then the ED80 will need even longer.

Your comparison of 15 mins ED80 and 5 mins Supernewt gives the newt more than a 2x advantage, maybe more if dark current is significant.

If you had an f5 newt and went 2x2 binning on your CCD I'd expect roughly the same performance. (given enough cooling)

EDIT: Doh!.. you're comparing your MN190 to the super newt, not the ED80... yes it's image scale overcomming CCD noise.. try 2x2 binning on the MN190... and make sure the CCD is as cold as you can get it.

Derek

[dph1nm]

Well the answer to the original question is they don't. Both telescope should receive the same number of photons per square arcsecond. So as Derek has said, if you (hardware) bin the f5.3 2x2 there should be little difference in the images (apart from FOV of course). If there still is, then you are left with the likelihood that the two scopes are not collecting the same amount of light (dirty mirror etc?).

NigelM

[harry page]

Hi

Not my words but

There's a number of factors operating here. All 8" primary scopes gather the same number of photons if we exclude the difference caused by different sized obstructions. A 10" primary will gather 100/64 more photons than the 8".

Once we've gathered the photons, the f/ ratio determines what we do with them. This becomes critical when imaging extended objects. The longer the focal ratio (for a given objective), the larger the image. An f/10 lens produces an image that is (linearly) twice as big as that produced by an f/5 lens. An image that's 2x as large occupies 4x the area as the smaller image. The difference for imaging is that the same number of photons will be illuminating 4x as many pixels, giving 1/4 the number of photons/pixel. We normally compensate for this by using an exposure that's 4x as long.

Another way of viewing this phenomenon is that the f/10 objective has a smaller field of view that the f/5 one. Using a focal reducer concentrates the available photons into a smaller area, illuminating less pixels, and putting more photons into each pixel. This lets us use shorter exposures for the target.

Image size is actually solely a function of focal length. That's one of the reasons why planetary telescopes are f/10, f/15, or longer focal rations. These scopes' long focal lengths, with the consequential narrow field of view and larger image make it easy to get high magnification with longer focal length eyepieces.

The next problem we encounter in this journey is the effects of seeing. The larger the objective; the worse the effects of seeing become. I've attended star parties where the owners of 14" scopes had poor viewing of planets while the 6" scopes had clear ones.

We all use our scopes for different purposes. It's up to us to learn enough before making a purchase to understand what will work best for our intended use.

Harry