Astrophotography f ratio's and barlows

[Essex nightsky]

I have a skywatcher 114mm 500mm focal length (f/4.3) and when I connect my camera (Sony nex 3 mirrorless system camera) there is not enough inward focus as my t adapter is for lenses and gives a 55mm offset to account for the sensor being directly behind the lens mount ring (as opposed to a dslr where the sensor sits behind the mirror arrangement) I have ordered a low profile adapter which will allow the sensor to be 55mm closer to the focal plain.

While I wait for the low profile t ring I have put a 2x Barlow into the optical train and can get focused but was wondering how this affects the telescopes f ratio.

I would assume that it would double the (apparent) focal length so my scope would be 114mm aperture 1000mm focal length f/8.7.

Is this understanding correct?

[carastro]

My understanding is it quadruples the F ratio by the inverse square law.  

Carole 

[Tommohawk]

I think that if you use a x2 barlow you're doubling the effective focal length, and this must mean you're doubling the f ratio.

But this means you'll need four times the exposure time.

Does that sound correct?

[carastro]

Yes that's probably the right explanation, I knew it was four times as long to get the same data.

Carole

[Essex nightsky]

Thanks for the replies, going to put the dsos on the backburner and do some solar system imaging until low profile adapter arrives and ill have an f/4.3 scope

[ollypenrice]

If you double the focal length with a Barlow you double the focal ratio and that quadruples the exposure time.

Or does it? Not necessarily.

If you were shooting a small target that fitted on the chip with and without the Barlow then it doesn't. Neither Barlows nor reducers alter the amount of light collected from the object. All they alter is the number of pixels onto which the object's light is poured.

But your plan seems sound. Get the low profile adapter and enjoy widefield at a nice fast F ratio.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[Tommohawk]

If you double the focal length with a Barlow you double the focal ratio and that quadruples the exposure time.

Or does it? Not necessarily.

If you were shooting a small target that fitted on the chip with and without the Barlow then it doesn't. Neither Barlows nor reducers alter the amount of light collected from the object. All they alter is the number of pixels onto which the object's light is poured.

Well, I suspect I'm gonna get torn to shreds here, cos I'm still a novice compared to Olly, but....

First, I was just trying to clarify the numeric effect of Barlows on F ratio. I think it must be true that if you use a 2x barlow you double the effective focal length and so double the F ratio.

I do agree that making the target image twice as big (linearly) doesnt alter the total amount of light which you collect on the chip/sensor - providing the image still falls within the size of the sensor of course. But because the same amount of light is spread over a larger area, the number of photons per square mm will reduce - by a factor of four.

This must mean that to get an image of the same brightness (per unit area) you need to quadruple the exposure time. Of course for a very bright image this is of no consequence - lunar or planetary images are quite bright, so increasing the exposure time is of no real consequence and a bigger, better resolution image can be had. My latest DSLR effort on Venus was done with x3 barlow and video settings of 50 fps 1/250 sec ISO 200, so hardly any issue with exposure time.

One practical illustration of this is that if I find an unbarlowed target such as Jupiter on Live View using say ISO 400 and 1/200 sec exposure, when I place the 3x Barlow in and refocus, the image is almost invisible - until I adjust the the exposure and/or ISO settings. This happens precisely because the light is spread over 9 times the area. Again, this doesnt matter in this case, but needs to be taken account of.

However, if imaging a faint small DSO, if the target is Barlowed, this becomes much more of an issue and exposure times will surely become very significant. To complicate things, with an increased focal length, if the guiding isnt spot on the resolution will suffer.

Of course all of this is less critical for CCD imagers - like Olly - but for us mere mortals with DSLRs, super long exposures mean less beauty sleep.

(dons fireproof overalls and waits for the heat!)

[dph1nm]

This must mean that to get an image of the same brightness (per unit area) you need to quadruple the exposure time.

Per unit area on the detector, yes. But what really matters is the brightness per square arcsecond on the sky, and this not changed. You could happily bin up your long f-ratio image 2x2 and get the same image as at the short f-ratio in the same exposure time.

Where this may fall down is that you get more read noise (and maybe more dark current) per sq arcsec. Whether this affects the final image depends on how much sky signal you are getting.

NigelM