Could someone please explain this to me?

[trudie]

Hi All,

I realise I am dealing with a complete minefield in this subject, but the more I read the more confused I am getting.

I am trying to get my head round 'f ' numbers and what they mean when doing astro photos, but I feel like my brain is about to explode.

I have read CC's post from January but still confused. I have an ED80, an spc900, and until today a 2x barlow.

Does the barlow make any difference to the f number, or am I missing the point here?

I have just received a 4x imagemate (thanks Steve) to have another go at Saturn among other things.

I am sure you are reading this and thinking I am totally stupid, I know my scope is f7.5, but what I am to find out is.

1) Does this number alter when I use different EP's or not?

2) Is high or low f number best for photos?

Plus about 500 other related questions whizzing round in my head at the same time.

Apologies for being so thick.

Best Wishes

Trudie

[Andrew*]

Trudie,

yes, a barlow will multiply the f/ ratio. That would effectively turn your f/7.5 scope into a f/30 scope.

1) no. Different EPs don't alter the f/ ratio, they just alter the magnification (focal LENGTH of scope divided by focal LENGTH of EP = magnification)

2) depends. To get the image scale (the size of the thing on the chip) for planets, you generally need a high f/ ratio. For fainter DSOs, however, you need as low an f/ ratio as you can get, because in imaging, the lower the f/ ratio, the shorter the exposure. For this, you can buy the opposite of a barlow - a focal reducer.

I hope that explains accurately what you need to knwo.

Keep those Qs coming!

ANdrew

P.S.

Apologies for being so thick.

:nono: don't do that - there's no question too thick on here!

[trudie]

Thanks Andrew,

That makes it perfectly clear, I was really struggling there, the more i read the more I did'nt understand.

2 more questions please,

Can you use 2 barlows together or is that no go?

if you can, does a 2x plus a 4x make 6x or 8x, hope that makes sense.

Thanks

Trudie

[Jamie]

Can you use 2 barlows together or is that no go?

You can use 2 barlows but it will be near impossible to focus as the focusing

point will be such a fine point...

if you can, does a 2x plus a 4x make 6x or 8x, hope that makes sense.

It would make it (theoreticaly) X8

There is a reason you don't hear folk talk about using more than one barlow

and you will find out why when you try (and i know you will)

[Indisguise]

I'm sure you could use two Barlows stacked up on top of each other, but why would you? In my (admittedly limited) experience, Barlows dim the image a fair bit so you there isn't much point. I was reliably informed that a smaller but brighter image is preferable than a larger but dimmer (and less defined) image). Also don't try and over magnify, because your scope probably wouldn't take it and your field of view would be so small, you'd see much anyways. The basic rule of thumb for your maximum magnification is the apature of your scope x2. Ie: a 150mm aparture scope doubled would give you a maximum magnification of x300.

[GordonCopestake]

People do stack barlows, but the resulting image is very dim and VERY hard to focus. Rather than stack barlows just get a 5x powermate if you need that high a magnification. I can't think of ANY target that needs more than a 5x powermate

[trudie]

Thanks all,

The barlow question was triggered by a debate between me and my other half, I won't try it.

Am hoping 2nite will be clear so I can try out my x4 imagemate

Trudie

[Kaptain Klevtsov]

Hi Trudie,

The f/ number relates to the dimness of the image when doing astro photos. Other than that you needn't worry about it. If you add a Barlow lens you just multiply your 'scope's f/ number by the Barlow ratio which is usually 2, or for the Imagemate 4.

The necessary exposure time is related to the square of the f/ number so doubling the f/ number using a 2X Barlow lens will make the exposure times 4 (2X2) times as long. The imagemate will need 16 (4X4) times the exposure compared to without the Imagemate.

Many astronomers use the f/ ratio as some kind of measure of something or other, it only helps work out if they used a Barlow lens if you know the unBarlowed f/ number of their 'scope. It doesn't bear any relationship to the image size on the Toucam.

I've found that filling the frame of a Toucam with Saturn needs a focal length of around 12000mm (which I can't manage) so I make do with 6000mm and a half frame sized Saturn. That lets me have some leeway before it falls off the edges so it's a good thing. I've also worked out that using my 300mm Newt. I get an f/ number of about 20 which is the focal length (6000mm) divided by the mirror size (300mm).

To get a focal length of 6000mm with an ED80 you would need a 10X Barlow and end up with an f/ number of 75. That would be too dim to image as there wouldn't be enough light to show up on screen.

You need to play with the equipment and find out what it can do as there are no hard and fast rules, only the laws of Physics and Optics.

You're not being thick at all, it's a very unweildy concept which should be got rid of and focal length quoted instead IMHO.

Good luck with the 4X tonight, and bear in mind that you will need the shutter speed on the slowest setting and the gain on maximum to find your target. Get it centred without the 4X, check the alignment of your finder is spot on, then fit the Imagemate and refocus until you can see it. Don't be tempted to reaim the 'scope until you are totally convinced that you can't find it. If that doesn't get it, try with the 2X to centre it better, then go for the 4X. Remember that at such a magnification you will need to use the slowest setting to slew your mount to centre anything, otherwise it will whizz off and never come back.

Another tip is to try in daylight to see which way you need to move the focuser from the normal position to the position with the 4X fitted as well as how much. Focus will be very fiddly with the 4X fitted so every bit of practise helps. Try this with the 2X as well so that you have a better chance of doing it in the dark.

Captain Chaos

[Jamie]

Trudie

To be honest you only need the higher F ratio when imaging the planets (they image better @ higher F ratios)

DSO's on the other hand image better at faster F ratios...

[trudie]

Thanks all,

And thanks to CC have printed out your advice and will have it to hand 2nite, sky looking great so far, fingers crossed, will post if any good.

THanks very much again to one and all

Trudie :D