Imaging - Magnification - Barlows

[Langy]

So every time someone mentions magnification where imaging is concerned they all get shot down, for which I can see why it is asked and why the answer always come back to the area of the sky rather than magnification. In fact there is a good reason for a newbie to know the magnification answer as it is easier to relate to at what magnification the scope was at when an image was taken. I'm not here to argue that question but ask something different, so hopefully that will not get mentioned again.

On with another question that does relate to this a bit.

With a scope you get a best usable magnification, however I think that I've seen it posted in the forums on more than one occasion in that really you don't want to be looking at much more of a higher magnification that the aperture size mm or at a very maximum of 1.5x the aperture here in the UK due to our climate.

That's all fine, I have the AstroMaster 130EQ and since replaced the stock 10mm EP with a 8mm BST which is by far so much better. The 10mm is now stuck in the bag probably never to see the light of day, or night sky for that matter. So my main collection now consists of the stock 20mm at 32.5x and the 8mm at 81.25x. I also have a 2x Barlows that I have to use for my DSLR, but it obviously doubles up my EP's to 65x and 162.5x the latter taking it past the aperture size. The Barlows also does get used with my cheap xBox webcam as and when required.

Since we do not talk about magnification when we talk about imaging, how do you work out what the maximum usable magnification is when something like a DSLR or WebCam is then used Prime Focus. I have the 2x Barlows and have thought at times about possibly getting something bigger. The 5x Barlows will be of no use for my EP as the 8mm would then go to 406.25x which I believe is even higher than what the manufacturer lists as the highest usable magnification let alone what is usable in the UK.

[ollypenrice]

Anybody saying that magnification in imaging is meaningless is telling the truth. Let's think it through.

To magnify is to make something bigger. I guess we agree? OK...

Bigger than what?

Bigger than the image your naked eye forms on your retina. Binoculars that magnify 8 times make the image on your retina 8 times bigger.

So now we use a camera and your retina (or mine) no longer appears in the story. So bigger than what? There is no starting point so we cannot talk about magnifying that starting point. I'll post some images I took.

Now here is the point. This is the same image. The big one is not, so far as the capture is concerned, 'magnified.' It has just been given more pixels on the PC you're looking at. The Mona Lisa doesn't get bigger as you walk towards it. (I realise that you know this!!!)

There is no 'equivalent' to an eyepiece view in imaging. You will sometimes read that such and such a focal length with a webcam is equivalent to such and such a magnification at the eyepiece but this is surely total nonsense. If anyone can persuade me otherwise then please fire away. When I look at Jupiter at 300x it looks very small. When I look at a Damian Peach image of Jupiter at full resolution on my it looks very large (and very impressive!  )

I don't want to shoot anyone down in flames but you need to understand image scale in astrophotography because it is the only meaningful measurement. How big an image does Object X in the sky form on your chip? You measure this in arcseconds on the sky turned into mm on your chip. Arcseconds per pixel, and that's yer lot. There is no other way.

Olly

[Langy]

I know all about image scaling etc. etc. etc. I've spent over 20 years in photography an IT.

The question was about the maximum usable magnification of a scope and how you determine that with imaging.

Sent from my Windows Phone 8X by HTC using Tapatalk

[ollypenrice]

Sorry, the builder arrived just as I was finishing this and I cliclked before answering you final question. The highest 'magnification' you can use (really the smallest number of arcseconds per pixel it is worth trying to capture) depends firstly on webcam or long exposure. Webcams take lots of fast exposures. Most are junk, wrecked by the atmosphere. A few are good and you keep these. You can, therefore work at a very small number of arcseconds per pixel. In long expoosure imaging (deep sky) you cannot beat the seeing so your best hope is about 0.5 arcseconds per pixel and many would say 2 arcseconds per pixel. It is much debated.

Olly

[ronin]

Ignore magnification for imaging you get an image size on the sensor.

What you do is relate the image size via the focal length to the pixel size.

Did read something from the SBIG site that gave a "guide" but cannot recall what.

Following Olly's post I think that SBIG said 1 arcsecond per pixel, then gave conditions when you may want more and when you would want less. So as Olly says open to debate.

Not exactly obvious when I read it.

[glappkaeft]

The question was about the maximum usable magnification of a scope and how you determine that with imaging.

Basically there is no easy way to do that. Generally resolution of an telescope is determined by the seeing, how well focused the the optics are and the diameter/type/quality of the optics. Note I said resolution, the best magnification is determined in part by our eyes and brain and is to large degree a personal preference. Different kinds off seeing (slow/fast, micro/macro) affects visual and photographic resolution in different ways and the answer depends on the target you a observing. A small fuzzy object can often tolerate more magnification than a contrast rich object.

[Langy]

Thanks for the replies. Sorry if i sounded a bit short but typing on a mobile is not like a trusty keyboard.

As i hope you have gathered it wasn't about magnification size of an image but more on the maximum performance of a scope for imaging.

I would say you have to take an image first to then calculate the fov the image has recorded and then divide that in your pixel resolution to calculate the coverage per pixel.

Sent from my Windows Phone 8X by HTC using Tapatalk

[ollypenrice]

In imaging the scope is only one player. Assuming a near perfect mount the other big player is the size of the pixels. There's no easy answer to any of this. What are you wanting to do?

Olly

[Langy]

It was just a case that I was thinking if to buy another Barlows. As I said I have the 8mm EP which gives me 80x and with the 2x Barlows 160x.

The specs on the scope say the Highest Useful Magnification is 306x so a 3x Barlows as far as I can tell may be of use with the EP but I think a 5x Barlows would be a waste of time on the EP.

So the question about maximum useful magnification on imaging was basically to see if a 3x or 5x would be of any use.

I know I can't do any long exposures through the scope so I either piggy back the camera with up to a 300mm lens or like tonight been out taking Lunar with webcam and DSLR (DSLR as to use the Barlows or I can't focus) and Jupiter with the webcam, with and without Barlows. Obviously Jupiter with a 2x Barlows is still very small. If a bigger Barlows is not going to gain anything then I wouldn't bother.

The scope I have is an entry scope and I don't expect to get miracles of hubble pictures, just looking to push the scope to its maximum.