Resolving power and arcsec/pixel.

[VikkiFord]

I have a question and it sort of stems from Mark Thompson's Astrophotography book where he's talking about arc seconds per pixel.

Now, my 200pds has a resolving power of 0.6" and I can work back from that to see what size of object, on the moon say, is visible to me. I don't have my figures with me some I'm going to pull some arbitrary ones out of thin air in order to aid the putting of my question, hope that's ok.

So, my telescope gives about 800m on the moon and that, apparently, should be doubled so I should discern things about 1.6km in size on the moon (mumble-mumble Hubble seeing a football stadium, mumble-mumble).

Now, if my RP is 0.6" and doubled is more like 1.2" how does a pixel resolution below that figure help me? I'm putting this badly. If I can only resolve 1.2" what good is an arcsec/pixel of 0.25"? OR! Are these two things not related at all?

In other words, 18 megapixel camera against a 1.2 megapixel camera taking exactly the same size-in-frame-shot - which should be better?

My ZWO has an effective FL of 6mm, my Canon, I believe, is 36mm effective FL. So ZWO takes a shot naked whilst Canon requires a Barlow to achieve same size image. Wish I had my notebook with me, I could give the proper figures.

I hope I've managed to put my question across in a way that it clicks with some of you lovely people.

Best regards.

Vikki.

[VikkiFord]

Drat! Why do I always think of a practical example after hitting POST? Senior moment.

M13 with ZWO requires 0.5 focal reducer, Canon requires 2X Barlow. Which gives the better image resolution? I would have said the Canon. I'm prepared to be surprised.

I got a thing about M13 at the moment. Tried twice with the ZWO and found 5s and 10s exposures too short. Get a nice little cluster but not many stars, not like other pics I've seen. I see some advocating 30s and some 2.5mins. I'm bursting to try again.

This is also way I'm interested in which camera will pick out the most detail (number of stars).

Vikki.

[Uranium235]

Firstly, try not to get hung-up on image scale, resolution and the use of barlows - bigger (scale) isnt always better. Also, try to work with prime focus only (no barlows) - it will yield better quality results, unless youre doing lunar or planetay.

Out of the two examples above - the canon with the barlow would have a higher (perhaps too high) resolution, but the use of a barlow has diluted the light to the point where the exposure length required is quadrupled (not good). So as you can see, a barlow is a no-no becuase when looking for DSOs, you need to be getting as much light as quickly as possible on to that chip - so stick to F5 (its pretty good!).

Your resolution (" p/p), neednt be below 1" p/p for 99.5% of your work (as you are limited by the atmosphere). In fact, anywhere from 1" up to 3.5" p/p will give you a decent image - with most people settling for somewhere between 1.4 and 2.6" p/p. Probably the best way to get more out of your camera pixels would be to get telescopes of varying focal length, rather than introducing more glass into the imaging train.

[VikkiFord]

Thank you, U235. That answered my question. I have a choice of 1000, 650 and 500mm focal length telescopes and by switching between cameras can cover quite a nice range without resorting to Barlows (although the focal reducer can come in to it).

So, is there really a great difference between 18mp and 1.2mp when it comes to imaging through a telescope? I think the ZWO is good for clusters as it closely matches their size whereas the Canon would require the Barlow. I was partly concerned that the lower resolution of the ZWO would hinder the gathering of detail.

My 'scopes are a 102mm refractor, a 130p hacked about a bit to allow my DSLR and ZWO to achieve focus and a 2000pds (which is gorgeous).

I have a little crib sheet I made that gives the FoVs of my cameras with Barlows, reducers and naked so I can pick the appropriate FoV for the object I'm trying to image. I'm trying to be methodical (I'm like that, hehehehehe).

I understand that the ZWO sensor is tiny compared to my Canon.

Oh, I've also entered the details into Stellarium so I can see on-screen what it might look like. That program is brilliant.

Vikki.

[Cornelius Varley]

If your telescope has a resolution of 0.6 arcsec the imaging resolution for lunar and planetary imaging should be between 0.3 and 0.2 arcsec per pixel. For DSO imaging a resolution of 1.5 - 2.5 arcsec/pixel is adequate http://www.ayton.id.au/gary/Science/Astronomy/Ast_Photography.htm

[Cornelius Varley]

If both cameras are imaging at the same arcsec/pixel ratio the level of detail should be similar, the larger sensor will cover a greater area. 

[VikkiFord]

Hypothetically, if an object fills the frame of my ZWO and, through nefarious curved glassy means, I get the same object to fill the frame of the Canon, I understand that the exposure time will go up astronomically (no pun intended), the Canon detail will be higher?

Buttttt, it will suffer because of the extra glass, longer exposure - soooo (slow thinking going on here) the ZWO image, although using fewer pixels could provide the better image (lets say I'm thinking of M13, The Great Globular Cluster in Hercules). It boils down to horses for courses? It becomes a suck-it-and-see?

This is fascinating and I am only to willing to give it a try (when cloud and true dark permits).

[Uranium235]

Yes, with a barlow - the damage done to the image will outweigh any benefits, it would basically be a trip to coma hell with only about 10% of the field being useable.

Youre still sort of missing the point of what I said earlier about getting hung-up on image scale (filling the chip), though I understand your train of thought becuase I made exactly the same assumptions when I first started out. Ok, try this - get both cameras and take an image of the same object using the same telescope. Next, crop the DSLR image to the same dimensions as the ZWO image..... now compare the two at 100% zoom (using whatever software you use), there will only be a slight difference between the two. From there you will be able to judge which is the better camera to use (ie: which one was more sensitive).

You will more than likely end up using both cameras, the ZWO for small things, the DSLR for big things - or multiple targets in one shot.

[VikkiFord]

Thanks, U235. I wasn't getting hung up, honest, I was just trying to understand something and you've explained it beautifully. I just needed some reassurance that one camera wasn't better over the other and each has its ideal place. Canon for big stuff, ZWO for little stuff and keep extra curvy glass out of the equation

I got your point but was a little unclear in a couple of places but that's made sense now. Thank you.

Vikki.

[Uranium235]

No problem   I certainly did get hung up on it when I first started...lol... under the incorrect assumption that bigger is better.... Im just making sure you dont take a trip down the same road I did

After much trial and error, Ive found its more a case of "faster is better" (but slightly more difficult) - the resolution kinda takes a back seat (in my case) because most of the time its only going to end up on the comparitively small screen of a laptop or computer. Only if you were commissioned or going for wall art would you be looking to get some proper image scale. I often shoot at 3.4" p/p, which is pretty chunky... but once you throw a few panes together for a mosaic, the impact of the bigger picture begins to outwieigh its lack of resolution at pixel scale.

To get the most out of your camera(s), I would stick with the 200pds and get a corrector. The SW corrector acts as a 0.9x barlow, so with that you will be operating at somewhere about f4.5 900mm focal length - which is pretty good! But - your sub length unguided will be quite limited.

One calculation (as an imager) that you may use often is the one used to compare the speed of two systems:

Example: How much faster is a reduced 200pds than a reduced 80ED?

80ED: F6.38

200pds: F4.5

6.38/4.5 = 1.41  x² = 2.01

So we get a figure of 2.01, meaning its about twice as fast.... thats a big difference! You can apply that calculation to any potential imaging rig you might have your eye on - just to see whether its worth switiching (or not). However, comparing an F5 newt to an 80ED isnt quite fair since each has its own merits (the 80ED has stacks more contrast).
 

[Uranium235]

PS: use this.... its brilliant

http://www.12dstring.me.uk/fov.htm

[VikkiFord]

Thank you U235, I shall enjoy playing and discovering wonderful things

[StargeezerTim]

Hi, this is all a bit complex for me, but I use an app (android) called FOV viewer deluxe free, and it is free!

[VikkiFord]

I have my sensors, telescopes and Barlows set up in Stellarium so I get a good idea of what an object will look like. It's really good.

[Uranium235]

Same here, just dont forget to switch stellarium to GEM mode so you get the right framing

It gets better.... if you connect stellarium to your mount (eg: HEQ5/NEQ6), you can use stellarium to point the mount - very useful for finding stuff quickly that isnt in the handset. Though that will depend on whether you got the data cable with your mount (the one that looks a bit like a phone cable at one end), and you would probably have to buy a USB > RS232 converter (about 20 quid).

[VikkiFord]

Yes, I got the data cable with the mount. I also have the data cable (that I paid for, fume) with the AltAz Goto. Never mind, got a spare haven't I 

O-o-oo, got the converter too, I avoided the Prolific one and went for the FTDI based one. Not tried it out yet though. Will do.