Maximum Magnification of Astrophotography Camera

[darthmoo]

I've recently acquired a Sky-Watcher Explorer 130M (900mm focal length, 130mm aperture / mirror size, maximum recommended magnification 260x).

I've also ordered a budget SV105 camera for getting more into astrophotography. 

As discussed in the thread here, the SV105 has a field of view equivalent to looking through a 4mm eyepiece. If I use a 4mm eyepiece that's a magnification of 225x. Adding a 2x Barlow lens the magnification would be pushed past the maximum limit to 450x.

I know that you can't really talk about "magnification" in the same way regarding cameras, but does the same limit apply? i.e. if the FOV of the camera is equivalent to a 4mm eyepiece, will a 2x / 3x / 5x Barlow result in a blurry image because of the maximum recommended magnification? Or will it be crisp because the camera doesn't work the same way as an eyepiece? 

[vlaiv]

8 minutes ago, darthmoo said:

I know that you can't really talk about "magnification" in the same way regarding cameras, but does the same limit apply? i.e. if the FOV of the camera is equivalent to a 4mm eyepiece, will a 2x / 3x / 5x Barlow result in a blurry image because of the maximum recommended magnification? Or will it be crisp because the camera doesn't work the same way as an eyepiece? 

Term magnification does not apply to cameras and camera FOV is only indirectly related to resolving power of telescope.

With cameras, maximum resolving power of a telescope is related to pixel size. SV105 has 3µm pixel size (from what I see via quick Google search), which means that optimum F/ratio for that F/11.7 or about F/12.

Your scope is F/6.92 so x2 barlow would be most suitable to raise your F/ratio to F/13.8. Barlow magnification changes depending on distance from optical element to sensor - so if you can bring it a bit closer, you could be able to get F/12 or F/11.7.

This is theoretical maximum "magnification" (although like I said - wrong term here). Using higher F/ratio, or longer focal length will result in larger image without any additional detail, so this is the limit. You can use shorter FL and besides maybe missing out on smallest feature - no harm is done.

In reality, maximum level of detail will be governed by atmospheric turbulence / seeing. If you want to observe Jupiter via computer - then just go ahead and dial in exposure length for best image, but if you want to image Jupiter - well, that is all together another matter.

Planets are imaged using something called Lucky imaging approach. Movie is made consisting of very short exposures - like 5-6ms each, and many frames are captured - often in tens of thousands. Then this video is fed into special software that selects best frames - ones the least distorted by atmosphere and stack is created (meaning average of these best frames) - that improves signal to noise ratio making images less noisy.

In the end, special sharpening algorithms are applied that reverse some of atmospheric influence and limitation of optics and one ends up with decent sharp image of planet - like this one:

This was captured with exact same scope (SW 130/900 Newtonian) and similar camera - QHY5LIIc, using above lucky imaging technique.

By the way - Welcome to SGL

 

 

[darthmoo]

Wow thanks for the detailed and informative response, that's been really helpful! And that's such a great image of Jupiter. 

I've read / watched videos about image stacking (PIPP, Autostakkert, RegiStax, etc.) but obviously haven't had a chance to try it for myself yet... 

For capturing that photo (or video frames) were you using a Barlow lens (and if so was it 2x) or just the camera straight into the scope? 

Thanks again! 

[barbulo]

Welcome to SGL @darthmoo 

After the precise explanation of vlaiv, as always, you can try astronomy.tools to experiment with different configurations. Use visual mode for eyepieces calculations and imaging mode for cameras’.

[vlaiv]

10 hours ago, darthmoo said:

Wow thanks for the detailed and informative response, that's been really helpful! And that's such a great image of Jupiter. 

I've read / watched videos about image stacking (PIPP, Autostakkert, RegiStax, etc.) but obviously haven't had a chance to try it for myself yet... 

For capturing that photo (or video frames) were you using a Barlow lens (and if so was it 2x) or just the camera straight into the scope? 

Thanks again! 

Given the size of the image, I think I was using x2 barlow at the time but not sure what magnification it gave. Probably this model:

https://www.365astronomy.com/GSO-2x-Barlow-2-Element-Achromatic-Barlow

It has barlow element with 1.25" filter thread that you can unscrew and screw onto nose piece of camera. I probably used it like that, since at the time I was not familiar with all the technical details like critical sampling rate (max F/ratio) and all of that.

By the way - placing barlow element further away gives larger amplification of focal length, and placing it closer - smaller. Therefore if you have such barlow with removable lens element - you can vary magnification just simply by using different 1.25" (in this case or T2 in some other cases) extensions.

We can actually calculate what sort of barlow I was using, from the image.

Jupiter on the image measures roughly 143px across. Image was taken in April 2015, and apparent diameter of Jupiter at that time was around 40".  If we divide the two we get 40" / 143px = 0.28"/px.

I was using 3.75µm pixel size camera which gives FL of ~2760. So barlow used 2760 / 900 = x3

At the time I did not have x3 barlow. I had x2 and x2.5 GSO barlow, so I probably used the later one with a bit more spacing to give it x3 magnification (not that I aimed for that - I just inserted nose piece of camera into barlow and that is what it gave me ).

[darthmoo]

Again great explanation, thanks for your help. Excited to try it out when it arrives! Really awful weather here in the UK though so might need to wait a few nights...