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Pixel size and F ratio

Planetary Camera Acquisition Solar System
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Marc avatar
Marc avatar
Hi,

first of all, sorry, there are answers to similar questions over on the Googlez, but I just need advice specific to my particular situation, so here goes. 

I am not a planetary imager, far from it, but I would like to dabble in it just a little bit. The scope I want to use is an Esprit 150 (6" APO, F7) - so decent, not great for this kind of application.

I have just ordered a QHY III 715C, which is the "new" planetary camera released by QHY - color version, with tiny pixels of 1,45um. Color version, because I don't se myself getting into RGB imaging just for planets (and I am not going to mess around with my Deepsky kit, it's currently working so well ;)) 


The math I was able to find says that, for good-to-OK skies (which is the best I can hope for on most nights), I should multiply the pixel size by ~5 to get my optimal F ratio. So that would be a little over 7 (7,25 to be exact). 

The way I read this means that I can use this camera with my APO without needing a barlow, correct?

Along the same line, would adding a 2x barlow give me any advantage, at all? I also ordered an ADC to squeeze a bit more out of my images, and these are said to perform best at F ratios > 10 (but apparently still work at faster speeds…). Or is this is a waste of money?

Clear skies,
Marc
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Pablo Petit avatar
Pablo Petit avatar
Hello Marc

I think the important factor here is your sampling and the resolution of your scope.
A 150ED, with a diameter of 150mm will have a maximum resolution of a little less than one arc second, depending on the criteria you use.
You can use this calculator to figure it out : https://astronomy.tools/calculators/telescope_capabilities
For example, with the Dawes' limit you are at 0.77.
Now for planetary you want your sampling to be a bit below that limit to get the best of your scope resolution.
With a 1.45um pixel, your sampling will be 0.28 arcseconds per pixel (https://astronomy.tools/calculators/ccd_suitability)
Which is more than enough for your scope.
If you reduce your sampling even more with a barlow, you will not get any more resolution because you already are at your scopes' maximum,
but you will get a fainter image, so I think its better not to use one.


Pablo
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Nick Grundy avatar
Nick Grundy avatar
Hi Marc, there's a lot of great planetary imagers that can probably give you a better answer, but…I got an ADC awhile back and it has really felt like a waste. The few times I used it, I couldn't see much of a difference in the images. (maybe someone else has better experience with it). 

Much like everything here, I would just try it out without the barlow and see how it goes. You can always get a barlow and ADC after a session if you are dissappointed.
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Marc avatar
Marc avatar
Thanks guys, this was really helpful . So yeah, I'll just see how it goes. The ADC was mostly me thinking that I wanted to get as much out of my measily 6" aperture as possible - and I am not sure I would really be able to tell one way or the other if I just start without one (without a clear frame of reference, that is). Food for thought, I suppose!
Nick Grundy avatar
Nick Grundy avatar
Marc:
measily 6" aperture as possible


the last thing i'm calling that Esprit 150 is measily! Although F10+ is probably best for planetary, I'd still be expecting some solid images from that scope. 

Looking forward to seeing how they turn out!
Marc avatar
Marc avatar
Yeah, fair enough smile  The Esprit 150 is a really nice scope, and I have seen some rather impressive planetary images over on Cloudynights taken with it. Obviously, they can't hold a candle to shots taken with a large SCT. I just hope that I will be happy what I get and not start thinking about getting a C11 next smile don't have room (or funds) for yet another scope…^^
James Tickner avatar
James Tickner avatar
For what it’s worth, my usual rig (102 mm F/7 ES apo with a mono camera with 2.9 um pixels) is significantly more undersampled than your setup, with a focal-ratio to pixel-size ratio of just 2.4. 

On several occasions I’ve tried collecting images with and without a 2x Barlow. I use a 3x drizzle in AutoStakkert on the Barlow-less image and a 1.5x drizzle on the Barlow image, so the final image size is the same. Honestly, I’ve never been able to convince myself that I could see any difference between the final images: it seems that the extra drizzle recovers the same detail that is obtained optically using the Barlow. And skipping the Barlow makes for an easier setup!

Maybe my setup or seeing isn’t great and your mileage might vary 😀
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enta avatar
enta avatar
I fiddled around a little with my 5" apo (950mm) and a 2x barlow I lend from a friend.
It looks better in the preview and I thought my image will be much improved.

The result was disappointing, Saturn 2x bigger and 2x less resolution.
Surely you can catch planets with your apo but it won't perform well no matter what you attach to it.
Marc avatar
Marc avatar
Hi,

thanks again for the feedback. I was able to take the setup for a short spin. I mean...I am not dissapointed. At the end of the day, 6" wont beat a C11, but hey - looks like Jupiter to me Need to learn more about editing planetary images, but I think this is a decent first shot. 


Esprit 150 with QHY715, no barlow.