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Larger Chip or Maybe Smaller???


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Hi All,


Apologies for thinking aloud however I would appreciate views from other astrophotographer's on the following and if anyone's trying/doing anything similar???


I appreciate larger imaging chips give wider fields for the same focal length and that historically a larger chip has given greater efficiency (more light to relevant internal collectors).


However the advent of back illuminated technology versus front illuminated technology has made a significant improvement in sensitivity which many of us are benefiting from.




I have also now become aware of the next step which is stacked back illuminated which improves it yet further.




I don't know how much of an improvement this has made however it sounds really beneficial...


And the crooks of the matter - cameras with larger chips are more expensive!!!


So if you dont need the wider field and I/we can get targets into the field of view of the smaller chip, and if the predominant factor that determines light collection is the scopes aperture size (i.e. more photons onto smaller chip area/fewer photons onto larger chip area) then should this mean astro cameras have a hope of coming down in price/we can use cheaper cameras (using smaller stacked back illuminated technology) or is there another optical factor that I'm missing?


I should say, I think larger chips will always likely be that bit more efficient with better associated adc's etc - however optically it sounds like there may be much less of a difference than previously with front illuminated technology...

I also appreciate longer focal lengths are slower and that the smaller targets are more prone to atmospheric disturbance...

Also that binning can effectively multiply the area up at the cost of resolution (both in analogue (better where available) and digital sides of chips)


I've got and am still trying my RPi HQ 12MP camera with stacked back illuminated technology gradually.


I originally was thinking the HQ camera was a little bit of fun - and with the cheap f2 lens I initially tried it didn't look the best - however I'm now thinking that was mainly down to the lens.

I need to get it on the back of a proper scope and see if it has any real legs - sorry for thinking a loud...


However eventually I've got a big mount and light bucket that gets me back some of the field size - making it sound more viable...

Also if it works out I'm thinking of eventually going larger aperture and putting the smaller camera where the secondary is, improving my optical path/reducing obstruction...

Then heading down the line of lucky imaging as per Astrobiscuit as I am sick of the atmosphere (optically)... I can do very long exposures currently however the atmosphere seams to be my limiting factor (dont live in Spain sadly) and few good nights - was going to drop astro sadly at one point - but I'm thinking there may be a way forward that isn't too crazy and may make some sense???


Comments appreciated thanks (good and bad).

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For many, cost is a major factor. If your setup can't quite fit an image you want to take then the cheapest option is to plan a mosaic. With CMOS needing a lot shorter exposures this makes it a lot better that waiting for X number of clear skies to get enough data with a CCD.


If money isn't an issue then multiple scopes and cameras cover all sizes of targets.


The RPI HQ cam sounds an interesting possibility for cheap imaging, would like to see some results when you have them.

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Hi Mark,

Mosaicing sounds like a really good addition to what I'm thinking - for larger targets - rather than trying to get all the light in at once split it up.

Also works with my intention to eventually go fully automated and big and sensitive with short exposures so I can maximise imaging time and chance of overcoming atmosphere without falling asleep at work and getting sacked.

I suppose I'll have to make sure I use a field flattener/check my optics and get good image at edges as well as centre.

Will let you know how HQ goes - some good/positive images on internet but doubt they were taken with anything spectacular - so hoping.


p.s. the link below aren't mine sadly but are the HQ on a very nice but small apo and are even from an alt/az...

Astrophotography using Raspberry Pi HQ Camera (neocities.org)


Thanks, Nick.

Edited by Demon
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Those images are pretty good, processing could be better but shows what can be done.

Looked up the specs of the HQ sensor and ideally you'll want a focal length between 200 and 400mm for the pixel size.

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Thanks Mark, That's exactly what I needed - I thought there was something else😀.

Pretty fundamental but slipped my mind.

Yep pixel size versus arc sec and after a little reading as you say - 300 fl for 1 arcsec per pixel. Or binned 2x2 for 2 which is typ for good viewing. So on longer focal length scopes technically overkill - but still cheap...

I'll run the numbers on my various scopes and Barlows and see how overkill it is and versus target sizes.

Fundamentally I need to try it - sounds like easiest is smallest shortest focal length anyway and see where it goes from there with actual results and Binning and as you say mosiacing.

But if I'd gone straight for big newt id have got confused.

thanks very much - that was exactly what I needed.


Edited by Demon
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