Choosing an Apo - Aperture vs pixel resolution vs FL vs FR

[parallaxerr]

So you're in the market for a new imaging apo.

Your current (non-apo) set up is f5 @ 600mm with a resolution of 1.34"/pixel with DSLR, which you are happy with (adequate guiding) and you plan to use the same camera with the new scope.

Due to light pollution, you will mostly image narrowband, OSC occasionally.

What would be the order in which you prioritise the following and why?:

Aperture - More is always better, but is it more important in NB? e.g. a lot of OSC'ers & RGB'ers use 80mm - OK for NB?

Focal length - image scale & pixel resolution trade off. What do we feel is minimum acceptable pixel res in UK - 2"? If so, would you regret the move from 1.34"?

Focal ratio (speed)

 

 I'd be really interested to hear peoples approaches to specifying a new scope to match an existing camera and what spec swings the deal.

TIA

EDIT: Oops, should have gone in the imaging section, mods pls move

[Jessun]

I'm in a severely LP affected area and with the SW80ED reduced using Atik 460s I was at 1.83"/pixel.

I opted for faster optics and landed at 1.92"/pixel, and looking at cameras that would push that to 2.53.

My rationale is that you can always increase the canvas size in PS or similar to be able to print big. I print something like 6200*4800 pixels from the little Atik and it looks great.

This has made me look less at resolution and more at speed.

/Jesper

[vlaiv]

I usually start with wanted resolution as set, then look at following:

What scope + camera that is in weight category my mount will carry will provide most aperture for given resolution and have widest aberration clear field (reducers / flatteners included). Aperture is always going to bring more photons in, NB, mono, color, OSC - it does not matter - more aperture at given resolution - less imaging time or more SNR for same imaging time.

So let's say you have your DSLR fixed and you want to maintain close to 1.34"/pixel resolution, but want to choose good OTA.

From info you've given I conclude that DSLR that you are using has 3.9um pixel size, and if you don't want to change resolution (or small shift) we are looking at 550-650mm FL range to do it in.

This is really simple for me: 6" F/4 Newton with matching CC or 8" F/4 Newton with CC that acts as focal reducer and brings down FL to below 700mm.

Now if you want to sacrifice a bit of FOV you can use 6" RC as well and bin your images x2 - that will give you same ~ 1.4"/pixel resolution, but your images (unscaled at "native" pixel resolution) will be x4 smaller (x2 in width and x2 in height). For Aps-c sized chip you will need field flattener.

These are all cheap but "fiddly" options (some people find mirrored systems and collimation fiddly process, I'm quite fine with that).

There is of course much more expensive option - go with good APO - something like TS 130 F/7 and good field flattener / reducer - like Riccardi x0.75 - but that combo will cost something like x4-x5 as much as mirror options mentioned.

[Thalestris24]

I think the theoretical max resolution (diffraction limited) is 1" with a 120mm aperture refractor. I guess you'd need excellent optics and perfect skies and seeing to achieve the 1". Maybe it's really about having a suitable focal length to frame your preferred targets for a given scope/camera combo. Of course, there's no reason not to have more than one camera, or even more than one scope  

Louise

[parallaxerr]

Thanks for the replies. So it sounds like I haven't missed anything in my considerations, it's just a personal choice as to what weight an individual gives each factor.

19 hours ago, vlaiv said:

From info you've given I conclude that DSLR that you are using has 3.9um pixel size, and if you don't want to change resolution (or small shift) we are looking at 550-650mm FL range to do it in.

Spot on, and 550-650mm is where I was initially thinking which, considering budget lead me to this Altair Astro ED doublet...https://www.altairastro.com/Starwave-102-ED-Doublet-Refractor.html

I'd previously considered an Esprit 100ED or the Altair 102 Triplet but both of these options are pushing affordability and it turns out the Esprit is heavy too.

BUT, I've previously imaged at 2.03"/pixel and I wasn't keen on the results, having said that it was alt/az imaging with 30s subs so SNR was poor. However, based on what's been said I think I could afford to give up a little resolution for a larger field of view but due to weight limitations of the mount that would mean dropping to an 80mm objective. It would mean I could get a better figured scope though so colour correction would likely be better, I'm thinking this...https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-80-480.html

So 80mm triplet or 102 doublet it is (at the moment). There's no reviews yet on the 2017 102mm doublet, but the 80mm triplet seems to be well regarded and has a nice flat field with the matching flattener........

[Filroden]

I image at 1.96”/pixel using an 80mm / f5 scope and I’ve never considered resolution to be limited. Between guiding accuracy (typically 1” RMS) and UK seeing, I think it’s about right for my setup.

[DaveS]

I can attest to the quality of the TS 130 f/7 triplet, with or without a reducer, for the money (It's a budget Apo after all) it's pretty good. Checking focus between R, G, B with a Bahtinov mask the focus shift is barely visible, Cooling shift is more of an issue.

I've quite happily imaged at 0.86" PP with the ASI1600.

[sloz1664]

I can also attest to the quality of the AA80 f/6 triplet, with a reducer, a nice flat field across the chip and for the money it's very good. A big solidly built scope, with a big solid focuser. As you are using an OSC camera you will be pleased to know there is no blue bloat whatsoever.

Steve

[ollypenrice]

I take objections to coarser pixel scales with a pinch of salt. You should be able to see this Ha Rosette full size by clicking on it. It was shot at 3.5 arcseconds per pixel as are all images from our dual Tak setup. (The stretch is rather hard because it was destined to be applied to the red channel in a colour image.)

Using this 9 micron pixel full frame camera in the TEC140 gives 1.8 arcsecs per pixel which gives a lovely result.  Using a small pixel camera in the TEC at 0.9"PP makes for a system which is very seeing-dependent, though when the seeing allows it you can do smallish galaxies happily enough.

If NB is your thing it's worth remembering that many of the targets are huge so field of view matters a lot.

Olly

[parallaxerr]

On 26/11/2017 at 08:33, ollypenrice said:

It was shot at 3.5 arcseconds per pixel as are all images from our dual Tak setup

Wow. Just goes to show how I am guilty of analysis by numbers, nothing like real world examples!

On 26/11/2017 at 08:33, ollypenrice said:

If NB is your thing it's worth remembering that many of the targets are huge so field of view matters a lot.

I think the case has been made for more fov at the expense of pixel resolution