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Testing for planetary imaging.

1parsec

By 1parsec
in Imaging - Planetary

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1parsec

1parsec

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I'd done mostly deep sky imaging in the past  but want to get some shots of Jupiter this opposition.

With the recent poor weather I have had only a couple of chances to get everything running and check focus and collimation etc.

The 2 videos show a recording of a star with a 10" f6.3 Newtonian with x4 powermate - 6360mm focal length.

What focal length do you think is most suitable for the ASI120mm camera ? Should I drop down an use a x2 powermate ?

Raw recording  2.5mb

https://flic.kr/p/quWcvP

Aligned 7.5mb

https://flic.kr/p/py7yTb

Thanks, Dave.

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JamesF

JamesF

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I'd say that x2 is a bit low.  x4 might be a shade on the excessive side.  Somewhere between the two would probably work quite nicely :)  From what I've picked up from others using the ASI120, focal rations in the high teens to low twenties seems to be where most people are aiming.

James

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I don't quite understand why F20 would be the optimal focal ratio.

A C14 @ f20 is about 7000mm focal length, but my newt at f20 would be 5000mm  ?

Isn't there an optimal focal length to go for ?

Thanks, Dave.

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Ah, just read - a scope operating at f20 is working at close to it's theoretical resolution. A higher f ratio and you'd be oversampling and not getting any further resolution.

So with a x4 powermate  at f25, would be better than a x2 at  f12.6.  So yes,ideally a x3 powermate would be even better!

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JamesF

JamesF

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I don't quite understand why F20 would be the optimal focal ratio.

A C14 @ f20 is about 7000mm focal length, but my newt at f20 would be 5000mm  ?

Isn't there an optimal focal length to go for ?

No :)

To understand why you have to think about why there might be an optimal "anything" at all.  What are you trying to achieve in deciding what needs to be optimal?  The logical choice (to me, certainly) appears to be that the camera should be capable of capturing the smallest detail that the telescope can resolve.  That is, the smallest detail the telescope can resolve should cover one pixel of the camera sensor.  As the C14 and your 10" newt by the very fact of their differing apertures will have different limits of resolution there can't be a "one size fits all" optimal focal length because that would mean that a single camera pixel would always correspond to the same angular dimensions of the sky regardless of the size of the smallest resolvable detail.

With the formulae for image scale and limits of resolution and a bit of maths it's possible to calculate this optimal arrangement and unexpectedly (to me, at least, when I did so) it works out that all you need to do is get the focal ratio correct and everything else takes care of itself.  There's a bit of hand-wavey stuff involved because actually it's dependent on the wavelength of the light being captured and even in RGB we're talking about quite a range of wavelengths, but for cameras with pixels the size of the ASI120 it works out giving an optimal focal ratio in the high teens or low twenties.  Or, as someone pointed out a while back, around five to six times the numerical value of the pixel size in um.  The ASI120 has a pixel size of 3.75um, giving a range for the focal ratio of 18.75 to 22.5.  It's still just a rule of thumb, but it's not at all a bad place to start.

James

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JamesF

JamesF

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Yes, I'd be tempted to give it a go with the 4x and if the image looks horrible perhaps accept that the seeing isn't really up to it and drop down to the 2x.  The powermates don't really give you many options otherwise.  It's one of the times when a true barlow can be handy because you can modify the distance between the camera sensor and barlow to change the power.

James

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skybadger

skybadger

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That's not quite true. Pick the right powermate and you can vary the distance just like a Barlow by inserting t2 spacers. Alternatively use eyepiece projection for greater control of achieved focal ratio and less faffing swapping eyepieces.

M.

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JamesF

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That's not quite true. Pick the right powermate and you can vary the distance just like a Barlow by inserting t2 spacers. Alternatively use eyepiece projection for greater control of achieved focal ratio and less faffing swapping eyepieces.

M.

I don't believe the effect is particularly significant with the two powermates that Dave has though.  From memory the smaller multiplier powermates need quite large extensions to achieve even small changes in power (100mm+ to achieve an increase or decrease of up to 0.5x I think).  The one that it really does work with is the 5x.  I think with that one  there's an increase of about 1x for every 30mm extension or so.

There's a graph on one of the Tele Vue pages.  I'll see if I can find it.

James

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James you're right. The x2 and x2 powermates have pretty much the same amplification where ever the camera is placed.

Now hoping for a clear spell to try this lot out :grin:

Skybadger - I collimated the scope visually and although it looked good the video shows it is slightly off. Do many people a camera for the final tweaks ?

Dave.

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I'm not sure many do, though I found it quite handy when I did my 127 Mak.  Once I got this far I decided I'd leave it as is...

diff-rings2.png

James

That looks as concentric as can be James.

Here's a stack of the capture showing the in-focus Airy disc & diffraction rings. I'm really surprised to see them as I didn't expect it to show any thing but a blob when stacked.

15903022837_68bee8765e_o.jpgStar-ClearFilter-Stack

Dave.

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ArmyAirForce

ArmyAirForce

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If you are using a filter wheel on a barlow, it will act like a spacer. I've found my Tal x2 barlow, then filter wheel and camera gives results more like x3 magnification, and my x3 Powermate and filter wheel is nearer to x4. I use them on an 8 inch reflector, and in poorer seeing, am limited to the x2 barlow giving x3 magnification. On my 200PDS, the two barlows with filter wheel give 3 metres focal length ( f15 ) and 4 mtrs, ( f20 ), so around the ideal focal ratio.

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skybadger

skybadger

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That is what metaguide is supposed to do out of the box and still hasn't for me ..yet I keep trying. Those perfect rings are taken on an out of focus star aren't they? In my experience tweaking on a real star will soon fix that to something less than perfect.

I take the point on the powermates, though the 2.5x is the one I have had for the longest time.

I do recommend eyepiece projection though. Lots of control.

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JamesF

JamesF

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That is what metaguide is supposed to do out of the box and still hasn't for me ..yet I keep trying. Those perfect rings are taken on an out of focus star aren't they? In my experience tweaking on a real star will soon fix that to something less than perfect.

I'd completely stripped the scope to fix a few little niggles and once it was all reassembled I collimated the OTA on the desk by eye (without even an artificial star) until I decided I couldn't get it any better.  Then I took it out and tested it on a real star with the camera in.  I decided that in the dark I stood no chance of doing any better than I already had.  To me the rings look ever so slightly tighter in the midnight to 3 o'clock region, but it's so close that I think I'd need far more skill than I have to improve upon it.

James

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JamesF

JamesF

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If you are using a filter wheel on a barlow, it will act like a spacer. I've found my Tal x2 barlow, then filter wheel and camera gives results more like x3 magnification, and my x3 Powermate and filter wheel is nearer to x4. I use them on an 8 inch reflector, and in poorer seeing, am limited to the x2 barlow giving x3 magnification. On my 200PDS, the two barlows with filter wheel give 3 metres focal length ( f15 ) and 4 mtrs, ( f20 ), so around the ideal focal ratio.

In my setup it works rather unkindly for me.  I have a C9.25 with a secondary focuser, filter wheel and 2x barlow and the final focal ratio is well over f/25.  I'm now trying to find a decent 1.6x-ish barlow to use instead.

It's just struck me that I could machine a new barrel for the 2x barlow lens group (it's the TV 2x barlow with a removable lens unit) with a shorter body.  I might have to post about that separately.

James.

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