ASI120, Detail and Focal Ratios - What effect does it have?

[ArmyAirForce]

I've had my ASI120MM since early January, and have got some pretty good images of Venus, Mars, Jupiter, Saturn and the Moon with it. All of those pictures have been using a S/W Explorer 200PDS with either a TAL x2 or Televue x3 Barlow. Once the filter wheel spacing is added, it makes the scope focal length about 3.2 or 4.2 metres. It is an F5 scope, so that's about F16 to F21.

When imaging the Sun, I have an off axis Baader filter of about 95mm diameter. At native focal length, that gives me a focal ratio of F10.5. This becomes F33 with x2 barlow and filter wheel, and F44 with the x3 Televue and filter wheel.

Am I on the right lines so far?

The reason is, my Solar white light images are a bit more grainy/pixelated, compared to my planetary images which turn out fine. For Solar, I've been shooting about 3000 frames and stacking 25%, or thereabouts. ( That works fine on the Moon ).

Is the more grainy result due to needing more frames, or is it to do with the focal ratio of the scope in combination with the ASI120? I had read here and elsewhere, that the ASI120 doesn't like going much above F25.

I've attached a couple of examples. They aren't massively grainy, and a slight Gaussian blur after wavelets tends to hide it, but curious as to the cause.

Thoughts anyone?

Thanks

[D4N]

What filters are you using for the solar?

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[JamesF]

The 200PDS has a claimed focal length of 1000mm.  You can't change that unless you add more optical components.  Filters should be optically flat, so adding filters and a filter wheel shouldn't change the focal length, or not by more than a couple of millimetres, anyhow.

So, with a 95mm aperture you'll have a focal ratio of about 10.5.  With a 2x barlow it will be just over f/21 and with a 3x barlow just over f/31.5 as long as the barlow is after the filter wheel.  If you put the barlow before the filter wheel the additional space between the barlow lens and the sensor will result in a greater effective focal length.  It's probable that the only way you can work out exactly what the focal length is at that point is to image something of a known size and work backwards from its dimensions in pixels.  I'm not sure from what you've written which it is that you're doing.  Are you using any filters in the filter wheel whilst imaging the Sun, or is it just present with an empty slot?

There's no big deal using the ASI120MM at more than f/25, but the main point is that beyond that sort of figure you're exceeding the telescope's ability to resolve detail.  That is, increasing the focal length any more will make features larger, but won't give you more detail in those features.

I'm not sure what the problem might be, to be honest.  I'd drop back to f/20 to f/25, then take runs of different lengths and stack various numbers of frames from each to see how they turn out.  I'd also check focus, though I can't say that it looks like there's too much wrong there.

James

[ArmyAirForce]

Focusser, barlow, filter wheel, camera. Currently shooting through the ZWO Lum filter, though I haven't tried my separate UV/IR block that I had on my old webcam. Shooting planets with this combination and Firecapture, it gives you the focal length based on object size. For a number of different sized planets, I get pretty much the same magnification, ie x2 barlow = x3.2 and x3 barlow - x4.2.

[JamesF]

FireCapture's focal length calculations look pretty reasonable to me.  It uses ephemeris data to calculate the target size and combines that with the image size to work out the focal length/focal ratio and has been acceptably close when I've looked at it.

James

[D4N]

Shooting the sun with a luminance filter isn't ideal.

The best for detail is a dedicated solar filter but failing that you could use a green filter or an Oiii filter to boost the contrast.

An IR cut will certainly help as well.

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[JamesF]

Surely a luminance filter is pretty much just an IR/UV cut filter?  Other than allowing UV through which I'd have thought might cause focus issues, why would an IR cut filter be better?

James

[ArmyAirForce]

It didn't like green at all. Very fuzzy, indistinct and significantly more dim.

[D4N]

Yeh I guess if it cuts IR then it will work fine.

Just you need too narrow the bandwidth down if you want clear details.

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[ArmyAirForce]

Of course seeing wasn't ideal when I took these anyway!

[D4N]

Hehe yeh that doesn't help

I've started fiddling with focus before then looked up to see very fine cloud had decided to form >

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[james_screech]

With my ASI120MM, ZWO filters / filter wheel and 8" SCT I use a 2.5x powermate for planets, this gives about f25.

For solar images I use no filter and no barlow / powermate (so about f10), that's with a full 8" solar filter. I did try the ZWO L filter once but the images were not as good as those without the filter.

James

[JB80]

I use a baader UV/IR cut with mine and this was my first attempt with a x3 barlow on a 6" f/8 newt.

And this with no barlow

Now both of these are very promising starts that I have not been able to replicate since.

Seeing plays a big part at long focal lengths too.

What gain and gamma setting are you using and how many fps?

[Grimmec]

I run the Asi120mm with my c9.25sct focal length of 2350 and the max I push mine too is a 2.5x powermate on nights of good seeing. Most nights just a 2x telvue Barlow. I have tried the 3x Barlow but even on a good night it was still washing out the detail.

The seeing plays a bigger part in what focal length you can use, only on those rare nights were seeing is good can you really push the FL.

These were from a night of pretty good seeing, running a 2.5x powermate, baader filters, IrRGB and my best if the season.

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[ArmyAirForce]

Grimmec, I don't have a problem with night time planetary or Lunar shots, where I'm running at 4.2mtr FL, F21, it's just the daytime solar that had a bit more grain to the image. My planetary images are similar to yours.

I'm going to try and shoot for longer next time as while 3000 frames shot and 25% stacked is working out fine on the Moon, the heat of the day means poorer seeing, so affecting the solar images. I try to go for the Sun mid morning, so it is high enough out of the murk around the horizon, but before the heat of the day builds up. I'm also going to experiment with my UV/IR filter, rather than the Lum. Shooting without any barlow causes no problems at all, so I'm guessing it is a combination of the high f numbers, low frame count and daytime seeing.

[ArmyAirForce]

.....What gain and gamma setting are you using and how many fps?

Off the top of my head, I don't recall the gain, about 60ish. Gamma is always 50 with the ASI cams. For a 1280x960 capture area, I'm getting 27fps. 960x960 was 36fps and 640x480 was 67fps.

[D4N]

I've been using gain on zero on the sun with my ASI120MM. I guess it depends on your setup but I didn't see the point in using gain on something so bright.

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[james_screech]

I use a gain of 25 and exposure of .6ms with a 8" SCT at f10 for the Sun.

[JamesF]

I've been using gain on zero on the sun with my ASI120MM. I guess it depends on your setup but I didn't see the point in using gain on something so bright.

My vague recollection from talking to Sam @ ZWO about the gain settings on the ASI120 is that 10 is unity gain.  Below that something unexpected and non-linear happens, but I can't recall what at the moment.

James

[D4N]

Sounds scary, nothing exploded and the image looked fine though

I guess it wouldn't hurt to use 10 gain and reduce the exposure if it's known to cause weird things to happen.

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[JamesF]

I'd say "counter-intuitive" rather than "weird". It still works perfectly predictably.  Just not how one might expect

I think (and I'm honestly not entirely sure about this -- I've had a face full of anaesthetic at the dentist this afternoon and I'm really not at my sharpest right now) that I discovered the effect whilst trying to set up the ASI120MM for solar Ha imaging with my PST.  I'll see if I can find out what was discussed at the time.

James

[JB80]

I had been turning the gain on mine right down but after reading what James said I used it at 11 today for a close up with the x3 barlow.

Again quite promising and I got in before the heat turned everything to mush.

It recorded at 100 odd fps which I didn't really want but nevertheless....

[JamesF]

You turned it all the way up to 11, eh?  How Spinal Tap

James

[JamesF]

I've found the information about low gain values now.

What I discovered was that the gain values from up to 8 limited the maximum pixel value in the output image.  That is, they effectively compressed the dynamic range of the image.

Sam confirmed that for gain settings up to 9 the actual gain is 0.5 to 1.  So, if you set the gain to the minimum value you're actually halving the potential dynamic range of the output signal.  To use the full potential dynamic range of the camera you must use a gain setting of at least 9 (which gives unity gain, not 10 as I misremembered above).

James

[D4N]

That's good information to know. I'll keep that in mind next time I'm using it. 9 is the magic number

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