Problems with proper use of ASI385MC

[George Gearless]

Ok, so this is a little embarrassing. A couple of months ago, I bought a Zwo ASI385MC. Particularly for planetary/moon photography on my Mak 180. But judging by results posted on Astrobin, it is very much possible to do some DS objects as well.

The past months have not offered much opportunity. If the skies weren’t cloudy, I could be sure I’d be working the nightshift. But a couple of days ago I got lucky and had somewhat clear skies (and managed to smash my Stellarmate in my uncontrolled excitement 😭).

I tried a globular cluster first with my trusty 80mm Evostar. The stars at the edges were extremely elongated and distorted. The center was fine. This wasn’t a problem when I used my DSLR. Or at least not as pronounced.

Then I switched to my Mak and pointed at the moon. When the center was focused, the edges were not. Once again I’ll point out that I never had this problem with my DSLR.

The only thing I’ve changed is substituting my DSLR with the 385MC. Why are the results so different? And what do I do about it?

 

George

 

PS: If anyone has an extra Stellarmate lying around that they don’t know what to do with, I’ll be happy to take it off your hands 😁.

[don4l]

Have you taken off the lens that comes with the ASI?

 

[vlaiv]

Posting resulting images can help determine the cause.

Also - a bit better description of each setup or possibly a picture of it would be good.

[George Gearless]

7 minutes ago, don4l said:

Have you taken off the lens that comes with the ASI?

 

Yes. But was actualy wondering if I should put it on. But I guess not then :).

[vlaiv]

Just now, George Gearless said:

Yes. But was actualy wondering if I should put it on. But I guess not then :).

No, you should leave it off. There are cases when you want to use that lens - like when using camera as all sky camera (without telescope to capture meteors or weather information), or when doing some fancy afocal things - combining eyepiece and camera lens into imaging system.

But for general imaging - either planetary or DSO - you should leave it off.

 

[George Gearless]

2 minutes ago, vlaiv said:

Posting resulting images can help determine the cause.

Also - a bit better description of each setup or possibly a picture of it would be good.

Not sure I saved any of them. I know I didn't for the moon pictures. I'll check.

I'll post some pictures of the setup when I get home tonight. Propably easier than to describe it.

 

[vlaiv]

3 minutes ago, George Gearless said:

Not sure I saved any of them. I know I didn't for the moon pictures. I'll check.

Having images would really help, I think.

Its hard to imagine what could be wrong because you mention that DSLR image is ok, while 385 image is not and behaves like there are optical aberrations in system (both refractor and mak) - field curvature or something else.

385 sensor is much smaller than DSLR sensor and it should be much less sensitive to aberrations that are inherent for particular optics (image is best in center of properly collimated scope).

Only thing that I can think of is that you are not used to seeing stars / image detail at that scale and that you have same level of aberrations (probably due to tilt, or defocus or similar) on both cameras but 385 being much smaller shows them larger (think crop factor here, although I don't really like to use that term).

What ever it is, I think we can sort it out, but like I said - best thing would be to record an image of star field / star (if you don't have any saved) and post it here so we get idea of what we are dealing with.

[George Gearless]

14 minutes ago, vlaiv said:

Having images would really help, I think.

Its hard to imagine what could be wrong because you mention that DSLR image is ok, while 385 image is not and behaves like there are optical aberrations in system (both refractor and mak) - field curvature or something else.

385 sensor is much smaller than DSLR sensor and it should be much less sensitive to aberrations that are inherent for particular optics (image is best in center of properly collimated scope).

Only thing that I can think of is that you are not used to seeing stars / image detail at that scale and that you have same level of aberrations (probably due to tilt, or defocus or similar) on both cameras but 385 being much smaller shows them larger (think crop factor here, although I don't really like to use that term).

What ever it is, I think we can sort it out, but like I said - best thing would be to record an image of star field / star (if you don't have any saved) and post it here so we get idea of what we are dealing with.

I get what you mean about crop factor, and your explanation makes sense.

The aberations were so severe that I remember thinking there was no reason to save the pictures until I got the problem sorted. But as I said, I'll check when I get home tonight and revisit this thread.

[George Gearless]

10 hours ago, vlaiv said:

Posting resulting images can help determine the cause.

Also - a bit better description of each setup or possibly a picture of it would be good.

Ok, here we go.

The moon picture. I only have one of the moon taken with the Mak. Even as compressed file it should be obvious that the edge is out of focus whereas the center is not. Unfortunately I don't have one with the refractor and the completely elongated stars. Same problem but much more pronounced. It was so bad that the stars at the edges looked like rainbow coloured comets. Tail and all (no, it wasn't drifting. The center was fine pinpoints of stars)

The three following pictures are of:

1. My diagonal mirror at the bottom of the Mak.

2. The nose/extension/eyepiece/camera coupling/camera laid out in order of assembly.

3. The whole setup assembled.

It is not clear on the picture because the camera is lying upside down, but the extra 150 deg lens is indeed off  the camera (and always was).

 

George

 

 

 

Edited December 11, 2019 by George Gearless

[vlaiv]

Ah ok, yes, what you see is indeed very severe aberration but only because you are using it wrong.

Well, not wrong per se, but in this case you don't want to use it like that.

You want to loose much of the bits between focuser and camera. In fact you don't want to have anything between focuser and camera, except maybe adapter to attach camera to focuser.

If I'm not mistaken, your focuser should have T2 thread on it (male T2 thread) and camera should have female T2 thread on it - so just simply screwing camera onto focuser tube will be enough.

Next option is to use 1.25" camera nose piece that is shown in one of the images by screwing that into camera and then using that in focuser in place of 1.25" accessory. This option is a bit better then above direct connection because it let's you rotate camera to align FOV of your camera the way you want it to be aligned.

Bottom line - you want your camera in prime focus - either straight or with barlow (depending on scope and application) and not in eyepiece projection mode.

In fact, you might want to try eyepiece projection thing at some point, but do it via afocal method (using both eyepiece and small lens that you've got with your camera) - that way you might try the EEVA on Mak for example - but that is another story.

[George Gearless]

Thanks Vlaiv.

The bottom of the Mak does indeed have a T2 male thread. And yes, I can screw the camera on directly to that. Good tip about the nose piece , btw.

But without eyepieces I will not be able to achieve different magnifications. Which was kind of the point of inserting the tube in the first place. Particularly for moon photography but also for planetary photography.

I understand your point about the smaller sensor on the 385 vs my DSLR. But it does still surprise me that the difference in aberation is so pronounced. In fact, I had not noticed it at all until the problems with the 385 came about. To be honest, even knowing what I know now, I'm still struggling to see it.

So, it still leave the question:

How do I achieve different magnifications for moon/planet photography with my 385?

 

[vlaiv]

For best image quality you want to avoid using eyepiece projection and go for prime focus like explained above.

There is a number of ways you can achieve different "magnification", but for start let's discuss why that is wrong term in this context.

Magnification is the term that we use for visual applications - it explains how much something is magnified in the sense of - what would it look like to naked eye if it was X times larger or closer. It can be technically described as ratio of angular size of object.

With imaging things are different - we no longer have two angular sizes to compare - angular size with naked eye and angular size with telescope and eyepiece, we now have a different process - mapping angular size to pixels (or sampling points). That is called sampling resolution. Given an image of certain sampling resolution - you can still make thing in the image appear small or large by using different projection on device that is used to display things - like this:

 

Above is the same image (it is Voyager 1 image of Jupiter in high resolution so credits to NASA for that one) but displayed at different scale. What is the "magnification" of that image?

Now do an "experiment" - stand really close to monitor and observe these two images, and then walk away 3-4 meters and again look at that image, compare "magnifications" of those images again (they will look less magnified from 3-4 meters away although we did nothing to them).

Above was written just to show that "magnification" is meaningless term in imaging - it is related to visual and should not be used when imaging.

Proper term for imaging is pixel scale, or sampling resolution and it is expressed in case of astro photography in arc seconds per pixel ("/px for short).

Ok, now that we know what we are working with - let's see what would be the proper answer to the question "how one might change magnification". It is really about two things - changing pixel scale and changing FOV.

First you need to understand that there is something called native sampling rate for camera and telescope. It depends on telescope focal length and size of pixels on camera chip. Native FOV depends again on telescope focal length and size of sensor. Since you can't physically change number of pixels sensor has - native sampling rate and native FOV are related in the same way pixel size, sensor size and number of pixels are related (sensor size = number of pixels x pixel size).

Native sampling rate is your "baseline" - that is basic "magnification" that we can modify via different methods to obtain other "magnifications". It is calculated as 206.3 * pixel_size / focal_length.

I would like to mention one more thing that is important - that is critical sampling rate. Due to physics of light there is only so much detail that a telescope of a given aperture will show, and if you are sampling too fine (high sampling rate) - you will be just "wasting pixels" simply because there is no finer detail to be recorded. In reality oversampling has both benefits and drawbacks, but that is another discussion. Once you match your sampling rate to level of detail that aperture can provide under ideal circumstances (not guaranteed that you will actually record that level of detail - it does depend on atmosphere and quality of optics) - we say you are at critical sampling rate. There is simply no benefit in detail capture if going with "higher magnification" - or rather higher sampling rate.

Here is guide formula for critical sampling - you want your focal length to be equal or less to pixel * aperture * 3.857281 (this last number is 510nm and 2.4 and 1.22 combined into single constant to make things easy).

For example - if your camera is 3.75um pixels and you have 150mm aperture (not sure if your mak is 150mm, but let's say it's that one) - max focal length is 2170mm - that is ~ F/14.5. In fact you will find that F/ratio for critical sampling depends only on pixel size of camera.

How to change sampling resolution - to answer finally your question on magnification:

- use barlow lens. You can change magnification of barlow lens by changing distance of barlow element to sensor. Add more distance - larger magnification you get (finer sampling rate).

- use of binning - this process joins few adjacent pixels into "larger pixel". This method will not change FOV but will change sampling rate

- use of mosaics - shooting multiple panels and stitching them together. This technique is useful for larger FOV - something you will want for the Moon for example.

Most planetary imagers opt to sample at critical sampling rate and make mosaics for lunar and solar - only two planetary targets that are not tiny (please make sure you have proper filters when trying solar imaging!).

 

[George Gearless]

35 minutes ago, vlaiv said:

For best image quality you want to avoid using eyepiece projection and go for prime focus like explained above.

There is a number of ways you can achieve different "magnification", but for start let's discuss why that is wrong term in this context.

Magnification is the term that we use for visual applications - it explains how much something is magnified in the sense of - what would it look like to naked eye if it was X times larger or closer. It can be technically described as ratio of angular size of object.

With imaging things are different - we no longer have two angular sizes to compare - angular size with naked eye and angular size with telescope and eyepiece, we now have a different process - mapping angular size to pixels (or sampling points). That is called sampling resolution. Given an image of certain sampling resolution - you can still make thing in the image appear small or large by using different projection on device that is used to display things - like this:

 

Above is the same image (it is Voyager 1 image of Jupiter in high resolution so credits to NASA for that one) but displayed at different scale. What is the "magnification" of that image?

Now do an "experiment" - stand really close to monitor and observe these two images, and then walk away 3-4 meters and again look at that image, compare "magnifications" of those images again (they will look less magnified from 3-4 meters away although we did nothing to them).

Above was written just to show that "magnification" is meaningless term in imaging - it is related to visual and should not be used when imaging.

Proper term for imaging is pixel scale, or sampling resolution and it is expressed in case of astro photography in arc seconds per pixel ("/px for short).

Ok, now that we know what we are working with - let's see what would be the proper answer to the question "how one might change magnification". It is really about two things - changing pixel scale and changing FOV.

First you need to understand that there is something called native sampling rate for camera and telescope. It depends on telescope focal length and size of pixels on camera chip. Native FOV depends again on telescope focal length and size of sensor. Since you can't physically change number of pixels sensor has - native sampling rate and native FOV are related in the same way pixel size, sensor size and number of pixels are related (sensor size = number of pixels x pixel size).

Native sampling rate is your "baseline" - that is basic "magnification" that we can modify via different methods to obtain other "magnifications". It is calculated as 206.3 * pixel_size / focal_length.

I would like to mention one more thing that is important - that is critical sampling rate. Due to physics of light there is only so much detail that a telescope of a given aperture will show, and if you are sampling too fine (high sampling rate) - you will be just "wasting pixels" simply because there is no finer detail to be recorded. In reality oversampling has both benefits and drawbacks, but that is another discussion. Once you match your sampling rate to level of detail that aperture can provide under ideal circumstances (not guaranteed that you will actually record that level of detail - it does depend on atmosphere and quality of optics) - we say you are at critical sampling rate. There is simply no benefit in detail capture if going with "higher magnification" - or rather higher sampling rate.

Here is guide formula for critical sampling - you want your focal length to be equal or less to pixel * aperture * 3.857281 (this last number is 510nm and 2.4 and 1.22 combined into single constant to make things easy).

For example - if your camera is 3.75um pixels and you have 150mm aperture (not sure if your mak is 150mm, but let's say it's that one) - max focal length is 2170mm - that is ~ F/14.5. In fact you will find that F/ratio for critical sampling depends only on pixel size of camera.

How to change sampling resolution - to answer finally your question on magnification:

- use barlow lens. You can change magnification of barlow lens by changing distance of barlow element to sensor. Add more distance - larger magnification you get (finer sampling rate).

- use of binning - this process joins few adjacent pixels into "larger pixel". This method will not change FOV but will change sampling rate

- use of mosaics - shooting multiple panels and stitching them together. This technique is useful for larger FOV - something you will want for the Moon for example.

Most planetary imagers opt to sample at critical sampling rate and make mosaics for lunar and solar - only two planetary targets that are not tiny (please make sure you have proper filters when trying solar imaging!).

 

What a great and useful reply!

It's a lot to process (pun intended), but I think I follow. 

It's like the Canon Powershot camera I have with 50X optical zoom lens. I can push the 'magnification' to over 50 with the digital zoom. But this is just the camera processing it in the same way I would use the magnifying glass in the Windows Paint program. I can digitaly magnify it ad infinitum or until I'm blue in the face. But the picture will just become grainier and grainier until I'm looking at one single pixel.

Sampling rate is a term that I've been missing in my vocabulary. While I may have had an intuitiv but somewhat misguided understanding of it, it is extrememly helpful to have it explained all the way down to a mathematical formula.

I guess the trick is to accurately distance the camera to the Barlow lense to achieve critical sampling. Or 'the sweet spot' as I've dubbed it in my mind :). I'm sure there is a mathematical formular to calculate the exact theoretical distance. But since I have no way of accurately determining or adjusting the distance between the camera and the lense, that would just be an academic exercise. Trial and error will suffice, now that I know what to look for.

So, armed with my Mak180, my Barlow, my 385MC camera AND my new gotten knowledge, I now stand a fighting chance in getting some 'award winning and never seen better before' pictures of the moon :).

Thanks a bunch Vlaiv.

[vlaiv]

You are welcome.

There is mathematical way to determine barlow magnification once you know focal length of barlow, but you are right, you can do it via trial and error all the same. Just shoot something that has feature of known size (like disk of particular planet at particular time or crater at the moon) and then measure in pixels of your resulting image the size - ratio of real angular size of the feature and measured pixel count will give you roughly sampling rate.

For mathematical way, use this: 1 + distance / focal_length, but you need to know focal length of barlow lens. Usually better barlow lens have that info published.

There is quite a bit to learn before you get to your award winning image of the Moon, but I do encourage you to just start recording. Here are some tips for planetary imaging in general:

And of course other threads offer good advice as well, so lookup optimizing planetary viewing/imaging (not all is related to gear used, ambient has quite an impact as well) and how to acquire and process planetary images.

[Merlin66]

It’s very easy to measure the focal length of a Barlow.

Draw a circle on a piece of card twice the diameter of the Barlow lens, focus the sun’s image on the piece of card - when the image fills the circle, the distance between the card and the lens of the Barlow is the focal of the Barlow.

 

[George Gearless]

10 minutes ago, Merlin66 said:

It’s very easy to measure the focal length of a Barlow.

Draw a circle on a piece of card twice the diameter of the Barlow lens, focus the sun’s image on the piece of card - when the image fills the circle, the distance between the card and the lens of the Barlow is the focal of the Barlow.

 

Brilliant. Just...brilliant.

[vlaiv]

32 minutes ago, Merlin66 said:

It’s very easy to measure the focal length of a Barlow.

Draw a circle on a piece of card twice the diameter of the Barlow lens, focus the sun’s image on the piece of card - when the image fills the circle, the distance between the card and the lens of the Barlow is the focal of the Barlow.

 

Not sure how that works, can you explain why it works, and if for example it works only for x2 barlow (how about x2.5 barlow and alike?).

[Merlin66]

Vlaiv,

It works 100% for every negative lens - like the Barlow.

I have the sketch somewhere...it's just thin lens formula/ theory.

[vlaiv]

8 minutes ago, Merlin66 said:

Vlaiv,

It works 100% for every negative lens - like the Barlow.

I have the sketch somewhere...it's just thin lens formula/ theory.

Ah yes, I get it now - twice diameter being related to two times focal length (simple triangle similarity) rather than magnification of barlow itself.

For anyone interested, here is why it works (provided that there is no vignetting in barlow element (but even small vignetting will not hurt):

[Merlin66]

Vlaiv,

Yes, that's it.

An easy and handy way of measuring Barlows.