26 replies to this topic

[mayhem13]

So I’ve started my AP journey and not ready to share any images yet as I’m still in learning mode and the weather here in the NE US hasn’t been great.....but that’s changing fast!!!!. For deep sky, I’m working with a Canon T7i APS-c and various lenses 18mm,50mm,200mm. My mount is an iOptron 3200eq Pro and the weight cap is 10lbs. My observing scope is a 12” Dobsonian. The question is

 

1.Get a dedicated Astronomy camera for the Dobsonian.....the ZWO ASI 178 would require a Mosaic approach......of which I have zero experience! Lol

 

2. Get a small 90-127mm Cass and Mount it to my Canon on the mount. I’ll be at 6.5lbs all in. Preowned the cheapest option

 

3. The most expensive option....a 400mm F4 prime lens and crop. 

 

4. A refractor scope and Barlow using my DSLR but it would have to be small to work with my mount......72mm and I’d need a 2x Barlow.

 

The goal here here is for really nice frame able prints 11x16 of the moon in various stages.

 

Thanks in advance

Edited by mayhem13, 08 March 2021 - 10:44 AM.

[SeymoreStars]

Number 1.

 

Not number 2.

 

Definitely not number 3.

[SeymoreStars]

I don't like the Moon through my Canon DSLR's (6D, 5DM4,7D). You will need a UV cut filter with the ASI178, it's Protect window is an IR-CUT window.

 

To be more accurate I am referring to an ASI178 MONO camera.

[SeymoreStars]

For further clarity. If you have the 12inch Dob, the ASI178 will not show the Moon's full  disc.

 

 

 

http://astronomy.too.../field_of_view/

[mayhem13]

For further clarity. If you have the 12inch Dob, the ASI178 will not show the Moon's full  disc.

 

 

 

 

http://astronomy.too.../field_of_view/

You are quite right!......I must have forgotten to adjust the FL when I was calculating the various FOV options. I’d have to do a Mosaic I suppose?......of which I have zero experience. I will edit my post to reflect . Thank you! 

[sg6]

How are you intending to image the moon?

The dedicated camera route would seem to imply a videao the stack of the best 10% of the frame and then a little processing.

The DSLR and prime would imply a single shot at say 1/100 sec, ISO 400.

 

2 somewhat different approaches.

[SeymoreStars]

You are quite right!......I must have forgotten to adjust the FL when I was calculating the various FOV options. I’d have to do a Mosaic I suppose?......of which I have zero experience. I will edit my post to reflect . Thank you! 

I recently sold my mono asi178, it's a great Moon camera.

[Sandy Swede]

Not sure how stable tracking is with your Dob, but if it is quite stable, go with that scope.  Fast glass is great for wide field (e.g., M42, M31, M44), but not for the Moon or planets.  A Mak (long FL) is ideal for lunar and planetary and a camera that can take video with a relatively high frame rate is the way to go.

Edited by Sandy Swede, 08 March 2021 - 11:04 AM.

[james7ca]

Something like a Celestron C90 (Mak-Cas) can take very good pictures of the moon and if you want to keep small and light that isn't too bad of an option (and they are fairly cheap). However, it won't cover the entire APS-C format and you'll have some fall off in image quality out toward the corners of the frame. So, if you want full-disk images you'll have to do a mosaic (Microsoft's free Image Composite Editor or ICE can make lunar mosaics with just a few simple clicks -- in fact it is one of the better apps for doing that, IMO even better than Photoshop).

 

In any case, you'd do much better with a dedicated astro camera and I'd agree that Sony's IMX178 sensor is a very good choice for lunar imaging. You could try either the QHY5III-178 or the ZWO ASI178 (I have both, the one-shot-color version from QHY and the mono from ZWO). Interestingly enough, given the small pixels on the IMX178 you'd be working near critical sampling with the C90 even without a barlow and in fact in terms of sampling it's nearly an ideal match for the one-shot-color version of that sensor (QHY5III-178C or the ZWO ASI178MC).

 

Below is a sample image that I took with a C90 using a different camera (ASI174MM).

 

There is a larger version on Flickr:  https://flic.kr/p/XpyrvF

 

And here are a variety of subjects that were taken with the C90:

 

  https://www.cloudyni.../#entry10392971

 

One thing to remember is that you'll need a large and fast disk to capture enough frames to make a good mosaic. It's not unusually for me to take several hundred gigabytes of lunar images in a single night. Samsung's external (USB-3/USB-C) T5 SSDs are good for that purpose.

 

Finally, here are is a link where I used a QHY5III-178C with the C90:

 

  https://www.cloudyni...c/#entry8530915

Attached Thumbnails

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Edited by james7ca, 08 March 2021 - 11:38 AM.

[mayhem13]

Thanks for the detailed response James!

 

The C90 os similiar seems like the way to go for starters. Any thoughts on retaining sharpness with my DSLR if I stack the images or do I need to move up to a 102mm scope?

[james7ca]

I think if you use the DSLR you'll have to crop the images and just use the center 50% or so of the entire frame (full-frame DSLR would need an even greater amount of crop). But, you'd be much better off with a small format, dedicated astro camera like the IMX178 (mono or color), or the IMX462 (color), or IMX290 (mono).

 

In all cases you'll have to do a mosaic to cover the entire moon, so with the DSLR crop down to just the center BEFORE you try to create the mosaic. Also, download a copy of Microsoft's ICE, it's very easy to use and it produces really good results.

 

Here is a link where I used an APS-C camera with the C90. In this case I didn't do a mosaic and you can see the image isn't that sharp near to the cusps of the moon. But, the larger frame covered the entire disk of the moon.

 

  https://www.cloudyni...e/#entry7736169

Edited by james7ca, 08 March 2021 - 01:39 PM.

[Borodog]

How fast can your dslr shoot raw?

I shoot the full disk with a Sony a6000 in a 1200mm dob. The results are not as sharp or low noise as a dedicated astro cam that can shoot thousands of high speed frames for stacking, but I have good results stacking of order 100 frames. A 6000 pixel resolution would be 10” at 600 dpi, 20” at 300 dpi.

I use an EQ platform to get more frames on target with the dob and Sony.

Edited by Borodog, 08 March 2021 - 12:52 PM.

[mayhem13]

How fast can your dslr shoot raw?

I shoot the full disk with a Sony a6000 in a 1200mm dob. The results are not as sharp or low noise as a dedicated astro cam that can shoot thousands of high speed frames for stacking, but I have good results stacking of order 100 frames. A 6000 pixel resolution would be 10” at 600 dpi, 20” at 300 dpi.

I use an EQ platform to get more frames on target with the dob and Sony.

My Canons sensor backset won’t allow focus on my Dob with prime focus method.....your Sony is mirrorless and the sensor is closer so it can focus. If I were to use a Barlow, I’d be in good shape with backset......but then the moon would be too large for a full disc frame.

[james7ca]

Unless you used a coma corrector the Dob would produce way too much coma to allow you to cover the entire moon (regardless of how large of a sensor). In fact, an uncorrected, "fast" Newtonian probably wouldn't be able to cover a relatively small sensor like the IMX178.

 

The coma-free field of a Newtonian can be estimated with the following formula:

 

field diameter = 0.022 x F^3 mm where F is the focal ratio of the scope

 

Thus, at f/4 we'd have: 0.022 x 4^3 ≈ 1.4mm

 

Whereas the linear size of the IMX178 is 7.4mm x 5mm

 

Of course, if you can add a coma corrector (and still reach focus) then you can probably use most or all of the APS-C frame.

[mayhem13]

Thanks James! I settled on the iOptron mc90 for lunar imaging. It’s the lightest available and 3.5 lbs and the flip mirror will be useful. 

[Borodog]

Unless you used a coma corrector the Dob would produce way too much coma to allow you to cover the entire moon (regardless of how large of a sensor). In fact, an uncorrected, "fast" Newtonian probably wouldn't be able to cover a relatively small sensor like the IMX178.

 

The coma-free field of a Newtonian can be estimated with the following formula:

 

field diameter = 0.022 x F^3 mm where F is the focal ratio of the scope

 

Thus, at f/4 we'd have: 0.022 x 4^3 ≈ 1.4mm

 

Whereas the linear size of the IMX178 is 7.4mm x 5mm

 

Of course, if you can add a coma corrector (and still reach focus) then you can probably use most or all of the APS-C frame.

Interesting. I did not realize this. I presume this applies to my 10" F/4.8 Dob and my Sony a6000 (APS-C sized sensor) as well? What kind of coma corrector do I need to look for?

[Tom Glenn]

Interesting. I did not realize this. I presume this applies to my 10" F/4.8 Dob and my Sony a6000 (APS-C sized sensor) as well? What kind of coma corrector do I need to look for?

James and I use the same coma corrector:

 

https://agenaastro.c...telescopes.html

 

This is absolutely critical with Newtonians, even if they aren't that fast.  My Newtonian is f/6 and won't even come close to covering the field of my ASI183 without a corrector.  The downside of the coma corrector is that you can't combine it with barlows in any fashion that seems to work, due to tight restrictions on the spacing from the lens element of the corrector to the sensor, and so it only makes sense for imaging at prime focus.  If you look at examples of lunar images taken with very large aperture, fast Newtonians, which use barlows, you will often see blurring of the image near the periphery of the field, even with relatively small sensors.  In most cases this is coma.  

[Borodog]

James and I use the same coma corrector:

 

https://agenaastro.c...telescopes.html

 

This is absolutely critical with Newtonians, even if they aren't that fast.  My Newtonian is f/6 and won't even come close to covering the field of my ASI183 without a corrector.  The downside of the coma corrector is that you can't combine it with barlows in any fashion that seems to work, due to tight restrictions on the spacing from the lens element of the corrector to the sensor, and so it only makes sense for imaging at prime focus.  If you look at examples of lunar images taken with very large aperture, fast Newtonians, which use barlows, you will often see blurring of the image near the periphery of the field, even with relatively small sensors.  In most cases this is coma.  

Thank you. I am quite surprised to here this, actually, as don't think I've ever noticed coma in any of the images with this camera/scope combination unless the Moon was significantly off-center in the frame. Is it different for mirrorless than a DSLR for some reason? I am especially surprised at the "coma free field size" calculation given above, which returns a very tiny field, much smaller even than the sensor on my planetary cameras. I've never noticed coma in those images, either. Do I just not have the calibrated eye? Would my images be significantly sharper with a coma corrector? 

 

Now that we're on the subject, what about an SCT? The full disk won't fit on the sensor in my C8, but I've never noticed any problems in the mosaics with the Sony.

Edited by Borodog, 09 March 2021 - 05:36 PM.

[Borodog]

Does the coma free field size scale with the increased focal ratio when using a Barlow?

[Tom Glenn]

The coma free field doesn't change appreciably with a barlow.  The size is based upon the native f-ratio of the scope.  A barlow reduces the angular field of view, but at the same time it magnifies the image, and these two factors offset one another, so while you might expect a barlow to reduce the impact of coma, it actually has no effect.  There are some barlows out there that are sold as combination units that incorporate a coma corrector.  These, of course, would reduce the coma in accordance to whatever the manufacturer specifies.  Regarding whether or not you can identify coma in a lunar image, this will depend on the quality of the image.  If the image is not very sharp to begin with, even in the center, then you would likely not notice much effect in the image.  With my f/6 Newtonian and ASI183, at prime focus, degradation of the image near the poles and limb of the Moon is very obvious without a coma corrector.  This would likely be misinterpreted as seeing-induced blur to the casual observer, except that it changes with distance from the center of the field, and responds to a coma corrector.  Also, if your scope is not collimated properly, then the coma free field will not be in the center of the optical axis, which means that you will have asymmetrical distribution of coma, and the corrector will not work as designed.  

[Borodog]

Sigh. What an expensive hobby I've chosen.

[Tom Glenn]

Sigh. What an expensive hobby I've chosen.

It's true, but it's also a good example of why it's important for people that are still in the beginning stages of imaging to slow down and take their time considering choices.  For example, several years ago, after much research, I decided to purchase my C9.25 Edge HD precisely because I knew that I wanted a large coma free field, and the Edge optics made sense.  There are a number of advantages to large and fast Newtonians, but obtaining wide field views with diffraction limited results do present some additional problems because of coma, and to a lesser extent, field curvature.  These are not considerations for planetary imaging, because the very center of the field is all that you care about.  But for lunar imaging, this can be problematic if you are attempting to image a wider field of view.  

[Borodog]

Well my favorite thing is definitely lunar.

 

What about my 1985 C8? Does it have coma/curvature that I've never noticed before?

[Tom Glenn]

Well my favorite thing is definitely lunar.

 

What about my 1985 C8? Does it have coma/curvature that I've never noticed before?

Coma in SCTs is about the same as a Newtonian of the same focal ratio.  So at f/10, it will be less, but present.  Field curvature I'm not sure off the top of my head compared to a Newtonian, but SCTs do have it.  If you were to image with a large sensor (ASI183 for example) with small pixels and directly compare your C8 to an Edge version in good seeing conditions, the image from the Edge will be superior away from the center of the field.  If you are doing mosaics, this would require more overlap and cropping, which negates the utility of larger sensors.  Small sensors likely won't be impacted, except on very fast Newtonians.  

[mayhem13]

Well my favorite thing is definitely lunar.

 

What about my 1985 C8? Does it have coma/curvature that I've never noticed before?

Fear not.....we’ll be able to use our fast reflectors on the distant planets in a few months where other scopes will struggle.....no coma worries and nice images with a 3x Barlow! Yes.....this hobby is expensive.....as a newbie, i figured one scope to rule them all.....Ha!