Lunar and Planetary Imaging

That’s indeed where I started.

I think most do: it is easier than DSO imaging, or at least it is easier to get your first decent moonshot than to get your first acceptable DSO picture.

Back in 2006-2007 I was using the Meade LPI with the Meade 2045D and the Meade ETX-80 (a sort of more or less “holy” Meade trinity of mine) with the help of a Meade 140 2x Barlow to image the moon and the available planets.

That gave me a 2000 mm maximum FL with the Meade 2045D (which was quite hard to focus properly, however) and, by using its built-in barlow plus an extension, a focal length of about 1600 mm with the ETX-80.

LPI’s pixels were however twice the size of those of a contemporary DSLR, and moreover it wasn’t particularly light sensitive, so the results were decent, but nothing more.

While I used again briefly the LPI with the Skywatcher MAK-90 in 2017, as soon as I had the Canon EOS 1300D in my hands, I realized it made for a much better camera than LPI, particularly when paired with APT’s Planetary features.

When used in planetary mode, APT essentially writes to file the video stream from the Live View feature of the DSLR as a sequence of JPEG files that can be as fast as 20-30 frames per second (on paper even more, but that’s what I get with my hardware).

One can then use programs like Autostakkert and Registax to stack the thousands of frames collected and process them to obtain one final picture of the imaged subject.

autostakkert

The great advantage of APT in planetary mode paired with a DSLR is that you can use the wide field of view granted by its big sensor to comfortably frame the subject (or part of it, when talking about the Moon) and then zoom-in through APT’s commands and start collecting frames.

Today my little great scope for lunar and planetary imaging is the Skywatcher MAK-90: of course its aperture has its limit, but I think I can still try to squeeze a little more out of it before moving to something bigger.

Also because this is not a big time for planets: from the Northern hemisphere in general and from my balcony in particular, they are very low or not visible at all.

There is always the moon, of course, but, again, from my position I can enjoy the waxing moon only six months each year (the waning one is definitely too late/early for me) and hence it will take to me some time before exhausting all the possibilities offered by the small Maksutov.

All the more so since it is rare to have days of outstanding seeing here in town and hence one keeps hoping and trying from year to year. Moreover a small bore scope improves things a bit with seeing, even if, of course, we all know a bigger scope can always take advantage of the rare moments of steady air, etc., etc.

I usually add a 1.4X Telextender to the optical train so that the effective focal length of the MAK-90 becomes 1750 mm. I have also used the triplet unit of the Meade 140 barlow (which is detachable from the extension body) to connect it directly to the DSLR through a 1.25 inch adapter, in order to achieve an approximate 2500 mm focal length.

The barlow is a 2X FL multiplier with the body extension, while the triplet should be a 1.5X unit, but the distance between the top of the 1.25 inch adapter and the sensor inside the DSLR is such that an overall magnification of about 2X is obtained.

Is that the correct sampling?

A 90 mm aperture scope has a resolving power of approximately 1.3 arcsec according to the Dawes limit (which is an approximation and/or a simplification in itself), so according to the usual application of the Nyquist-Shannon theorem the correct sampling would be reached with each pixel covering 0.65 arcsec.

Applying the well known formula

R = 206.265 * P / f

or

f = 206.265 * P / R

where

R : sensor resolution (arcsec/pixel)

P : Pixel size (microns)

f : Focal length of your scope (mm)

to our case (P= 4.3, R=0.65), we find that the optimal FL for the MAK-90, when paired with the EOS 1300D, should be:

f = 206.265 * 4.3 / 0.65 = 1365 mm

BUT

According to some expert :

For normal deep-sky astrophotography of stars, the pixel size for the Nyquist sampling rate is basically 1/2 the size of the detail you want to record. So if your seeing limits you to 5 arc seconds in long exposures, your pixels need to sample about 2.5 arc seconds.

But for planetary detail, you need to go a little bit smaller, and you need to sample at about 1/3 of the size of the detail you want to record. If you want to record 1 arc second of detail, you need a pixel that covers 0.33 arc seconds of size based on the focal length you are using.

And also in this paper we may read:

So, to sum up, the Nyquist Theorem says that if we follow all the rules, we need only sample an image at a rate that is twice the highest spatial frequency we want to be able to faithfully reconstruct.
Weʼll bend the rules a bit at times and we have things like how f-ratio affects star shapes, how CCDs are imperfect, etc. that will place other limits on our performance. So, if the utmost fidelity in spatial resolution is the goal, going a bit beyond the 2x rate is a good idea.

So, we should indeed put in our formula R = 1.3 / 3 = 0.43 and hence the ideal focal length becomes:

f = 206.265 * 4.3 / 0.43 = 2063 mm

The focal lengths I usually use (1750 mm and 2500 mm) would then be slightly undersampling or more significantly oversampling, respectively.

However I have found in practice that a 2500 mm FL provides useful detail when the seeing allows it.

However, given the average seeing I experience, 1750 mm is normally the most balanced focal length I can employ with my MAK-90 when paired with the Canon EOS 1300D.

Of course, when using the ASI ZWO 178MM camera, with its 2.4 microns pixels, things change dramatically and the native focal length of 1250 mm is more than enough:

f = 206.265 * 2.4 / 0.43 = 1152 mm

 

Back to Contents

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s