Lunar mosaic not fitting - Earth rotation theory

[spaceman_spiff]

Morning SGL,

I am planning on getting my Mak back under the moonlight tonight to do some imaging (it's been so long!). In previous imaging sessions I would move along the terminator taking ~30 video recordings to stack. More often than not, the stacked images would not perfectly overlay. I made sure I refocused roughly every 90 mins, so it's unlikely to be the telescope's fault.

It has just occurred to me that as the Earth rotates I am moved towards the Moon (as it rises) and then away again (as it sets). I now think this is causing a slight change (about 20 arcsec change in Lunar diameter) in my image scale and making my mosaic not fit together.

Do you agree? Or it is something else?

Dan.

[michael.h.f.wilkinson]

I don't think this would make much difference. Mismatches are more likely due to some distortion in the imaging system. Much also depends on how you stitch. Programs like AutoStitch have yet to fail me finding a good solutions to stitching lunar mosaics. I do take the images in quick succession using a planetary camera, and it takes me less than 30 minutes to capture all the data I need for a complete moon mosaic. If you take much longer, changes in the illumination may have an effect.

[spaceman_spiff]

3 minutes ago, michael.h.f.wilkinson said:

I don't think this would make much difference. Mismatches are more likely due to some distortion in the imaging system. Much also depends on how you stitch. Programs like AutoStitch have yet to fail me finding a good solutions to stitching lunar mosaics. I do take the images in quick succession using a planetary camera, and it takes me less than 30 minutes to capture all the data I need for a complete moon mosaic. If you take much longer, changes in the illumination may have an effect.

Thanks for that,

Maybe some type of mirror shift is happening, I hope not! I use Gimp to stitch the panels together (using the difference between layers to get a precise match).

I'll see what happens tonight.

[michael.h.f.wilkinson]

Just now, spaceman_spiff said:

Thanks for that,

Maybe some type of mirror shift is happening, I hope not! I use Gimp to stitch the panels together (using the difference between layers to get a precise match).

I'll see what happens tonight.

The distortion is most likely some pin-cushion or barrel distortion, not necessarily mirror shift

[vlaiv]

If you are refocusing (due to temperature drop) then most definitively mirror shift is happening and it affects FL of scope

[spaceman_spiff]

1 minute ago, michael.h.f.wilkinson said:

The distortion is most likely some pin-cushion or barrel distortion, not necessarily mirror shift

Hmmm, I image using only a very small 640x480 array of pixels in the centre of the sensor. I would have thought barrel/pin cushion distortions would be minimal there..

2 minutes ago, vlaiv said:

If you are refocusing (due to temperature drop) then most definitively mirror shift is happening and it affects FL of scope

Thanks, I try to reach focus from the same direction to reduce the mirror shift but it could always be happening anyway.

[vlaiv]

20" change in diameter?

Let's put that into percentage just to see if it can give us some better picture.

That would be third of arc minute, and moon is roughly 31 arc minutes - that means that change is ~ 1/93 ~ 1.07%

Now due to earth rotation change in distance is max 1 earth radius ~ 6300km, and distance to the moon is 384.400km - huh interesting 63 times and that would mean 1/63 for full radius (from moon rise to moon being at highest point).

Actually earth rotating is plausible theory!

[spaceman_spiff]

Stellarium shows the moon's distance (and scale) oscillating each day.

[vlaiv]

8 minutes ago, spaceman_spiff said:

Hmmm, I image using only a very small 640x480 array of pixels in the centre of the sensor. I would have thought barrel/pin cushion distortions would be minimal there..

Thanks, I try to reach focus from the same direction to reduce the mirror shift but it could always be happening anyway.

Well, actually focusing from same direction won't make a difference - Mak and SC focal length depends on primary to secondary distance - so changing focus moves primary and changes the distance between mirrors.

For your Mak with FL of 1800 - 1% would be 18mm, now this does not translate into 18mm focuser movement, because secondary is amplifying and it actually means less focus travel to make 18mm difference in focal length.

Look at this thread on cloudy nights: https://www.cloudynights.com/topic/532166-effective-focal-lenght-in-a-maksutov-cassegrain/

there is even spread sheet, and factor is about x3.9 for IntesMicro M715, so that I guess translates to about 4-5mm focus travel to get 18mm change in FL.

[spaceman_spiff]

Thanks vlsiv, interesting link,

So does changing the focus point in the Mak will then change the image scale? I am not changing the back-focus distance, only the distance between the primary and secondary mirrors.

[vlaiv]

22 minutes ago, spaceman_spiff said:

Thanks vlsiv, interesting link,

So does changing the focus point in the Mak will then change the image scale? I am not changing the back-focus distance, only the distance between the primary and secondary mirrors.

Yes, by changing distance between primary and secondary mirrors you are changing effective focal length, and image scale depends on focal length.

I don't believe that this is the cause for effect that you noticed - earths rotation most certainly is the reason, but over the course of night as the OTA cools - exact focal plane position changes (as well as distance between the mirrors) - so you are forced to refocus. I don't believe that change in distance between mirrors is exactly matched by refocus needed (because all parts of scope change length somewhat and not only OTA between mirrors) so you will most certainly end up with a bit different focal length when you refocus. I also believe that this effect on imaging scale is almost negligible - fraction of arcsec at most (this is because change in distance between mirrors is somewhat compensated by refocusing - also changing the distance between mirrors, and in general on OTA that long, changes in length are fraction of millimeter - so effective focal length changes by fraction of percent)

[spaceman_spiff]

Good to know vlsiv. When I get my stacked images, I'll play about rescaling them and see if the fit better. I'm sure the scaling factor could be predicted but I'll do it by trial and error.

Something to keep in mind when imaging the moon over the course of a night!