Size of the Moon in recent pictures [Solar System] Acquisition techniques · Jean-David Gadina · ... · 12 · 382 · 3

macmade 3.01
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This might be a dumb question or a facepalm moment for me, but there's something I don't understand with my latest moon pictures.

I shot the mandatory Super Blue Moon on August 19.
I wasn't super happy with my data, but I still managed to get a picture out of it.
I usually don't like imaging the full moon, as the direction of light makes it seem a bit flat.

So, four days later, I took other pictures in the waning gibbous phase.
My data was much better, but surprisingly, the moon was also much bigger.
Here's a comparison:

Moon-Size.jpg

The equipment used was:

 - Celestron NexStar Evolution 6 (1500mm)
 - Celestron 0.63 Reducer
 - Canon EOS R7

Yes, I did use the reducer both times.

According to Sky Safari, here are the Moon's data on these dates:

August 19, 2024 - 11:08 PM:
  • Distance: 359674 km
  • Size: 33.2 arcmin
  • Altitude: ~18°

August 24, 2024 - 04:19 AM:
  • Distance: 358051 km
  • Size: 33.4 arcmin
  • Altitude: ~55°

I don't think the difference in distance would result in such a change, nor would atmospheric conditions, right?

On August 19, I left my filter drawer and extension tubes from a previous session with another camera.
On August 24, I removed them, so the Canon was directly attached to the T-adapter.

But this shouldn't affect the focal length, or does it?
Maybe I haven't tightened the focal reducer on the first night? Could that be the cause?

What am I missing here?
And more importantly, what did I do wrong so I don't repeat the same mistake?

Sorry if this is obvious, but I just can't stop thinking about this.
Thank you for your time.
CS!
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JanvalFoto 4.51
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A Supermoon event mostly indicates a full moon when its at or near its closest point to us, but it doesn't necessarily appear on the closest point when the moon is full. I haven't looked into the recent one, but it seems like the explanation is that the moon is actually closer now after the event itself.
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Jeff_Reitzel 2.15
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The moon changes apparent size/distance about 12-15% throughout it's orbit so that could absolutely be part of what you are  seeing. I think more likely, with the changes you mentioned in your image train, the focal reducer ended up being a small amount closer to the back of the telescope in one of the images. That will definitely result in a small change in the reduction factor/focal length and contribute to the change in moon size you are seeing. Even just 1mm can make a noticeable difference. 
CS,
Jeff
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jschella 0.00
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The difference in apparent size that you are seeing is due to the fact that the August 24th image was taken at a much lower altitude than the August 19th image. The close the moon is to the horizon, the larger it will look (due to light diffraction). Nothing to do with the distance to the moon. 

Jason
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Jeff_Reitzel 2.15
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Jason Schella:
The difference in apparent size that you are seeing is due to the fact that the August 24th image was taken at a much lower altitude than the August 19th image. The close the moon is to the horizon, the larger it will look (due to light diffraction). Nothing to do with the distance to the moon. 

Jason

*** Jason. That is purely an illusion or trick our mind plays on us. Here is a NASA article for reference. https://science.nasa.gov/solar-system/moon/the-moon-illusion-why-does-the-moon-look-so-big-sometimes/  .  Although the article does mention "a little squashed in the vertical direction in images" at low altitudes which could be part of what is shown. 
CS,
Jeff  ***
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Gondola 8.11
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I believe that it's the focal reducer. If the spacing changed at all then your focal reduction will change. For this kind of shot you can do better by shooting without the reducer and learn how to make mosaics.
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macmade 3.01
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Jason Schella:
The difference in apparent size that you are seeing is due to the fact that the August 24th image was taken at a much lower altitude than the August 19th image. The close the moon is to the horizon, the larger it will look (due to light diffraction). Nothing to do with the distance to the moon. 

Jason

It was higher on August 24.
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macmade 3.01
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A Supermoon event mostly indicates a full moon when its at or near its closest point to us, but it doesn't necessarily appear on the closest point when the moon is full. I haven't looked into the recent one, but it seems like the explanation is that the moon is actually closer now after the event itself.

It was a little bit closer, yes, but the difference should be 0.2 arcmin.
I think it’s too small to produce such a difference in the final images.
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macmade 3.01
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Jeff Reitzel:
The moon changes apparent size/distance about 12-15% throughout it's orbit so that could absolutely be part of what you are  seeing. I think more likely, with the changes you mentioned in your image train, the focal reducer ended up being a small amount closer to the back of the telescope in one of the images. That will definitely result in a small change in the reduction factor/focal length and contribute to the change in moon size you are seeing. Even just 1mm can make a noticeable difference. 
CS,
Jeff

Thanks. I’ll definitely have to try on the same night with these different setups to see what difference it makes.
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macmade 3.01
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I believe that it's the focal reducer. If the spacing changed at all then your focal reduction will change. For this kind of shot you can do better by shooting without the reducer and learn how to make mosaics.

Thanks! I’ll try to double check the reducer next time and see what kind of difference it can make.
Mosaics are definitely on my list of things to try and learn!
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jschella 0.00
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Sorry,  I mistook the ~ for a minus sign. And I had no idea about the "illusion". It must some change in your optical train.

Jason
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JO_FR_94 6.49
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@Jean-David Gadina it is highly probable that the change in the optical train has caused such a difference in your field of view. The distance between the reducer and the sensor determines the multiplication factor of the reducer (see charts for the CCDT67 of Astro Physics). Just a 10 mm change (and filter drawers for example are about that thick) will greatly affect this factor.


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macmade 3.01
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Jérémie:
@Jean-David Gadina it is highly probable that the change in the optical train has caused such a difference in your field of view. The distance between the reducer and the sensor determines the multiplication factor of the reducer (see charts for the CCDT67 of Astro Physics). Just a 10 mm change (and filter drawers for example are about that thick) will greatly affect this factor.

Thanks a lot for this data!
Yes, it's most likely that! I'll do some tests to confirm.
I had no idea this could have such a difference.
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