Mewlon 180 for Planetary Imaging?

[Sunshine]

As some may have heard, I have been mulling over purchasing a larger refractor but as I take into consideration my yearning to continue planetary imaging I have had a change of heart, somewhat. Lately, my curiosity over a Mewlon 180 has me considering one, but a thought hit me like a brick regarding diffraction spikes, these are not present in SCT’s and Mak designs. How would diffraction spikes produced by the Mewlon show in planetary imaging? I certainly would not like planetary images with glaring spikes, I am not sure how and if they would present themselves. Pardon my ignorance but I have no experience imaging planets in Newtonian or the like which have secondary veins, though I am aware that many do planetary imaging with Newtonians and even dobs, yet I haven’t seen images of Jupiter with spikes anywhere so am I missing something? do they show up in planetary cameras? Thanks.

Edited January 15, 2023 by Sunshine

[vlaiv]

Most people imaging with newtonians don't in fact have curved spiders.

We see spikes in deep sky images because of vast difference in brightness between stars producing them and objects that we try to image so we stretch our data a lot.

Planets are on the other hand much much brighter and don't have nearly as much dynamic range as DSO images. Spider effect will be there - but won't be visible in images, so you don't have to worry about that.

[Sunshine]

27 minutes ago, vlaiv said:

Most people imaging with newtonians don't in fact have curved spiders.

We see spikes in deep sky images because of vast difference in brightness between stars producing them and objects that we try to image so we stretch our data a lot.

Planets are on the other hand much much brighter and don't have nearly as much dynamic range as DSO images. Spider effect will be there - but won't be visible in images, so you don't have to worry about that.

Thank you! I was wondering about a Jupiter image with four bright spikes, not so pretty.

[Craney]

I might be wrong here ..... often am .....   but I thought that extended object like planets  did not give significant diffraction patterns whereas 'point' objects like stars did produce them.  All to do with image formation and superposition of waves.

That's why I thought you could only use stars for Bahtinov masks as well.

[dweller25]

The Mewlon 180 has three spider vanes which visually show up as 6 very weak diffraction spikes on the planets - they are much weaker than the four you see in a Newtonian.

Fedele has posted planetary images on here using his M180, it would be worth checking those out.

Visually if the seeing is good in my experience the M180 will outperform your TSA102 on the planets, in poor seeing they will show the same planetary detail.

 

 

 

[Sunshine]

55 minutes ago, Craney said:

I might be wrong here ..... often am .....   but I thought that extended object like planets  did not give significant diffraction patterns whereas 'point' objects like stars did produce them.  All to do with image formation and superposition of waves.

That's why I thought you could only use stars for Bahtinov masks as well.

Bright objects like Mars, Jupiter do give diffraction spikes, hefty wide ones but not as bright or dense as stars, my 8” dob produces obvious and wide spikes on Jupiter.

[vlaiv]

43 minutes ago, Craney said:

I might be wrong here ..... often am .....   but I thought that extended object like planets  did not give significant diffraction patterns whereas 'point' objects like stars did produce them.  All to do with image formation and superposition of waves.

That's why I thought you could only use stars for Bahtinov masks as well.

It does not quite work like that.

What spider supports actually do is alter PSF of a telescope. PSF is short for point spread function and you can deduce how it behaves from its name (with a bit of imagination and help of math ). It describes how light "spreads" from each point.

Stars are single points of light so star image is good representation of PSF itself.

Extended objects behave differently - like they are composed out of vast number of tiny points placed next to each other, and each of those points is affected by same PSF as above star is.

However, this process differs from waves in that it won't be susceptible to interference - but rather it is simple addition. Where spikes from multiple points cross - there won't be cancellation on phase (like you get with interference) - but always overlap / addition.

For several points this is what happens:

In each place spikes overlap - you get just a tad brighter spot.

It took some searching online to find suitable image - but look at this crop:

Now - we need to "extrapolate" the effect on entire planet - where each little bit of planet's image is single "star" and has its own spikes that overlap. That reduces contrast on the features just a tiny bit and leaves one "gigantic" cross that looks like this:

which is really just "tightly" packed pattern like this:

If I find suitable image and adjust exposure - we should be able to see it in the images as well. Let me try.

This is probably the best example that I could find - not sure if this is imaging attempt or phone at the eyepiece - but it shows the effect:

[dweller25]

Here is a composite image by Fedele…..

[Sunshine]

14 minutes ago, vlaiv said:

It does not quite work like that.

What spider supports actually do is alter PSF of a telescope. PSF is short for point spread function and you can deduce how it behaves from its name (with a bit of imagination and help of math ). It describes how light "spreads" from each point.

Stars are single points of light so star image is good representation of PSF itself.

Extended objects behave differently - like they are composed out of vast number of tiny points placed next to each other, and each of those points is affected by same PSF as above star is.

However, this process differs from waves in that it won't be susceptible to interference - but rather it is simple addition. Where spikes from multiple points cross - there won't be cancellation on phase (like you get with interference) - but always overlap / addition.

For several points this is what happens:

In each place spikes overlap - you get just a tad brighter spot.

It took some searching online to find suitable image - but look at this crop:

Now - we need to "extrapolate" the effect on entire planet - where each little bit of planet's image is single "star" and has its own spikes that overlap. That reduces contrast on the features just a tiny bit and leaves one "gigantic" cross that looks like this:

which is really just "tightly" packed pattern like this:

If I find suitable image and adjust exposure - we should be able to see it in the images as well. Let me try.

This is probably the best example that I could find - not sure if this is imaging attempt or phone at the eyepiece - but it shows the effect:

I knew that 😂

[Captain Scarlet]

24 minutes ago, vlaiv said:

It does not quite work like that.

What spider supports actually do is alter PSF of a telescope. PSF is short for point spread function and you can deduce how it behaves from its name (with a bit of imagination and help of math ). It describes how light "spreads" from each point.

Stars are single points of light so star image is good representation of PSF itself.

Extended objects behave differently - like they are composed out of vast number of tiny points placed next to each other, and each of those points is affected by same PSF as above star is.

However, this process differs from waves in that it won't be susceptible to interference - but rather it is simple addition. Where spikes from multiple points cross - there won't be cancellation on phase (like you get with interference) - but always overlap / addition.

For several points this is what happens:

In each place spikes overlap - you get just a tad brighter spot.

It took some searching online to find suitable image - but look at this crop:

Now - we need to "extrapolate" the effect on entire planet - where each little bit of planet's image is single "star" and has its own spikes that overlap. That reduces contrast on the features just a tiny bit and leaves one "gigantic" cross that looks like this:

which is really just "tightly" packed pattern like this:

If I find suitable image and adjust exposure - we should be able to see it in the images as well. Let me try.

This is probably the best example that I could find - not sure if this is imaging attempt or phone at the eyepiece - but it shows the effect:

Presumably this is just a background star in your planet image?😁

[vlaiv]

6 minutes ago, Captain Scarlet said:

Presumably this is just a background star in your planet image?😁

No, that is Ganymede for added realism

 

[Craney]

Thanks @vlaiv

Seems reasonable.  Never thought about it beyond my own understanding.

I can see where your brightness argument is coming from now.

Cheers, Sean.

[dweller25]

Also @Sunshine check out Neil Philips’ 8” Classical Cassegrains planetary images - that has 4 spider vanes….

 

Edited January 15, 2023 by dweller25

[CraigT82]

Here is a grossly overstretched image of Mars (I was trying to capture Phobos  and Deimos) which shows the fat spikes well. The planet itself was the same width in the image as the spikes.

In normal planetary imaging we don’t really stretch the data so the spikes don’t show up in normal exposure times (few milliseconds).