https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818

On 23/12/2021 03:02, RichA wrote:

> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818

It is quite a clever piece of mathematics to make that work. It just
goes to show that thinnest supports isn't always the best solution.

The scope I grew up with had a three way spider mount and so a fainter
six spike diffraction pattern. I preferred the 4 way cross spikes.

--
Regards,
Martin Brown

On Thu, 23 Dec 2021 10:23:19 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 23/12/2021 03:02, RichA wrote:
>
>> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
>
>It is quite a clever piece of mathematics to make that work. It just
>goes to show that thinnest supports isn't always the best solution.
>
>The scope I grew up with had a three way spider mount and so a fainter
>six spike diffraction pattern. I preferred the 4 way cross spikes.

Of course, there's no fooling Mother Nature. This trick increases the
amount of energy lost to diffraction... it just distributes it
differently so it's less obvious. Not sure it doesn't come at a cost,
though, in terms of what it does to the MTF of the system. Especially
if you've got any scientific intent with your images.

(Personally, diffraction spikes don't bother me. In the rare cases
I've wanted to remove them, I've done it by taking subs at two
orientations, which allows them to be almost perfectly removed.)

WONDERFUL NEWS!!!!!!!!!!!
Another Chicom launch with TWO new spy satellites added:
https://www.youtube.com/watch?v=4EoAHdwGBvU

On Thursday, 23 December 2021 at 09:24:43 UTC-5, Chris L Peterson wrote:
> On Thu, 23 Dec 2021 10:23:19 +0000, Martin Brown
> <'''newspam'''@nonad.co.uk> wrote:
>
> >On 23/12/2021 03:02, RichA wrote:
> >
> >> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
> >
> >It is quite a clever piece of mathematics to make that work. It just
> >goes to show that thinnest supports isn't always the best solution.
> >
> >The scope I grew up with had a three way spider mount and so a fainter
> >six spike diffraction pattern. I preferred the 4 way cross spikes.
> Of course, there's no fooling Mother Nature. This trick increases the
> amount of energy lost to diffraction... it just distributes it
> differently so it's less obvious. Not sure it doesn't come at a cost,
> though, in terms of what it does to the MTF of the system. Especially
> if you've got any scientific intent with your images.
>
> (Personally, diffraction spikes don't bother me. In the rare cases
> I've wanted to remove them, I've done it by taking subs at two
> orientations, which allows them to be almost perfectly removed.)

The loss is marginal, based on the comparisons the guy did. Certainly stars are less obscured by a few more % obstruction than a spike.

On Thursday, December 23, 2021 at 4:40:03 PM UTC-7, RichA wrote:

> The loss is marginal, based on the comparisons the guy did. Certainly
> stars are less obscured by a few more % obstruction than a spike.

Yes, but only those stars that are under the spike are obscured by it,
whereas _all_ the stars are affected by a little more obstruction.

So it matters what you are looking for. But that means it's great that
this kind of apodization mask exists, because it means you have a
choice.

John Savard

On Thu, 23 Dec 2021 15:40:01 -0800 (PST), RichA <rander3128@gmail.com>
wrote:

>On Thursday, 23 December 2021 at 09:24:43 UTC-5, Chris L Peterson wrote:
>> On Thu, 23 Dec 2021 10:23:19 +0000, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>>
>> >On 23/12/2021 03:02, RichA wrote:
>> >
>> >> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
>> >
>> >It is quite a clever piece of mathematics to make that work. It just
>> >goes to show that thinnest supports isn't always the best solution.
>> >
>> >The scope I grew up with had a three way spider mount and so a fainter
>> >six spike diffraction pattern. I preferred the 4 way cross spikes.
>> Of course, there's no fooling Mother Nature. This trick increases the
>> amount of energy lost to diffraction... it just distributes it
>> differently so it's less obvious. Not sure it doesn't come at a cost,
>> though, in terms of what it does to the MTF of the system. Especially
>> if you've got any scientific intent with your images.
>>
>> (Personally, diffraction spikes don't bother me. In the rare cases
>> I've wanted to remove them, I've done it by taking subs at two
>> orientations, which allows them to be almost perfectly removed.)
>
>The loss is marginal, based on the comparisons the guy did. Certainly stars are less obscured by a few more % obstruction than a spike.

There is more diffraction. Whether that is "marginal" or not depends
on your imaging intent.

On 24/12/2021 00:07, Quadibloc wrote:
> On Thursday, December 23, 2021 at 4:40:03 PM UTC-7, RichA wrote:
>
>> The loss is marginal, based on the comparisons the guy did. Certainly
>> stars are less obscured by a few more % obstruction than a spike.
>
> Yes, but only those stars that are under the spike are obscured by it,
> whereas _all_ the stars are affected by a little more obstruction.

More accurately the background sky brightness is compromised very
slightly so that some marginally detectable stars might be lost. There
is no free lunch - the diffracted energy has to go somewhere.

> So it matters what you are looking for. But that means it's great that
> this kind of apodization mask exists, because it means you have a
> choice.

It is an interesting take on an old problem born of modern synthetic
aperture work. Ultra thin spiders are not necessarily optimum method of
supporting the secondary depending on what you want to do.

--
Regards,
Martin Brown

On Wednesday, December 22, 2021 at 7:02:14 PM UTC-8, RichA wrote:
> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818

What happened to curved spider vanes?
https://www.cloudynights.com/uploads/gallery/album_4746/gallery_217007_4746_176849.jpg

On 25/12/2021 04:22, StarDust wrote:
> On Wednesday, December 22, 2021 at 7:02:14 PM UTC-8, RichA wrote:
>> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
>
> What happened to curved spider vanes?
> https://www.cloudynights.com/uploads/gallery/album_4746/gallery_217007_4746_176849.jpg

This trick is a lot more sophisticated than any simple adjustment of the
thin spider supports. They exploit the way the aperture works to
effectively neutralise the diffraction pattern to first order.

--
Regards,
Martin Brown

On Sat, 25 Dec 2021 08:38:36 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 25/12/2021 04:22, StarDust wrote:
>> On Wednesday, December 22, 2021 at 7:02:14 PM UTC-8, RichA wrote:
>>> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
>>
>> What happened to curved spider vanes?
>> https://www.cloudynights.com/uploads/gallery/album_4746/gallery_217007_4746_176849.jpg
>
>This trick is a lot more sophisticated than any simple adjustment of the
>thin spider supports. They exploit the way the aperture works to
>effectively neutralise the diffraction pattern to first order.

What I would say is that they transfer much of the energy from the
diffraction spikes to a halo around the sources. They don't really
"neutralize" anything. In essence, they shift the diffracted light
into a pattern that doesn't jump out at us the way linear structures
do.

On 25/12/2021 14:31, Chris L Peterson wrote:
> On Sat, 25 Dec 2021 08:38:36 +0000, Martin Brown
> <'''newspam'''@nonad.co.uk> wrote:
>
>> On 25/12/2021 04:22, StarDust wrote:
>>> On Wednesday, December 22, 2021 at 7:02:14 PM UTC-8, RichA wrote:
>>>> https://www.cloudynights.com/topic/803119-removing-diffraction-spikes-on-a-newtonian-telescope/#entry11569818
>>>
>>> What happened to curved spider vanes?
>>> https://www.cloudynights.com/uploads/gallery/album_4746/gallery_217007_4746_176849.jpg
>>
>> This trick is a lot more sophisticated than any simple adjustment of the
>> thin spider supports. They exploit the way the aperture works to
>> effectively neutralise the diffraction pattern to first order.
>
> What I would say is that they transfer much of the energy from the
> diffraction spikes to a halo around the sources. They don't really
> "neutralize" anything. In essence, they shift the diffracted light
> into a pattern that doesn't jump out at us the way linear structures
> do.

I don't think it is a halo so much as a wider raised baseline, but I
would have to run a more detailed numerical simulation to be sure.

it is certainly one of the more interesting and effective apodising
tricks born of modern mathematical analysis for a very long time.

BWT Merry Christmas everybody!

--
Regards,
Martin Brown