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Sky-Watcher 200p - Do you really need the PDS version?


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Nice video.

Just a few remarks, if you don't mind:

- PDS version sports larger secondary besides dual speed focuser and more back focus. This is important to provide larger fully illuminated field for astrophotography, but planetary contrast will suffer somewhat because of that.

- thin spider vanes throw same intensity spikes as thick spider vanes. Intensity of spikes depends on length of diffraction edge - rather than thickness of light blockage.

- You can image larger targets like Rosette and M31 with such scope if you do mosaics.

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On 07/03/2021 at 16:57, vlaiv said:

Nice video.

Just a few remarks, if you don't mind:

- PDS version sports larger secondary besides dual speed focuser and more back focus. This is important to provide larger fully illuminated field for astrophotography, but planetary contrast will suffer somewhat because of that.

- thin spider vanes throw same intensity spikes as thick spider vanes. Intensity of spikes depends on length of diffraction edge - rather than thickness of light blockage.

- You can image larger targets like Rosette and M31 with such scope if you do mosaics.

Thanks vlaiv! Darnit! I completely forgot about the secondary being a bit larger on the PDS version, and I didn't know the thickness of the spider vane didn't effect intensity of diffraction as always thought otherwise. I know now at least, cheers! Skywatcher must make them thin purely for increased contrast then :)

EDIT: lol relieved I got one thing right 🤗

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45 minutes ago, RobertI said:

Great vid Chris. The 200P is actually more compact than I was expecting. Looking forward to the seeing how you get on. 

Thanks Rob! hehe the opposite for me 😄 When I opened it up I thought it was bigger than I remembered! I think it should be the perfect size for the obsy though and I really hope I get on with it well :) 

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On 07/03/2021 at 17:10, Lockie said:

Thanks vlaiv! Darnit! I completely forgot about the secondary being a bit larger on the PDS version,

LOL - I always thought that this was the primary difference, for the reasons Vlaiv stated :)

 I experienced issue with imaging with the stock 200P (new design as shown in your video) due to the smaller mirror, and the way it was silvered.  The resulting unsilvered flat spots on the minor axis gave me additional rouge spikes on bright stars.  The larger secondary overcomes this for the reason Vlaiv mentions...  Making the PDS the scope to get if you intend to image.  I resolve my issues by replacing the stock secondary in the 200P with one of the same dimensions as the secondary fitted in the PDS version.

 

Other than that - excellent unboxing video

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On 07/03/2021 at 16:57, vlaiv said:

 

- thin spider vanes throw same intensity spikes as thick spider vanes. Intensity of spikes depends on length of diffraction edge - rather than thickness of light blockage.

I thought the same until recently, but there seems to be a consensus in the literature that thicker spider vanes do in fact produce shorter brighter stubbier spikes, although same total energy. And thinner spikes stretch out the spikes longer but fainter.

Qualitatively that makes sense to me, as it “allows” the spikes to disappear completely  as vane thickness becomes infinitely thin.

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Good video Chris.  I image with a 250px, the old blue tube one, I guess very similar to the 200p.  I fitted a Baader clicklock, use a MPCC and can still get focus with it - just - a few mm spare.  I havent noticed an issue with the secondary mirror size and lack of illumination.  So my answer to the question would be no lol

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11 minutes ago, Captain Magenta said:

I thought the same until recently, but there seems to be a consensus in the literature that thicker spider vanes do in fact produce shorter brighter stubbier spikes, although same total energy. And thinner spikes stretch out the spikes longer but fainter.

Qualitatively that makes sense to me, as it “allows” the spikes to disappear completely  as vane thickness becomes infinitely thin.

By that logic - bright star should change appearance of its spikes as it moves across the field of view, because spider thickness changes with incident angle.

image.png.c732722c917fdb304a6755492fb7f3d3.png

Take dob and let bright star drift across the field of view and observe if diffraction spikes change depending on position of the star in FOV.

Spiders tend to be vertically thick to provide enough support:

image.png.11de64aff867d97022ab59cf88cfe0d5.png

Another example would be Bahtinov mask - you can't make spikes brighter by making slots wider - but you can make spikes bigger by adding more grooves - length of edge will put more energy in spikes.

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The first two books I opened, Suiter and R N Wilson, each state that spikes get thinner longer and fainter as vanes get thinner., and vice versa.

as for bahtinov, making the slots wider reduces the number of edges so the two effects will tend to counteract.

Ill have to check on a star as it drifts across...

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25 minutes ago, Captain Magenta said:

The first two books I opened, Suiter and R N Wilson, each state that spikes get thinner longer and fainter as vanes get thinner., and vice versa.

In that case - I stand corrected, I believe those authors certainly did calculations and that their claims are correct.

26 minutes ago, Captain Magenta said:

as for bahtinov, making the slots wider reduces the number of edges so the two effects will tend to counteract.

Actually - groove density has large / measurable impact on diffraction spikes.

https://www.dpreview.com/forums/thread/4454760

If there is counteracting action - it is not enough to offset increase in diffraction due to longer edge

28 minutes ago, Captain Magenta said:

Ill have to check on a star as it drifts across...

Not sure that it will be visible although effect will be real.

If we take spider to be 1mm wide and say 15mm deep and we take 1° of incident light angle (2° FOV) - that will make total thickness be cos(1°) * 1mm + sin(1°) * 15mm = ~1.2616338 mm. That makes increase in thickness of only 26%. Doubt that change will be visible to eye.

We could make it more by using scope like 130PDS and very wide field eyepiece (up to 2° of incident angle) - but I'm afraid that coma will be much worse and will distort both star image and spikes. Maybe with use of Paracorr or similar visual coma corrector (one from ES?).

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On 08/03/2021 at 10:35, barbulo said:

I have the 150P and I really miss the dual speed focuser of the PDS. I use a home-made Bathinov mask but focusing is still a pain.

Yeah I agree dual speed is definitely a nice have, I've witnessed this myself on the 130pds and 150pds. I think the single speed can be tinkered with to make it a bit smoother, but either a larger focus wheel or one of these might be better if not eventually upgrading the focuser:

https://www.firstlightoptics.com/skywatcher-focusers/skywatcher-auto-focuser.html

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2 hours ago, malc-c said:

LOL - I always thought that this was the primary difference, for the reasons Vlaiv stated :)

 I experienced issue with imaging with the stock 200P (new design as shown in your video) due to the smaller mirror, and the way it was silvered.  The resulting unsilvered flat spots on the minor axis gave me additional rouge spikes on bright stars.  The larger secondary overcomes this for the reason Vlaiv mentions...  Making the PDS the scope to get if you intend to image.  I resolve my issues by replacing the stock secondary in the 200P with one of the same dimensions as the secondary fitted in the PDS version.

 

Other than that - excellent unboxing video

lol Thanks :) I really dropped the ball with that one hey :D 

I actually remember your SGL thread about the diff spikes! It was really memorable partly because you went to great lengths to try and solve the issue! I'm glad you finally got the bottom of it :)

When I imaged with a 150p I can't say I noticed any unusual artefacts, but I still have an ancient image of the Rossette nebula circa 2012 taken with a 150p and Modded Canon 350D, so I'll dig that out and take a look.

It could be that it's a specific thing to the 200p? I'll definitely investigate the stars produced by my 200p and see what the deal is. I think that would be interesting.    

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1 hour ago, tooth_dr said:

Good video Chris.  I image with a 250px, the old blue tube one, I guess very similar to the 200p.  I fitted a Baader clicklock, use a MPCC and can still get focus with it - just - a few mm spare.  I havent noticed an issue with the secondary mirror size and lack of illumination.  So my answer to the question would be no lol

Cheers! That's very good to hear! I'm sure my old 150p was pretty good for imaging too. Maybe a bit of vignetting but you really need to take flats with most setups anyway :) Sounds like the coma corrector may help with reaching focus perhaps? Have you tried without? 

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6 minutes ago, Lockie said:

Cheers! That's very good to hear! I'm sure my old 150p was pretty good for imaging too. Maybe a bit of vignetting but you really need to take flats with most setups anyway :) Sounds like the coma corrector may help with reaching focus perhaps? Have you tried without? 

I havent tried without, but I think you are right, it pushes back the focus point a bit.  The clicklock then eats away that gain plus some more!

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1 hour ago, Captain Magenta said:

The first two books I opened, Suiter and R N Wilson, each state that spikes get thinner longer and fainter as vanes get thinner., and vice versa.

as for bahtinov, making the slots wider reduces the number of edges so the two effects will tend to counteract.

Ill have to check on a star as it drifts across...

Phew! Relieved I didn't give mis information in my video, thanks so much for digging this out! :) I read something saying pretty much the same years ago now and it made sense to me so I've always just accepted it.   

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8 minutes ago, Lockie said:

haha you can't win!

Hope it’s ok to post a photo in your thread of my 250 👍🏻 It’s a dual speed focuser but that exactly the same height at the standard single speed one I replaced, it just worked better with the focuser motor.  That is it focused. Not a lot of spare room. 

0F7C9291-F875-4BCB-95BE-6E74BDEF053F.jpeg

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2 hours ago, Captain Magenta said:

The first two books I opened, Suiter and R N Wilson, each state that spikes get thinner longer and fainter as vanes get thinner., and vice versa.

Out of interest, I ran sim of thin vs thick spider vanes to see energy distribution and sim does not really support this position.

image.png.271df490d63460e3020ea6536c2f48c4.png

I created two apertures - one with thin and one with thick vanes (thickness is enlarged by factor of x2) - can be thought of 1mm vs 5mm spider vane thickness  comparison. CO is 25%

Then I produced PSF of both systems and normalized it in total energy and then I measured encircled energy close to star. Reasoning being - more energy in star - less energy in spikes. Turns out that thick vanes produce more energy in same area around the star - which means that there is less energy in spikes themselves. It looks like thick spider vanes actually make spikes less pronounced.

There is one thing that thick spider does - it "clumps" wavelength into more discrete chunks. This is monochromatic light so it is obvious - but for real light that is full spectrum - diffraction spikes will be solid. Thick spider vanes make them more "rainbow" like in images (clumps for different wavelengths are in different places):

image.png.7d9c02aeb83d470d3bb016ffb77c8a98.png

This was taken from image by @Allinthehead

As far as I can tell - Tak 130 epsilon does not have very thin vanes:

image.png.f42d52a87a9da627c6b0dfd30b03c868.png

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31 minutes ago, tooth_dr said:

Hope it’s ok to post a photo in your thread of my 250 👍🏻 It’s a dual speed focuser but that exactly the same height at the standard single speed one I replaced, it just worked better with the focuser motor.  That is it focused. Not a lot of spare room. 

0F7C9291-F875-4BCB-95BE-6E74BDEF053F.jpeg

Love it! There's a lot going on there and it looks like an engineering feat to get it all in👌 I'm planning on buying or making a motor focuser, but might go the finder guider route :)

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Very good investigation vlaiv! There can't be many threads out there that discuss thicker vanes causing rainbow effect diff spikes? Looking at the sim, the thinner vanes would be less obtrusive to my eye purely because they're more uniform. What do people think?  

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23 minutes ago, Lockie said:

Very good investigation vlaiv! There can't be many threads out there that discuss thicker vanes causing rainbow effect diff spikes? Looking at the sim, the thinner vanes would be less obtrusive to my eye purely because they're more uniform. What do people think?  

First, sorry for slightly derailing the thread, after all - it is about your video more than diffraction spikes.

I completely understand why you mentioned in your video that thin spider vanes produce smaller spikes - it is iterated over and over as, in my opinion - a marketing trick. Here is screen shot from Skywatcher website:

image.png.d5f982540fa4ce5f3fe78b87ae5185c9.png

And another one probably referring to the same thing, although this one makes even less sense:

image.png.9d31bff5f1f8fd1022a62ed491d3528c.png

Not sure how secondary mirror support can be diffraction limited, but hey, sounds cool, right?

Back on the issue of spikes - I don't think people will notice rainbow effect while observing - for several reasons. "Gap" spacing in spike for each wavelength depend on the wavelength. Since we observe roughly 400-700nm there will be quite a bit of overlap in those gaps - as 400 is very close to half of 800. At those light levels - we don't really see colors and gaps in spikes will simply blend into one long spike. Add a bit of seeing to all of that to blur the things further and there you go - single nice uniform spike.

If we want to know if there will be difference in brightness - we must look at energy distribution. More energy in spikes means higher brightness and more obvious and intrusive spikes. Sim shows that there is rather small difference - about 0.07%, or 7 parts per 10,000 - no way we will be able to distinguish that as we usually notice about 7-10% change in brightness (amount of light).

I could also talk about dual slit experiment and Heisenberg uncertainty principle that I used as basis for intuitive conclusion that spider vane thickness has nothing to do with spike intensity - but don't want to further derail the thread.

Anyway, that is my 2p worth on this topic.

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