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Replacement secondary mirror Skywatcher 200p


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Hi All.

So... in sorting my secondary dew heater I scratched the miror. Grr. Not badly, but am looking at a replacement anyhow.

The current one is 52mm minor axis, though I'm told the new 200PDS had a slightly wider mirror (58mm?) - apparently this is better for photography. I do mostly AP, so am wondering if I would do better with a slightly wider mirror. That said, I think the later scope had a different tube length, so may not be appropriate for me. Looking at the various design sites though, a larger mirror may indeed be better.

Also I gather some mirrors like Hilux have higher reflectivity - some 5% more which is significant I would think.

Anyhow, all of this conjecture may be immaterial as I cant find a supplier of 52mm or 58mm mirors. It seems to jump from 50 to 63.

Does anyone have any thoughts on this? I guess I could remirror the existing one, but dont want to decomission. (cos the sky will clear if I do )

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Hi Tommo. The size of the diagonal in a Newtonian is not really ultra critical. Lots of opinions out there on that.....however -

My own take on that (for what it's worth) is I'd sooner have a fractionally larger than strictly necessary diagonal, because some are not

the best towards the edges. If the diagonal is the minimum necessary size then the whole surface is being used. Also you want to make sure that all of the cone of light from the primary is being intercepted, a slightly larger diagonal gives a bit of leeway.

There's loads more theory of course.

Your scratched secondary will work ok, but I'd probably want to fix it like yourself.

A supplier of Newtonian diagonals here - http://www.image-optics.fsnet.co.uk/telescope6.htm - they also recoat.

Hope you sort it, Ed.

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Hi Ed.

Thanks for the link. Funny, I saw that site when I was thinking of recoating the existing mirror - didnt notice they had 56mm flats for sale. Just the job!

Do you happen to know anything about this supplier? Reliable? Good quality?

Thanks

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Hi Tommo.    I've had good and friendly service from Image Optics ( formally known as Galvoptics) over many years, for mirror recoating.  They did my 10" about 3 years ago.

As they are only 10 miles from me, I can deliver and collect, saves me securely packing and posting, and sweating on whether it gets dropped or lost in transit.

My local club used them for coating our 16" Dob, and one member uses them for his 20".   Not heard any complaints.

But I've never actually purchased mirrors, so sorry, but I'm afraid I cannot comment on how good they are.

Hope you sort the problem, Ed.

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Hi Ed. OK great thanks.

At the moment I'm trying to figuer out whats the best mirror size for my purposes ie AP.  If I'm going to replace it, I want it to be bigger/better/brighter ...

I've been using "Newt for the web" to play around with the design data - its quite informative, and a great tool, but I dont think my brain is up to it!!

I know that the later SW200 ie 200PDS has a slightly larger mirror then the 200P, but to some extent that was just to offset the fact that the tube was shortened in order to be sure of getting focus with cameras.

So far as I can tell, its actually better to have the earlier 200P (albeit with the single speed focuser), so long as you can get focus, because with the longer tube the light cone is smaller when it strikes the secondary ie you dont need such a large secondary and therfore less obstruction.

That said, this NFTW app appears to suggest there may be some advantage in a larger secondary in terms of greater illumnation.

My existing (early) mirror is 52mm, the newer one is 58mm. Maybe if I go with the Galvoptics 56mm that would be a happy medium?

I'm probably splitting hairs here... !!

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Don't worry Tommo about splitting hairs, I used to be champion at that myself....!

I'd say do your homework, get the best compromise you can ( all optical systems are a compromise to some degree ) and then relax and enjoy your kit.

Best regards, Ed.

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I replaced the stock secondary with a 58mm secondary from Orion Optics.  Cost was around £100 inc vat and delivery, and have been really pleased with the results.

Here's the thread (rather long) which detailed why I opted to replace it with a non-stock item.  http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/

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Don't worry Tommo about splitting hairs, I used to be champion at that myself....!

...Used to be..??? What was the cure??!

I replaced the stock secondary with a 58mm secondary from Orion Optics.  Cost was around £100 inc vat and delivery, and have been really pleased with the results.

Here's the thread (rather long) which detailed why I opted to replace it with a non-stock item.  http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/

O...M...G...!  Moderators - please attach a government health warning to this thread!!

What a thread! I read this from start to finish - didnt realise how many pages it was going to run to, or I'd have started from the far end!

Really interesting though, and some of this relevent to the issue of secondary design and lens size which I'm currently wrestling with.

I have a couple of specific points which I'm hoping you both might have some thoughts on. Will post again later, busy now.

PS Malcolm - did you ever find a use for that HEQ5 MCB?

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OK. A couple of things I'm struggling to understand.

When considering the diameter of the secondary you want the minor axis to have at least the same width as the light cone at that point. Any smaller, and there will be vignetting ie some rays will miss the edges and be lost.

The SW 200P has a cone diameter of 50mm at this point - I measured/calculated this figure, and someone has posted similar. So if the standard secondary is 52mm, this should be OK - assuming its correctly positioned and has no defects.

However, the design guides seem to suggest that having a larger diameter is better for AP, because it gives a greater image size with 100% illumination.

Q1. If a 52mm collects any and all rays that are reflected from the primary, what possible gain is there from a larger secondary?

The SW200PDS apparently has a slightly shorter tube, ie a reduction in the distance from the primary to the secondary. As I understand it, this was because some folk found they couldnt get the camera to focus because it wouldnt go close enough. So the new design means the light cone is intercepted earlier by the secondary, and theres a greater distance from the secondary to the focal plane. So the mirror would need to be a bit wider to allow for this - but probably not 6mm.

Q2. Does anyone know the difference in Primary - Secondary distance between these scopes?

Q3. Also, Malcolm, youve been there, done that (and how!!) Apart from the rogue spike problem, did you notice any benefit from the larger secondary?

Also, Orion dont seem to list a 58mm - was this special order??

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I think that we need to clarify something here.

Your question 1 is needs qualification. A secondary that is the exact diameter of the converging cone will collect all the light FOR THE CENTRAL POINT ON THE FOCAL PLANE. As soon as you move away from that point the light will drop off because you will not be seeing the whole primary mirror. For photography you need to calculate the secondary diameter needed to illuminate the corners of the chip to the degree that you require. This can be done with  http://www.bbastrode...om/diagonal.htm

I cannot comment on the SW200PDS or any changes over the years as I have never been in the position of owning or  using them.

Nigel

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Tommo, yes i should of warned you that the thread was 22 pages - Sorry :embarassed:

For me it was the fact that the mirrors were not perfect in that the flat edge on the minor axis was so close to this edge of the light cone that it was causing me problems.  As I tend to image rather than visual work the next step up in "standard" mirror sizes was 58mm. 

For me I get better illumination across more of the Canon 400d's sensor and no strange spikes.  I can't say I've seen a lack of contrast that you would expect when having a larger obstruction in the light path, but then this may be down to the quality of the secondary from Orion Optics being better than the stock mirror.

For me changing the mirrors transformed the optical qualities of the 200P and resolved the issue I was having.   Visually I'm guessing that unless you have perfect vision you may not notice anything, or the object may seem a tad brighter ?

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I think that we need to clarify something here.

Your question 1 is needs qualification. A secondary that is the exact diameter of the converging cone will collect all the light FOR THE CENTRAL POINT ON THE FOCAL PLANE. As soon as you move away from that point the light will drop off because you will not be seeing the whole primary mirror. For photography you need to calculate the secondary diameter needed to illuminate the corners of the chip to the degree that you require. This can be done with  http://www.bbastrode...om/diagonal.htm

I cannot comment on the SW200PDS or any changes over the years as I have never been in the position of owning or  using them.

Nigel

hi Nigel

OK thanks for that. I'm 100% confident in what you say, just struggling to visualise this! The problem I have is that when looking at a ray trace, such as that shows on Newt for the Web, there doesnt seem to be scope for any other incident rays to make there way to the image plane. It appears that all possible incident rays already make there way to the point of focus. Makiing the mirror larger seemto cause vignetting at the front aperture, which apears to improve if the OTA diamter were to be increased.

My best guess is that there is vignetting for the extreme rays only, and the majority do "make it". ??

From what yo have told me, and the very useful link, it is the case that a 58mm diagonal would be better.

I also plan to move the mirror down the tube using the adjusters - I can reach focus with about 12mm to spare so this wont be a problem.

Tommo, yes i should of warned you that the thread was 22 pages - Sorry :embarassed:

For me it was the fact that the mirrors were not perfect in that the flat edge on the minor axis was so close to this edge of the light cone that it was causing me problems.  As I tend to image rather than visual work the next step up in "standard" mirror sizes was 58mm. 

For me I get better illumination across more of the Canon 400d's sensor and no strange spikes.  I can't say I've seen a lack of contrast that you would expect when having a larger obstruction in the light path, but then this may be down to the quality of the secondary from Orion Optics being better than the stock mirror.

For me changing the mirrors transformed the optical qualities of the 200P and resolved the issue I was having.   Visually I'm guessing that unless you have perfect vision you may not notice anything, or the object may seem a tad brighter ?

Malcolm - like you I'm doing photography exclusively, so I hope to see a similar improvement. I cant see a 58mm flat though on Orion web site - only 63mm. Was it special order??

Thanks both

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Tommo,

I've juts looked back and checked the actual invoice.  This was for a 63mm secondary with Hilux coating.  The stock mirror on the 200P is 52mm minor axis, the 200PDS has a 58mm secondary.  At the time Orion offered a 57mm or 63mm minor axis secondary, both for the same amount, so I opted for the 63mm.  I also sent the stock mirror with the central boss to Orion Optics so they could mount this on the new mirror with the correct offset and square in all direction to the mirror back.

Sorry for the confusion over mirror size.  - My best advice would be to drop them an e-mail and discuss your requirements.

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Tommo,

I've juts looked back and checked the actual invoice.  This was for a 63mm secondary with Hilux coating.  The stock mirror on the 200P is 52mm minor axis, the 200PDS has a 58mm secondary.  At the time Orion offered a 57mm or 63mm minor axis secondary, both for the same amount, so I opted for the 63mm.  I also sent the stock mirror with the central boss to Orion Optics so they could mount this on the new mirror with the correct offset and square in all direction to the mirror back.

Sorry for the confusion over mirror size.  - My best advice would be to drop them an e-mail and discuss your requirements.

Hi Malcolm. Mindreader or what! I was going to ask how you mounted the mirror and forgot!!

OK thanks I'll give them a call.

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