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

[jetstream]

15 minutes ago, vlaiv said:

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.

waiting, waiting

[Davey-T]

45 minutes ago, vlaiv said:

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.

I once did a not very scientific experiment on a 150mm refractor using from thin fuse wire to 15mm wide wood and not much discernible difference, not sure if it applies to refractors though

Dave

[bottletopburly]

I had a 200p now have the 200pds tbh other than the shorter tube on the pds version and the focuser dual speed there no difference in images , I use with a dslr  and baader CC , I flocked mine and added a primary fan and added a autofocuser and a carry handle  , you can always upgrade the focuser which a few people do anyway on the pds when using a heavy imaging train I haven’t yet , still to do is trim the focus tube slightly and mask the primary clips .

Edited March 10, 2021 by bottletopburly

[Chris]

18 hours ago, vlaiv said:

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

I never worry about that, carry on it's all interesting stuff

Edited March 10, 2021 by Lockie

[vlaiv]

9 hours ago, jetstream said:

waiting, waiting

 

1 hour ago, Lockie said:

I never worry about that, carry one, it's all interesting stuff

Well, in the end, I must say I was wrong. In addition to those two things mentioned - I forgot to consider diffraction grating.

Energy in spikes does remain the same - there is no difference there, but difference in gap size is telltale sign. Dispersion depends on grating density and that means on distance between gaps - or thickness of grating.

With thinner spider vanes - diffraction spike is "stretched" - it is longer and since it has the same amount of light - that light is spread over larger surface and becomes less bright. Similar thing happens with spectrum on diffraction grating - use more grooves per mm - spectrum gets more resolved - or spread over larger area - but also becomes weaker.

For visual that is definitively a plus - you want weaker looking spikes even if they are longer as this means better contrast on planets. For long exposure AP - well, I'm not sure which one is better. Here we stretch our data and faint things become equally visible. Maybe shorter spikes are less "damaging" to the image here?

[malc-c]

12 hours ago, vlaiv said:

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

 

I think it should be me who should apologise, not you Vlaiv,  as it was my reference to my 200P experience that took us down this road ! (soz)

Lockie, I can't answer your question.  There were one or two other members who reported similar things and found their secondary had the same flat spots on the minor axis of the secondary of their 200P.  I don't recall anyone with 150P etc reporting similar issues.  It may well have been that the tolerances these are made to are so tight that only a few were affected when used for imaging. 

Getting this back on track, I salute you for the video.  I've been documenting the events on UK railways in my home county (and a few hobby videos) but never had the guts to stand in front of the camera rather than being behind it.  

[Chris]

On 09/03/2021 at 19:01, malc-c said:

The resulting unsilvered flat spots on the minor axis gave me additional rouge spikes on bright stars.

Here's a quick test shot from the 200p using a stock Canon 40D and Baader coma corrector. In order to check out the star shapes I pointed the scope at M45 to get decent diffraction spikes. Just a very quick 10 second light frame because of the high winds but what do you think? 

Edited March 18, 2021 by Lockie

[malc-c]

Looks good to me...

[Soligor Rob]

Excellent Post Lockie, I have just taken ownership of a Sky Watcher Dobsonian 200P and have learnt so much from this, I'm currently waiting for the Dual Speed Crayford Focuser to be available, that will be my next purchase.

I spent a good hour last night observing the moon, great views with the light showing the landscape shadows, tried a few photos with my Canon 60D attached to the Dob but really struggled with the focus, note to self, "Must try Harder".

[Chris]

14 minutes ago, Soligor Rob said:

Excellent Post Lockie, I have just taken ownership of a Sky Watcher Dobsonian 200P and have learnt so much from this, I'm currently waiting for the Dual Speed Crayford Focuser to be available, that will be my next purchase.

I spent a good hour last night observing the moon, great views with the light showing the landscape shadows, tried a few photos with my Canon 60D attached to the Dob but really struggled with the focus, note to self, "Must try Harder".

Thanks Rob. The Skyliner has a different focal length, I think it's an f/6 so 1200mm instead of 1000mm, so it may behave a bit different to the 200p f/5 In terms of reaching focus, I'm not sure? A lower profile focuser will definitely help, and pushing the primary mirror up the tube a few mm with the primary mirror adjustment screws. Also once you have enough focus travel a focus mask will really help you nail the focus I need to do a quick vid on that at some point. 

Edited March 18, 2021 by Lockie

[Soligor Rob]

28 minutes ago, Lockie said:

Thanks Rob. The Skyliner has a different focal length, I think it's an f/6 so 1200mm instead of 1000mm, so it may behave a bit different to the 200p f/5 In terms of reaching focus, I'm not sure? A lower profile focuser will definitely help, and pushing the primary mirror up the tube a few mm with the primary mirror adjustment screws. Also once you have enough focus travel a focus mask will really help you nail the focus I need to do a quick vid on that at some point. 

Point taken, with the reaching focus range I have decided to purchase the Low Profile Dual Speed Crayford Focuser, Alex from FLO recommended this a few weeks ago.

Also like you mentioned in the video I don't like the adaptor that holds the eye piece with those screws, I hoping the new focuser comes with a compression type fitting, I have also ordered a Bahtinov mask so that should help my focussing.

Thanks for the tips and also for making me think.