Imaging with the 130pds

[Grierson]

1 hour ago, Adam J said:

What edition was it?

March. It's in Sainsbury's so I would guess most other places. - John

[Adam J]

2 hours ago, gazza said:

Hi,

disappointingly my new 130pds is showing triangular stars. The focuser has been trimmed and is not impinging upon the mirror. Anyone else had this? Pinched primary in its cell? Secondary problem? Using Skywatcher f4 coma corrector. It is consistent right across the field. Also note the reflections on smaller stars. Have never seen this before....It was focused using a bahtinov mask, yet looks almost out of focus

Suggestions appreciated.

 

cheers

Gary

It looks like you have poor focus to me, its possible that the focus shifted after removing the mask, maybe combined with something else, maybe mirror pinch. I have not seen anyone report an issue with triangular stars with the 130PDS before. 

Edited February 21, 2017 by Adam J

[Skyline]

check to make sure the clips are not tight on your primary mirror.

[Stub Mandrel]

That does look just like pictures of 'pinched optics' that I have seen - have never seen this first hand though.

[rotatux]

20 hours ago, gazza said:

I wonder where you heard this? I've never heard or seen any evidence of this factory testing for optical centre? On the contrary I've seen many scopes that were impossible to critically collimate accurately using a factory placed spot, which became easy once a centre spot was accurately placed - indicating that the optical centre was the mechanical centre......Could you please provide a reference, I'd love to know how they determine the optical centre.....

I didn't explicitly read about it, just 0/ having a relatively strong mathematical and physics background (just as many people here ;-)), 1/ implicitly understood the thing after several months studying articles about "how to grind your mirror yourself" with rotation-based grinding machines and 2/ watching photos of how the pro-equivalent machines are made (basically the same principles as amateurs, just less rudimentary). You will note in those setups the mirror chocks / holders position can have some liberty in their blocking position depending on small mirror plate irregularities on the edge, which is the source of the shift discussed.

For a factory it's not as difficult as you may think, they don't HAVE to optical-test the mirrors, just know its center of rotation, which is easy for them because THEY grind the mirror. Of course it needs a bit of organisation and QA, so depending on brands and periods YMMV and I admit there must be exceptions For the 2 serious newtons I had in my life (130PDS and Lightbridge 12") both spot's position were perfect (i.e. could be used for collimation).

[Adam J]

My new DIY dew shield, the classical camping mat design.

[gazza]

1 hour ago, rotatux said:

I didn't explicitly read about it, just 0/ having a relatively strong mathematical and physics background (just as many people here ;-)), 1/ implicitly understood the thing after several months studying articles about "how to grind your mirror yourself" with rotation-based grinding machines and 2/ watching photos of how the pro-equivalent machines are made (basically the same principles as amateurs, just less rudimentary). You will note in those setups the mirror chocks / holders position can have some liberty in their blocking position depending on small mirror plate irregularities on the edge, which is the source of the shift discussed.

For a factory it's not as difficult as you may think, they don't HAVE to optical-test the mirrors, just know its center of rotation, which is easy for them because THEY grind the mirror. Of course it needs a bit of organisation and QA, so depending on brands and periods YMMV and I admit there must be exceptions For the 2 serious newtons I had in my life (130PDS and Lightbridge 12") both spot's position were perfect (i.e. could be used for collimation).

Hi,

These mirrors are not made in the same way that mirrors were ground in the old days.  They use machine generate blanks that are accurately ground, certainly not millimetres out of shape and use high speed grinding/polishing machines that are worth lots of money. These things are not likely to grind an off centre optic. Pre these machines no-one measured the centre of rotation then because the grinding process averaged out the centre position and it would have taken some appalling technique to get an off centre optic. You are lucky that your centre spots are centred! Mine never have been and I've had half a dozen GSO and Skywatcher newts over the years. I've also ground and polished numerous mirrors from 8-12" f4-f6 over the years, done by both hand and machine, and always centre spotted the physical centre, and always achieved spot on collimation.

A slightly offset donut can be used for collimation - If all you are using is a sight tube, laser and cheshire. A scope can look like it is collimated in this case. If you add an autocollimater in to the mix then you may start to see problems. These can also show up as tilt in the focal plane - which may not show up on small chips, but starts to show up on Kaf8300's and above in size.

In the end, if using the factory spot works for you, that's great - me I'll keep measuring and spotting myself if it is off centre!

YMMV

cheers

Gary

 

[gazza]

8 hours ago, Skyline said:

check to make sure the clips are not tight on your primary mirror.

Had loosened them when re-spotting the primary - perhaps not enough...Will loosen them some more. The mirror is also a very tight fit in the cell - tighter than I have seen before. Does anyone know if this is normal for a 130PDS? I may put the cell in the lathe and skim a couple of thou off it to give the mirror a bit more room.

Puzzled

Gary

[gazza]

7 hours ago, Stub Mandrel said:

That does look just like pictures of 'pinched optics' that I have seen - have never seen this first hand though.

Me either!!!!

cheers

Gary

[gazza]

8 hours ago, Adam J said:

It looks like you have poor focus to me, its possible that the focus shifted after removing the mask, maybe combined with something else, maybe mirror pinch. I have not seen anyone report an issue with triangular stars with the 130PDS before. 

I thought the focus was slipping also, however I put the Bahtinov mask back on and checked - it still showed perfect focus so I'm not sure what is going on here.

cheers

Gary

[gazza]

Hi,

just checked my mirror cell - the mirror was a very tight fit - it was impossible to rotate it using finger pressure. I have machined a tiny bit off the inside of the cell and the mirror now freely rotates. It looks like the casting was very slightly out of round. Should have checked this when I re-spotted the primary, but I didn't :-( I had loosened the primary clips, so I don't think these were the problem. Slight possibility of a clear night tonite for testing...

Hopefully this will fix the triangular stars.

cheers

Gary

[gazza]

Machining the cell has fixed the triangular stars - thank goodness!!

Now have to play with the spacing of the F4 coma corrector to optimise correction. It is not working as well at f5 at the sam spacing as provides perfect correction at f4 - can anyone who may be using this corrector  with the 130PDs please advise their spacing?

cheers

Gary

 

[Skyline]

That looks lots better doesn't it. Most centre spots are quite accurate centered, another way of checking if centered, draw around the mirror using a pencil, cut out the shape of the mirror on a plane piece of paper or a thin film of plastic, fold the paper 2 or 3 times, and make a center hole in the sheet of paper/plastic. Align it with mirror careful so you don't scratch the mirror then check where your centre aligns on the mirror. You could also use a Cats Eye mirror marking template.

Edited February 22, 2017 by Skyline

[Stub Mandrel]

11 hours ago, gazza said:

Machining the cell has fixed the triangular stars - thank goodness!!

This needs to go down in the great store of 130P-DS lore!

[Susaron]

Dear all

I have been working on the set up of the DIY project MyFocuserPro, from Robert Brown, adapted to my SW130PDS Moonlite tuned.

Today I have received a motor bracket I designed last week , manufactured with a 3D printer, if anybody is interested in the stl file done in FreeCad, please let me know.

[Adam J]

Nice one, my only worry would be how do you fit a dew shield?

 

[Susaron]

According to Robert´s project the steeper has to be attached either directly or through a printer belt to the hard part of the focuser.

The original SW130PDS focuser is symmetric around the focus axis, so you can turn the focuser 180 degrees having  the fine focuser knob pointing outwards and using the steeper from barckwards, that will allow to use a dew shield.

 

[gazza]

Wondering about choice of coma corrector?

I have a Skywatcher F4 coma corrector (used for my 8" Quattro), but despite spending an evening changing spacings I was unable to get good images to the edge of a KAF8300 chip with the 130PDS.

I tried a Skywatcher F5 coma corrector and this worked well (interestingly, at the same spacing as the F4 works in the Quattro), however I'm also interested in the Baader Mk111, as I've read of reflection issues with the Skywatcher.

Can anyone advise as to the best choice here or have comments on the best coma corrector for the 130PDS?

TIA

cheers

Gary

[Adam J]

8 minutes ago, gazza said:

Wondering about choice of coma corrector?

I have a Skywatcher F4 coma corrector (used for my 8" Quattro), but despite spending an evening changing spacings I was unable to get good images to the edge of a KAF8300 chip with the 130PDS.

I tried a Skywatcher F5 coma corrector and this worked well (interestingly, at the same spacing as the F4 works in the Quattro), however I'm also interested in the Baader Mk111, as I've read of reflection issues with the Skywatcher.

Can anyone advise as to the best choice here or have comments on the best coma corrector for the 130PDS?

TIA

cheers

Gary

Cant help with a comparison but may people including myself use the MPCC MKIII with great results even on APS-C sized sensors and the KAF8300 is sub APS-C so you will be good. Worked great on my 1000D, having some teething problems with my 550D but am sure ill get it working well with some tweaking. The F4 will most likely work well too...its just that the optimal spacing changes with F-number.

[Uranium235]

Yep, get the MPCC MkIII.

The SW correctors suffer from internal reflections on bright stars.

[Stub Mandrel]

8 hours ago, Uranium235 said:

Yep, get the MPCC MkIII.

The SW correctors suffer from internal reflections on bright stars.

The one time I've had this be a problem was the flame and horsehead where each sub had a 'ghost' of Alnitak as a blue patch diagonally opposite the star, which stretched into a faint blue blur (as the exact centre of the image moved over 3 hours) after stacking. I assume this is the sort of thing you mean?

[Uranium235]

1 hour ago, Stub Mandrel said:

The one time I've had this be a problem was the flame and horsehead where each sub had a 'ghost' of Alnitak as a blue patch diagonally opposite the star, which stretched into a faint blue blur (as the exact centre of the image moved over 3 hours) after stacking. I assume this is the sort of thing you mean?

Yes, exactly that! I also experienced the same issue on M45, I managed to clone brush it out but I'd rather it not be there in the first place

[Stub Mandrel]

11 hours ago, Uranium235 said:

Yep, get the MPCC MkIII.

The SW correctors suffer from internal reflections on bright stars.

Thanks - once I know what the issue is I can at least work round it

[Susaron]

A single shot from M42 from yesterday evening, I had problems with guiding, though my polar allignment was under 2 arc minutes on both axes.

60 seconds at ISO1600 with a unmodded Canon 70D

 

[Nova2000]

That's a nice image. I think there is a little of more blue in it. 

IT may be my screen.