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

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

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

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

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

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

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

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

10 hours ago, rotatux said:

 

Beware not to touch it unconsciously : the spot doesn't mark the geometrical center of the mirror, but it's optical center. The two are identical for a spherical shape but can be distinct for a parabolic shape such as the 130PDS'. It's supposed to be setup correctly at factory, so be sure to test your mirror on optical bench before thinking about moving it.

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.....

cheers

Gary

11 hours ago, Adam J said:

I've actually set it up this way so that I can do a mixture of visual and imaging. I want to look at an object, take a pic, record my observations. Having the c11 for visual gives me aperture, and the piggyback scope for imaging without having to disassemble anything so I can go whereever I fancy on a given night.

The spot was a bit over 2mm out of centre. As Have a Catseye template it was easy to check, and easy to replace. I've had half a dozen GSO/Skywatcher newts an only one was accurately centre spotted. I learned to check after hours trying to collimate and getting tilted fields and an edge out of focus. Once I centre the spots collimation is easy - I use a webcam to centre the secondary mirror, A Glatter laser to roughly point the secondary, and Catseye tools - cheshire and autocollimator to nail it. If your laser is collimated itself, get a Glatter Parallizer which will ensure the laser sits correctly in the tube.

cheers

Gary

 

Very nice, so will you be using both scopes at the same time to get place detailed objects within a wider field? 

I am starting to wonder if my centre spot is actually central....its just such a pain to remove the primary. How far out was it?

Either way I need to get shot of my terrible laser collimator as the laser is not parallel to the focuser before I do anything else. Anyone got a suggestion for a quality collimator....preferably still costing less than the scope?

 

 

Hi,

placed my first ever order with FLO last Thursday. Order shopped Friday, have received a text that item is "on board for delivery" today - and I'm 12000 miles away i Australia!

Fantastic.

cheers

Gary

Hi,

just got a 130PDS and have attached it piggyback to mt CPC1100 - see pic.

Had to cut 20mm off the focuser, and fit a reatining ring to the bottom of the focuser tube so that it would not  fall out. The centre spot was not centred, so removed it and replaced it with a Catseye spot. Put a piece of flock opposite the focuser. Also had to shift the secondary position as it was way to low. Scope is now collimated and ready to go. Will be using the Skywatcher (GPU) F4 coma corrector. Hope this works so I don't have to buy a Skywatcher f5 coma corrector......

cheers

Gary

Thanks Martin, have purchased adapter, got your email and will be buying more in the not distant future. Cheers, and thankyou immensely for sorting things out for me so quickly....The compliments you get are obviously well deserved.

cheers

Gary

Martin is on the case

The SX filter wheel is 100% compatible with Baader filters, the thicker cells are not a problem.

HTH

Aahh ... I see what has happened. Australia isn't one of the options in the drop-down list of available countries. It is one we manually add when requested. It is the same for the US. It is my fault for not letting Martin know.

HTH,

Steve

Steve,

could you please email me a quote for the item asap to the email address in the ticket? Once I have the reducer/corrector working I will be after the USB Starlight express Filter wheel for 2" filters, so could you add that as a separate quote to the reply? I can raise a separate query, but while we are here, is this filter wheel OK with Baader filters, which are a bit thicker than others?

cheers

Gary

We 'do' deliver to Australia. It depends on the item of course. We have even sold cameras to Japan. Please email us at the address below for a quote.

HTH,

Steve

Steve,

I was after :

FLO Adaptor for Skywatcher Focal reducers

My ticket number was 7391

the reply was in part:

"We dont ship to your destination "

cheers

Gary

It's a pity they will not mail order to Australia, even though their web site refers to international orders. We are not exactly a third world country!!! All I needed was an adapter! If that purchase had gone OK I was going to buy a Starlight Express CCD and filter wheel combo as well as they seemed to get favourable reviews as a supplier here....but will have to go elsewhere.

cheers

Gary