Problem with SW Coma Corrector

[mattastro]

Hi,

I finally got round to buying a SW Coma Corrector for my SW Explorer 150PDS.  I screw it directly into a T-Ring fitted to my Canon 1100D.  I then screw a UHC filter onto the Coma Corrector.  So Camera -> T-Ring -> Coma Corrector -> UHC filter.

Unfortunately I believe the coma has now got worse.  Instead of having it all pointing to the middle, the tear drops now all point in one direction in every picture I do, and all over the picture.  The direction varies from picture to picture.

I have attached an example from last night.  This was made up of around 25 x 4 minute exposures stacked.

Any help appreciated!

Thanks,

Matthew

[ronin]

The SW CC mentions a DSLR adaptor that seem to correctly space the DSLR from the CC. You do not mention one in the optics path so the spacing may be incorrect between the CC and the DSLR.

This in turn means that the UHC filter may be in the wrong position as I would then assume CC -> Adaptor -> DSLR. Meaning the UHC needs to be before the CC or it may sit inside the Adaptor.

That is all I can suggest, but spacing on these is normally critical.

[Kevstew]

I've also been having this problem but I don't know why. I checked my collimation and it was out so adjusted, but not had a chance to see if it made any difference. I've had the sw coma corrector a while but only recently seen this problem. I've been having guiding problems with a new mount, so it's not been at the top of my list of problems. I Will be interested if someone knows what's causing this problem.

[mattastro]

Thanks for the replies.  I collimated last night before imaging and did the star test - and it seems correct.  So I've ruled that out.  However I think I need to try the UHC filter before the CC as you suggest.

[Lonestar70]

Hi Matthew,

I assume you are using the SW coma corrector with 0.9x focal reduction which is the one SW specify for the PDS scopes.

You must use the special M48 camera adaptor ring with this (it is not a standard T2 camera adaptor) or you will not get the required spacing of 55mm necessary to achieve a flat field.

Filters screw directly into the front of the coma corrector (focusser end).

I have one on my 150pds and have found a problem with it which is due to the x0.9 focal reduction... this focal reduction creates a problem where, with the camera in focus, the focuser is almost all the way in (on my 150PDS the focuser only has approx 3 or 4 mm of inward travel left)... thus the focuser barrel protrudes too far into the scope body.

This results in a small nibble being taken out of any star images (all stars are effected) making them look elongated.

When I zoom in on your image I can see just such a nibble on all the stars in it (just on the left side of each star)... which is making them look elongated vertically.

This is a known problem with this coma corrector/PDS scope combination and SW have not appeared to have done anything about it... they have really dropped the ball on this one.

The only possible cure I have read of (other than cutting a bit off the inner end of the focuser barrel) is to re-collimate the scope with the main mirror as far up the tube as possible... this pushes the focal point a little further out and the focusser barrel can then be a little further out when the camera is in focus.

The better cure would be for SW to make the coma corrector either without the 0.9x focal reduction which would bring the focus point further out or to re-design it in some other way so that it does not cause the problem... I won't hold my breath on either option being done.

I am going to try the re-collimation method and see if it does enough to cure it... failing that I will change over to a Baader coma corrector which has no focal reduction and also has a longer backfocus distance.

I hope this helps.

Best regards.

Sandy.

[mattastro]

Thanks everyone for your help.

I have checked the adaptor I am using and it is the Skywatcher M48 - so hopefully no problem there.  I have also been using the filter in the correct place by the sound of it.

Therefore I will now try moving the main mirror up the tube as you suggest Sandy.  

Thanks again - I will let you know how I get on.

[Pompey Monkey]

Hi,

I finally got round to buying a SW Coma Corrector for my SW Explorer 150PDS.  I screw it directly into a T-Ring fitted to my Canon 1100D.  I then screw a UHC filter onto the Coma Corrector.  So Camera -> T-Ring -> Coma Corrector -> UHC filter.

Unfortunately I believe the coma has now got worse.  Instead of having it all pointing to the middle, the tear drops now all point in one direction in every picture I do, and all over the picture.  The direction varies from picture to picture.

I have attached an example from last night.  This was made up of around 25 x 4 minute exposures stacked.

Any help appreciated!

Thanks,

Matthew

I too have the SW coma corrector on a 150 PDS and get the same effect:

The nibble, however, makes the side of the star flat, rather than elongating it. I suspect that there is also an element of tracking drift going on in your image too. 

Also, from looking at my star close-up, the draw-tube is causing a lot more damage to the image than just the nibble. I think it's hacksaw time!!!

[Lonestar70]

I too have the SW coma corrector on a 150 PDS and get the same effect:

nibbled star SW coma corrector.PNG

The nibble, however, makes the side of the star flat, rather than elongating it. I suspect that there is also an element of tracking drift going on in your image too. 

Also, from looking at my star close-up, the draw-tube is causing a lot more damage to the image than just the nibble. I think it's hacksaw time!!!

Be very carefull if you take a hacksaw to the draw tube... take too much off and you can end up with the draw tube disconnecting from the support rollers when you rack outwards... the rollers are quite widely spaced on this focuser.

Moving the primary mirror is the better option, although neither should/would be necessary if SW had designed the corrector properly... it is not a particularly cheap item after all.

Yes I agree there could be a small element of drift included in the OP's photo, but I think the main problem is the draw tube protrusion.

Clear skies everyone.

Sandy.

[Pompey Monkey]

Be very carefull if you take a hacksaw to the draw tube... take too much off and you can end up with the draw tube disconnecting from the support rollers when you rack outwards... the rollers are quite widely spaced on this focuser.

Moving the primary mirror is the better option, although neither should/would be necessary if SW had designed the corrector properly... it is not a particularly cheap item after all.

Yes I agree there could be a small element of drift included in the OP's photo, but I think the main problem is the draw tube protrusion.

Clear skies everyone.

Sandy.

I have considered this and would be very cautious. However, I may just experiment with a little blacking up (not in "that" way ) of the outside of the tube first to see what happens. Interestingly, SW have darkened the inside of the draw-tube, but not the outside. That thing is diffracting/reflecting all over!

[Lonestar70]

Certainly worth a try it can only improve things by minimising reflections... not sure it will help much with the diffraction though.

Moving the primary and blacking up may cure both problems with any luck.

Keep Happy.

Sandy.

[mattastro]

I've moved the mirror as far up as it will go, and re-collimated.  I'll do some testing tonight hopefully (won't need too much clear sky for the purposes of this).  I'll let you know how it goes :-)

[lensman57]

I too have the SW coma corrector on a 150 PDS and get the same effect:

nibbled star SW coma corrector.PNG

The nibble, however, makes the side of the star flat, rather than elongating it. I suspect that there is also an element of tracking drift going on in your image too. 

Also, from looking at my star close-up, the draw-tube is causing a lot more damage to the image than just the nibble. I think it's hacksaw time!!!

You can take away about 10mm off the draw tube on the safe side and no more. The problem with cutting a little too much off the draw tube is that the spacing of the ball bearings in relation to the draw tube. Cut too much and the most inward bearings loose contact with the tube and it will then flops. To be honest this is bad design on the part of SW and it should really be put right.

A.G

[Ceames]

I'm having exactly the same problem with the SW coma corrector with a very similar setup (although I have the 150P with the single speed Crayford focuser rather than the PDS).  I'd be interested to see how you get on moving the mirror, I've had a quick play with mine and I've managed to get a few extra mm change in the focal point but I don't think it will be enough to cure the problem (may have to resort to the hacksaw method!).

C.

[mattastro]

Hi.  

I moved the mirror as far forward as it would go - basically so that the screws that lock it in position after collimation could still make contact.  

I am not a collimation expert by any means but I checked it by focusing on Deneb looking through a 9mm eyepiece and then unfocusing slightly - and I did get the concentric circles (airy effect).  But interestingly when I defocused the other way I got a distorted airy effect - so not sure it is perfect.

I then tried again.  What I noticed was that if I make the image unfocused by having the focuser tube too far in, and then move towards focus from that side, I don't get the coma problem.  However if I approach focus the other way - from having the focuser too far out, then I do get it.  Not sure why that is - I can only link it to the possible collimation issue?

For the purpose of testing I set it so I was still getting the problem, and then took the UHC filter off - still got the problem.  I then removed the CC altogether, and didn't get the problem (or to be precise just got the usual coma as expected all facing towards the middle).

For me I am now just going to focus from having the focuser too far in and moving outwards - and it is good enough for me.  I think there is still very slight coma, and possibly I am not perfectly in focus, but it is more than good enough for my level.

Hope that helps anyone else with this problem, and thanks for everyone's suggestions.  :-)

[Lonestar70]

Hi Matthew,

I am pleased to read you have made some improvements... well done, thats a good few steps in the right direction.

The difference in airey rings on either side of focus could be caused by a slight rocking of the focuser draw tube.

It would certainly pay you to check your focuser out and adjust the spindle alignment and tension... both could be slightly out which could cause a slight rocking as the drive is applied in opposite directions... especially if the tension is too little.

It is not very difficult to do and these guides will help: -

Adjusting_Crayford_focuser.pdf

https://www.youtube.com/watch?v=ofOSAMlqZWw

Keep up the good work, your almost there.

Best regards.

Sandy.

[Ceames]

Glad to hear you are making progress Matthew!

I bit the bullet and had a go with the hacksaw, taking about 10mm off the end of the focuser draw tube.  This does seem to have solved my problem - here are the before and after shots (excuse the poor tracking - these were both pushing the limits of what I can get from my setup!):

Cheers,

C.

[mattastro]

Looking at the image I took - I'm not so sure anymore.  I think it is better - but the stars are still cut off.  I might have to get the hacksaw out myself.  Ceames - how did you do this - did you unscrew the whole focuser and saw about 1cm off the tube, and then screw it back on?  I'm not really a DIY type ...!

I've attached the image.  It's a not great effort at the Pelican nebula - made of around 12 x 210s shots.

[Ceames]

I found I could remove the focuser draw tube by just lossening the four fixing screws in the four corners of the plate holding the focuser shaft and knob against the draw tube.  I have the single speed crayford focusser so it may be a bit more complicated for you with the dual speed, although looking at the PDF Lonestar70 posted the same technique should work for you.  After loosening the four screws most of the way I found there was enough room to slide the whole silver tube out of the focuser assembly without having to remove the plate fully.

I sawed ~10mm of the non threaded end with a hacksaw (not too hard to do but you'll need a vice or  something similar to carefully hold the tube stable and a bit of care and elbow greece!) and then filed down the cut edge to get a smooth-ish finish.  The video link posted above by Lonestar70 shows how to take the focusser apart, although they just file down the flat edge of the tube rather than sawing off the end.

I plan to coat the exposed shiny edge with some matt black paint when i get some, but have settled temporarily with careful application of some matt black tape.  The tape has also helped ensure the shortened tube stops before loosing contact with the bearings when the focuser is racked all the way out (which you may need to do to focus with an eyepiece for visual use when you're not imaging).  I'd be quite careful here when deciding how much to saw off -10mm was just about right for me but again I don't know if the tube in your dual speed focusser is the same starting length as mine.  My long term plan is to use some tape or filler material to shortern the length of the flat edge on the tube so that the mechanism stops the tube moving out automatically before it gets to the point it would lose contact with the bearings and flop down.

Hope that helps and good luck if you go for it!

PS I like your image of the Pelican!