Mpcc Not Working With 130Pds

[Ishan Mair]

Hi guys. I've been getting odd shaped stars with my MPCC III, the Skywatcher 130PDS and Canon 1100D. I have drilled a third screw into 2" eyepiece holder as suggested by Rob elsewhere on the forums. I checked collimation too. It looked near perfect to me. I thought I'd post cropped images to get insights of the more experienced imagers. Its a single guided test sub of 2 seconds so that pretty much rules out polar alignment and tracking error. The top-right corner does show a portion of bright star being cut off by the focuser drawtube.

And yes, does MPCC III cause noticeable decrease in light transmission? I suspect that images without using MPCC in the past have shown much more faint nebulosity even without guiding. Though this could be due to worsening seeing conditions and ever increasing light pollution here, has anyone noticed this or otherwise? 

Thanks.

Note: All corner crops are from the same image but were stretched separately using Curves to show some stars. I took care not to clip the highlights.

Bottom Left

Bottom Right

Top Left

Top Right

Center

[Tinker1947]

You could have a bit of tilt in the Focuser/DSLR.......

[Skyline]

have you got the spacing correct using the mpcc ?

[Ishan Mair]

You could have a bit of tilt in the Focuser/DSLR.......

That sounds like more trouble for me !! Is there a way to check it?

have you got the spacing correct using the mpcc ?

I use a standard t-ring which measures 11 mm in thickness.

[Skyline]

I found the mpcc very picky with spacing it should 55mm  +/- 1mm, my preferred tool is a digital precision caliper.

Al.

[Ishan Mair]

I found the mpcc very picky with spacing it should 55mm  +/- 1mm, my preferred tool is a digital precision caliper.

Al.

 I'll try adjusting the spacing more accurately. Thanks.

[Tinker1947]

That sounds like more trouble for me !! Is there a way to check it?

I use a standard t-ring which measures 11 mm in thickness.

I have a Moonlite Focuser these have 4 Allen screws one in each corner so if it needed adjusting these would do the job, if you focuser is without these then shimming would work, but try every thing else first.....spacing the MPCC i just used a T-Ring on my Canon work perfectly

 

[Owmuchonomy]

All of the above but you could well be at the limits of a flat field with your APS-C sensor.  I think Rob uses the 83000 chip so its a bit smaller I believe.

[jsmoraes]

With my coma corrector and Canon I needed to experiment to find the best distance. And my coma corrector doesn't a perfect correction. There are a  little coma as result. It is a GSO model.

And your images show very similar to mines. Better shape of star at one corner and worse shape at opposite corner.  I got a little correction taking care when attaching  the camera in the focuser. The screws do a little tilt on camera.

take a look at http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1507/photoset-measure.jpg and  http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1224/ASI120MC_1A.jpg

Can you image what tilt I have with this assembly http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1984/adapter-FE-CanonT3.jpg

Despite of I am using here a Focal Extender (like barlow) I loose the corners. I need crop the image.

[Tinker1947]

To stop any tilt on the Focuser to DSLR i used a Baader Click-lock they have (i believe) 3 rollers that tighten as the lever is pushed round, so there's no chance of tilt or the whole lot falling out of the Focuser.....

[Ishan Mair]

I have a Moonlite Focuser these have 4 Allen screws one in each corner so if it needed adjusting these would do the job, if you focuser is without these then shimming would work, but try every thing else first.....spacing the MPCC i just used a T-Ring on my Canon work perfectly

My focuser does have tilt screws. Any step-by-step instructions to follow for the adjustment? I get scared when I read "adjustments". I've a knack of generating huge waste materials by doing small adjustments.

With my coma corrector and Canon I needed to experiment to find the best distance. And my coma corrector doesn't a perfect correction. There are a  little coma as result. It is a GSO model.

And your images show very similar to mines. Better shape of star at one corner and worse shape at opposite corner.  I got a little correction taking care when attaching  the camera in the focuser. The screws do a little tilt on camera.

take a look at http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1507/photoset-measure.jpg and  http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1224/ASI120MC_1A.jpg

Can you image what tilt I have with this assembly http://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1984/adapter-FE-CanonT3.jpg

Despite of I am using here a Focal Extender (like barlow) I loose the corners. I need crop the image.

That's an astonishing setup Jsmoares. I am thinking of getting a GSO CC if this Baader doesnt work ever. I'll try being careful when I attach it now. And I dont know how I could experiment the spacing. It might take months before I receive spacers from shops abroad.

To stop any tilt on the Focuser to DSLR i used a Baader Click-lock they have (i believe) 3 rollers that tighten as the lever is pushed round, so there's no chance of tilt or the whole lot falling out of the Focuser.....

It looks like it will push the camera far out. Will it come to focus then?

All of the above but you could well be at the limits of a flat field with your APS-C sensor.  I think Rob uses the 83000 chip so its a bit smaller I believe.

I hope its not that. Though, I've looked all over astrobin. Most user images show some coma or have squarish/blocky stars when using MPCC with a DSLR. Very few may have got it perfect.

[Owmuchonomy]

To stop any tilt on the Focuser to DSLR i used a Baader Click-lock they have (i believe) 3 rollers that tighten as the lever is pushed round, so there's no chance of tilt or the whole lot falling out of the Focuser.....

What order have you got the various components in that set up please?

[Tinker1947]

Starting from the DSLR, there is the T-Ring, into this screws the MPCC, this slides into the Baader Click-Lock, the Click-lock is held in place by the Moonlight screws (3) the focus worked ok with the Baader in the chain, since upgrading to a CCD and Filter Wheel the MPCC is still used with the CCD the FilterWheel then a Baader Vari-Lock to get the 55mm spacing the the MPCC this  goes into the Moonlite....never had anything but round stars with either set-up....

[Owmuchonomy]

Starting from the DSLR, there is the T-Ring, into this screws the MPCC, this slides into the Baader Click-Lock, the Click-lock is held in place by the Moonlight screws (3) the focus worked ok with the Baader in the chain, since upgrading to a CCD and Filter Wheel the MPCC is still used with the CCD the FilterWheel then a Baader Vari-Lock to get the 55mm spacing the the MPCC this  goes into the Moonlite....never had anything but round stars with either set-up....

OK.  Its the click lock to focuser part that is throwing me.  What size is the Moonlite to take the click lock.  It won't fit inside my standard SW focuser.

[Tinker1947]

Its a 50mm hole...i have just tried a Click-lock in my Skywatcher Dob Focuser and its a sliding fit.....

[Owmuchonomy]

Its a 50mm hole...i have just tried a Click-lock in my Skywatcher Dob Focuser and its a sliding fit.....

Yes, I've figured it now.  I need the smaller click-lock.  I checked with FLO but as you say its only a push fit which defeats the object of getting one.  I think I'll try Rob's mod instead.

[Uranium235]

Cant use a click lock with the CC using the stock SW focuser, it will push the imaging train out too far.

Two things need to be checked:

1) Collimation, good collimation = good correction

2) Focuser tilt, if the focuser is tilted you wont be able to get the above collimation correct and it will also result in mis-correction.

You have at least one or two good-ish corners, so for now you can cheat the focus by moving the star you use for focusing towards the dodgy edge/corner and then focus using the b-mask. That will help balance out the field.

However, you seem to have a little residual coma after looking at the test subs - this would indicate that your spacing is too short. Try using a delrin spacer to increase the spacing, I found that I had to add a couple of mm to my spacing in order to get it perfect. Having said that, you are using a bigger chip than I do - so there will be a point where it might be unrealistic to expect a 100% perfect field, its more than likely you will get to about 98% - at which point its probably best to settle for that becuase that last 2% will be the hardest to achieve.

Lastly, the MPCC does not reduce the light throughput - its still f5 and if anything, any filters used will be blocking light (albeit a very small amount).

Its not that the MPCC "isnt working" - it most certainly is, because if you remove it your coma will be a whole lot worse. Its just a case of maximising your corrected field. Also, I would only suggest persuing my internal mounting mod if you are using a heavy imaging traing (1.5kg+) - a DSLR isnt heavy, so no need for it.

[Ishan Mair]

On ‎22‎-‎01‎-‎2016 at 18:44, Uranium235 said:

Cant use a click lock with the CC using the stock SW focuser, it will push the imaging train out too far.

Two things need to be checked:

1) Collimation, good collimation = good correction

2) Focuser tilt, if the focuser is tilted you wont be able to get the above collimation correct and it will also result in mis-correction.

You have at least one or two good-ish corners, so for now you can cheat the focus by moving the star you use for focusing towards the dodgy edge/corner and then focus using the b-mask. That will help balance out the field.

However, you seem to have a little residual coma after looking at the test subs - this would indicate that your spacing is too short. Try using a delrin spacer to increase the spacing, I found that I had to add a couple of mm to my spacing in order to get it perfect. Having said that, you are using a bigger chip than I do - so there will be a point where it might be unrealistic to expect a 100% perfect field, its more than likely you will get to about 98% - at which point its probably best to settle for that becuase that last 2% will be the hardest to achieve.

Lastly, the MPCC does not reduce the light throughput - its still f5 and if anything, any filters used will be blocking light (albeit a very small amount).

Its not that the MPCC "isnt working" - it most certainly is, because if you remove it your coma will be a whole lot worse. Its just a case of maximising your corrected field. Also, I would only suggest persuing my internal mounting mod if you are using a heavy imaging traing (1.5kg+) - a DSLR isnt heavy, so no need for it.

Thanks Rob for the analysis. Sorry for the late reply. I didn't get a notification so didn't see this thread for quite a while. I have decided to get a sight tube with Cheshire for collimation. I am very doubtful about the secondary position under the focuser and its rotation. Its difficult to judge those things. Focuser tilt.. Is there some easy way to confirm if its not orthogonal?

[Uranium235]

6 hours ago, Ishan Mair said:

Thanks Rob for the analysis. Sorry for the late reply. I didn't get a notification so didn't see this thread for quite a while. I have decided to get a sight tube with Cheshire for collimation. I am very doubtful about the secondary position under the focuser and its rotation. Its difficult to judge those things. Focuser tilt.. Is there some easy way to confirm if its not orthogonal?

There is no simple (to mind) that you can confirm focuser tilt, but the two giveaways are uneven correction or an inability to centre the secondary despite numerous careful attempts (eg: once you have made your final collimation tweaks the the secondary is not centred). The hardest part of collimating a secondary is ensuring it does not rotate when adjusting the collimation screws (I usually keep a grip in the secondary holder to prevent that happening).

[Ishan Mair]

11 hours ago, Uranium235 said:

There is no simple (to mind) that you can confirm focuser tilt, but the two giveaways are uneven correction or an inability to centre the secondary despite numerous careful attempts (eg: once you have made your final collimation tweaks the the secondary is not centred). The hardest part of collimating a secondary is ensuring it does not rotate when adjusting the collimation screws (I usually keep a grip in the secondary holder to prevent that happening).

That means I should try collimating the secondary with highest accuracy possible. I think its better to ask for advice on the collimation tool. Given that we use the same scope, how to collimate with the MPCC in the focuser? And what's your preferred tool for collimation? Especially for getting the secondary adjustments spot on. Also you mentioned the delrin spacers to get spacing right between the MPCC and the chip. But the mechanical noob with fingers on the keyboard here cant figure out how to do it. Images would be of great help.

[Tinker1947]

If you get the secondary set-up and there still iffy stars it will be the MPCC shoulder in the focus tube the addition of a third screw won't help much if the MPCC and Focuser tube are a sloppy fit to start with, it requires a sliding fit or when you tighten the screws how ever evenly you do it the MPCC will tilt, a quality focuser may be the best option if you aren't a bit DIY handy......

[Uranium235]

On 12/02/2016 at 10:57, Ishan Mair said:

That means I should try collimating the secondary with highest accuracy possible. I think its better to ask for advice on the collimation tool. Given that we use the same scope, how to collimate with the MPCC in the focuser? And what's your preferred tool for collimation? Especially for getting the secondary adjustments spot on. Also you mentioned the delrin spacers to get spacing right between the MPCC and the chip. But the mechanical noob with fingers on the keyboard here cant figure out how to do it. Images would be of great help.

You dont collimate with the MPCC fitted, all you need is a cheshire EP. I have no images of the procedure as such becuase they would not adequately describe what is happening. But the key thing is making sure the edges of the secondary are concentric with the bottom opening of the combined cheshire/sight tube - just rack the focuser in or out so you can just see the edges of the secondary at the bottom of the sight tube. Placing a bit of white paper behind the secondary and illuminating the inside of the OTA (but not pointing the light at the primary) will help you see whats going on.

On 12/02/2016 at 11:41, Tinker1947 said:

If you get the secondary set-up and there still iffy stars it will be the MPCC shoulder in the focus tube the addition of a third screw won't help much if the MPCC and Focuser tube are a sloppy fit to start with, it requires a sliding fit or when you tighten the screws how ever evenly you do it the MPCC will tilt, a quality focuser may be the best option if you aren't a bit DIY handy......

No 3rd party focusers are available for this telescope, you have to work with what you are given. So its a case of playing with the tension screws, taking out any tilt and ensuring the corrector sits squarely (by whatever method works). I fitted my MPCC internally by building an M54-M48 adaptor, the only one of its kind I believe (therefore all threaded):

[Ishan Mair]

A cheshire sighttube combo is on its way.  Meanwhile, the other night, the stars in the top left corner looked squeezed as if over corrected for coma and in the bottom right corner showed coma. Sign of some tilt, isn't it?

[richyrich_one]

Hi Ishan

Did you resolve this?

Make sure you are using the ring that protects the 2" thread, it gives a good face to sit against the focuser but more importantly it makes sure the lock screws locate well inside the undercut region on the barrel of the MPCC. Otherwise they tighten right on the lip and things can become unsquare very easily.

I had a similar problem and along with the 3rd screw this solved it.

[Ishan Mair]

On ‎24‎-‎02‎-‎2016 at 16:28, richyrich_one said:

Make sure you are using the ring that protects the 2" thread, it gives a good face to sit against the focuser but more importantly it makes sure the lock screws locate well inside the undercut region on the barrel of the MPCC.

are you referring to this stopper collar?