130PDS Coma Corrector

[JoshHopk]

Hi all, 

I know there's a lot of these threads going around, but I thought I should start a new one instead of trying to revive an old one.

I'm going to get a 130PDS soon and I'm aware I'm going to need a coma corrector. I will be using my Canon 100D (APS-C). I think I'd be right in saying the SW 0.9x will need me to move the primary up or cut the focus tube, but has some reflection problems on bright stars, so would I be better of going for the Baader MPCC?

However, some have said that the Baader is fussy about spacing, so how would i go about altering this?

Thanks in advance, Josh

Edited May 30, 2020 by JoshHopk

[JoshHopk]

Think I’ve decided for myself 

Im going for the SW 0.9x as it’s nowhere near as fussy as the Baader and the reflections it produces are easily fixed in post processing. 

[alacant]

1 hour ago, JoshHopk said:

SW 0.9x

Good choice.

You'll need to cut a length from the end of the focus tube if you want round stars. But try it first. It could be you'll not notice;)

HTH

[JoshHopk]

1 hour ago, alacant said:

Good choice.

You'll need to cut a length from the end of the focus tube if you want round stars. But try it first. It could be you'll not notice;)

HTH

I’ll try putting off assaulting my new scope for as long as I can, but I’m sure I’ll get to it 

[Pitch Black Skies]

On 30/05/2020 at 18:16, alacant said:

Good choice.

You'll need to cut a length from the end of the focus tube if you want round stars. But try it first. It could be you'll not notice;)

HTH

Would the Baader have been a better choice?

I'm trying to decide on a CC for my 130P-DS too.

I use a Canon EOS 800D.

[alacant]

4 minutes ago, Pitch Black Skies said:

the Baader

Hi

We tried four CCS.

Assuming affordable is important...

If you want corner to corner over aps-c out of the box, the GPU. The GSO comes a close second with the added advantage of not needing the focuser barrel cutting, the disadvantage being that it increases the focal length to over 700mm. 

Then comes the sw 0,9 which widens the field to a nice 585mm with the Baader the least satisfactory and prone to astigmatism.

No theory. Just our hands on.

HTH

[vlaiv]

15 minutes ago, alacant said:

Hi

We tried four CCS.

Assuming affordable is important...

If you want corner to corner over aps-c out of the box, the GPU. The GSO comes a close second with the added advantage of not needing the focuser barrel cutting, the disadvantage being that it increases the focal length to over 700mm. 

Then comes the sw 0,9 which widens the field to a nice 585mm with the Baader the least satisfactory and prone to astigmatism.

No theory. Just our hands on.

HTH

All that was on F/4 or F/5 scope (and is there any difference?)

[Pitch Black Skies]

47 minutes ago, alacant said:

Hi

We tried four CCS.

Assuming affordable is important...

If you want corner to corner over aps-c out of the box, the GPU. The GSO comes a close second with the added advantage of not needing the focuser barrel cutting, the disadvantage being that it increases the focal length to over 700mm. 

Then comes the sw 0,9 which widens the field to a nice 585mm with the Baader the least satisfactory and prone to astigmatism.

No theory. Just our hands on.

HTH

Thank you,

Is the GPU the same as the Sky-Watcher f4 Aplanatic Coma Corrector?

[Clarkey]

Not officially, but yes....

I have a Baader MPCC and the SW F4 Aplanatic and the latter is much better - however this is with an F4 scope.

[Elp]

I'm looking at the gso . What other parts do I need to connect my canon t ring to the 2 inch gso?

[Clarkey]

I think if it is an M48 T ring it will screw in directly. If not TS sell one.

Edited October 21, 2021 by Clarkey

[Pitch Black Skies]

I believe the Sky-Watcher 0.9x Coma Corrector is supposedly modified for the P-DS range.

But if I mount that CC in my 130P-DS, I am going to have to hacksaw the end of my focuser tube to achieve good results? 🤔

Surely that can't be right..

 

Increasing the fl isn't ideal either with the GSO CC, I would have gotten the 150P-DS in that case, with an increase in light gathering too. The shorter fl and lighter weight is what drew me to the 130.

The GPU might be best choice it seems.  It's stated to be optimised for F4 though.

It's not so straight forward, glad there is forums like this to get some advice.

 

[alacant]

13 minutes ago, Pitch Black Skies said:

not so straight forward,

Of the ones we've tried on the 130, the GPU is by far the best. Yes, it works well at f4 too.

For round stars, you need to cut the focuser on all but the GSO. But hey, you may get lucky. Some never notice;)

HTH

Edited October 21, 2021 by alacant

[Pitch Black Skies]

8 minutes ago, alacant said:

Of the ones we've tried on the 130, the GPU is by far the best. Yes, it works well at f4 too.

For round stars, you need to cut the focuser on all but the GSO. But hey, you may get lucky. Some never notice;)

HTH

Oh right, I get you now. So I would need to cut it a little bit with the GPU, and more with the GSO as the GSO decreases the fl thus protrudes the focuser tube a little deeper into the OTA.

Would moving the primary up not be a better option with longer collimation screws?

Will cutting the focuser tube have any negative affect if switching to visual, i.e, will it be long enough to reach back focus?

I suppose the extension tube could be used to compensate for the reduced back focus maybe?

[alacant]

13 minutes ago, Pitch Black Skies said:

GSO decreases the fl

No. The GSO increases the FL and pushes the point of focus away from the tube so no need to cut anything.

13 minutes ago, Pitch Black Skies said:

moving the primary up

The secondary only just about covers a dslr sensor as it stands. 

But by all means try it. A few M5 bolts and stronger springs and you're in business although of the two ideas, cutting the focuser would be our choice. At the rock bottom budget end of the market, very little is optimised out of the box.

Cheers

 

Edited October 21, 2021 by alacant

[Pitch Black Skies]

Oops, got mixed up. Where I wrote 'GSO' I meant 'SW 0.9x'. Too late to edit now.

12 minutes ago, alacant said:

The secondary only just about covers a dslr sensor as it stands. 

 You've lost me there.

11 minutes ago, alacant said:

At the rock bottom budget end of the market, very little is optimised out of the box.

Indeed, still a great little scope FWIW.

[alacant]

22 minutes ago, Pitch Black Skies said:

lost me

The secondary mirror intercepts the cone of light reflected from the primary mirror.

Move the cone further up the tube... Now the secondary can't catch all the light as the diameter of the cone has increased; the bits around the edge now escape reflection. 

In this game, you need every photon you can lay hands upon!

 

[Pitch Black Skies]

I see, so the secondary just about covers the sensor, and then if the primary is moved up it's resulting in less reflected light on a just about covered sensor. We need to make every photon count 🧐

1 hour ago, alacant said:

Of the ones we've tried on the 130, the GPU is by far the best.

Is that implying the GSO isn't as close a second as I thought you were saying?

The reason I ask is, the 715mm or so doesn't seem to make a huge difference in the fov calculator.

[Jamgood]

I never wanted to cut the focuser on my scope. I used the Baader originally and lived with pacman shaped stars, although I didn't think they were too bad at the time.

I got rid of the Baader and went for the TS-OPTICS GPU CC. This CC pushes the focus out by 20mm so no more focuser intrusion in the OTA and lovely round stars. I found the Baader a complete nightmare with spacing and tilt. The GPU just worked, no fuss.

Add a baffle to the primary mirror to block the mirror clips and you get even better looking images. No more horrible diffractions from the mirror clips.

My images have gone from this.....Baader MPCC and no baffle.

To this.....TS-OPTICS GPU CC with baffle.

Amazing what a little change and modding can do. 👍

Edited October 22, 2021 by Jamgood

[alacant]

11 hours ago, Pitch Black Skies said:

Is that implying the GSO isn't as close a second as I thought you were saying?

Optically it's a close second. The main problem in using it with a pds is the focus position. It is so far out from the tube that it is both very difficult to balance and I have serious doubts about the sw focuser's ability to maintain a camera at the required distance without tilt.

 

[Pitch Black Skies]

1 hour ago, Jamgood said:

I never wanted to cut the focuser on my scope. I used the Baader originally and lived with pacman shaped stars, although I didn't think they were too bad at the time.

I got rid of the Baader and went for the TS-OPTICS GPU CC. This CC pushes the focus out by 20mm so no more focuser intrusion in the OTA and lovely round stars. I found the Baader a complete nightmare with spacing and tilt. The GPU just worked, no fuss.

Add a baffle to the primary mirror to block the mirror clips and you get even better looking images. No more horrible diffractions from the mirror clips.

My images have gone from this.....Baader MPCC and no baffle.

To this.....TS-OPTICS GPU CC with baffle.

Amazing what a little change and modding can do. 👍

Awesome, I can clearly see the primary mirror clips interference, not so much the Pacman affect though.

There seems to be more nebulosity in the first, but I can also see vignetting. Is that down to total exposure time or post processing?

[Pitch Black Skies]

9 minutes ago, alacant said:

Optically it's a close second. The main problem in using it with a pds is the focus position. It is so far out from the tube that it is both very difficult to balance and I have serious doubts about the sw focuser's ability to maintain a camera at the required distance without tilt.

 

Thanks, it's great that you had an opportunity to try a variety of CC's with it. Makes it a lot easier for people like me to decide. The GPU/Aplanatic it is, and I think the pictures above have really sealed the deal.

[alacant]

7 minutes ago, Pitch Black Skies said:

The GPU/Aplanatic it is

Correct choice. You got there in the end!

--- --- ---

***EDIT: I think maybe we owe @JoshHopkan apology. Here's hoping it's been useful for all.

Edited October 22, 2021 by alacant

[Pitch Black Skies]

A little systematic investigation of a topic nevers hurts 🙂

 

Yes, although I think Josh will agree the findings have been worth it.

 

Clear skies

[Jamgood]

31 minutes ago, Pitch Black Skies said:

Awesome, I can clearly see the primary mirror clips interference, not so much the Pacman affect though.

There seems to be more nebulosity in the first, but I can also see vignetting. Is that down to total exposure time or post processing?

Yeah the exposure times are different as are the cameras/filters used.

The first image was 3hrs with a Canon 60Da and a CLS filter. The second image is only 50mins with the ZWO ASI294MC Pro and an UV/IR Cut filter.