130PDS Coma Corrector

[Elp]

Well, finally got my GSO coma corrector and managed to test it earlier this week on Pleiades:

1. Only on an alt az mount, no tracking.

2. Used a bahtinov mask for focusing.

3. Photos taken with a DSLR, no filter. Images 1 second exposures roughly 100 frames per image stacked.

4. No spacers used, just GSO into the 2 inch focuser and T ring on the end for my DSLR to connect to.

From this very rough test I don't really see any difference, very marginal, in fact I'd say the photo without the corrector looks better.

Thoughts?

[Pitch Black Skies]

1 hour ago, Elp said:

Well, finally got my GSO coma corrector and managed to test it earlier this week on Pleiades:

1. Only on an alt az mount, no tracking.

2. Used a bahtinov mask for focusing.

3. Photos taken with a DSLR, no filter. Images 1 second exposures roughly 100 frames per image stacked.

4. No spacers used, just GSO into the 2 inch focuser and T ring on the end for my DSLR to connect to.

From this very rough test I don't really see any difference, very marginal, in fact I'd say the photo without the corrector looks better.

Thoughts?

That's odd. What does a single exposure without the CC look like?

[Elp]

Single exposures as follows, can see some difference toward the edge. I guess a better test is to autoguide and expose for longer so there's more stars to look at.

[Pitch Black Skies]

17 minutes ago, Elp said:

Single exposures as follows, can see some difference toward the edge. I guess a better test is to autoguide and expose for longer so there's more stars to look at.

Can definitely see an improvement with the CC.

Without it, it looks like seagulls in the top left corner.

Much better corrected in top left with the CC.

Without CC ⬇️

With CC ⬇️

This is in a 130P-DS yeah?

How long were the exposures and do you know if the options are full collimated.

The reason I ask is, I think the pics with the CC should be a little sharper.

[Elp]

Guess I was expecting it to be much flatter like with my WO refractor flattener. The GSO is a nicely machined chunk of metal and has the advantage of not requiring to chop up the focuser tube.

130PDS yes, 1 second exposures as couldn't do more due to star trails on alt az mount and the telescope focal length, primary is collimated didn't need to adjust the secondary as it looks centralised in the collimator cap.

I was expecting it to be sharper, the bahtinov spikes were definitely centralised before imaging, I don't know if it's an issue with my DSLR (or backfocus setup) as most astro images tend to look a bit blurred.

[Pitch Black Skies]

1 hour ago, Elp said:

Guess I was expecting it to be much flatter like with my WO refractor flattener. The GSO is a nicely machined chunk of metal and has the advantage of not requiring to chop up the focuser tube.

130PDS yes, 1 second exposures as couldn't do more due to star trails on alt az mount and the telescope focal length, primary is collimated didn't need to adjust the secondary as it looks centralised in the collimator cap.

I was expecting it to be sharper, the bahtinov spikes were definitely centralised before imaging, I don't know if it's an issue with my DSLR (or backfocus setup) as most astro images tend to look a bit blurred.

It should be much sharper, mine is. Are you sure you've the GPU? It doesn't change the focal length, thus you would still need to chop the focuser tube, I did.

Back focus is 53mm for F4 but F5 can get away with 55mm which is basically just an 11mm t ring attached.

How is your imaging train clamped together? Are thumbscrews, compression ring, etc?

Edit: It does actually push the focus point out by a little if screwed directly onto the focuser tube.

Edited November 25, 2021 by Pitch Black Skies

[Elp]

Is the GSO (Guan Sheng Optical), I bought it as people said it's the only coma corrector whereby you do not need to modify the focuser tube.

Simply insert it into the 2 inch aperture focuser tube, tighten the two screws, insert the m42/t-ring step down ring into the top of the coma corrector (its a tight fit) and screw the dslr in via it's t-ring adaptor. I had to rack the focus out around 10mm from fully closed.

[alacant]

22 hours ago, Elp said:

No spacers used

The back focus is 75mm. With a DSLR, you need around 20mm between the t-ring and the shoulder of the GSO. The m48-m42 adapter I had took 5mm, so needing a 15mm spacer for the cc to become active.

Better to lose the adapter and use a 20mm m48 extension to a m48 t ring.

HTH

Edited November 25, 2021 by alacant

[Pitch Black Skies]

48 minutes ago, Elp said:

Is the GSO (Guan Sheng Optical), I bought it as people said it's the only coma corrector whereby you do not need to modify the focuser tube.

Simply insert it into the 2 inch aperture focuser tube, tighten the two screws, insert the m42/t-ring step down ring into the top of the coma corrector (its a tight fit) and screw the dslr in via it's t-ring adaptor. I had to rack the focus out around 10mm from fully closed.

Apologies, I mis-read your post. I thought you were referring to the GPU.

Once your spacing is rectified, consider upgrading the adaptor with the two thumbscrews to a compression ring with three thumbscrews. It gives a much more even and concentric fit, minimising the chance of tilt in the optical train.

[Elp]

On 25/11/2021 at 19:08, alacant said:

The back focus is 75mm. With a DSLR, you need around 20mm between the t-ring and the shoulder of the GSO. The m48-m42 adapter I had took 5mm, so needing a 15mm spacer for the cc to become active.

Better to lose the adapter and use a 20mm m48 extension to a m48 t ring.

HTH

I don't know if the manufacturer guide of 75mm backfocus for the gso coma corrector is accurate (maybe it's only accurate for dslr?). Tried it last night with my 224mc which I believe has a sensor to front face distance of 12.5mm. Made the balance of the 75mm up with extension rings (61.5mm I couldn't make up the remaining 1mm), camera didn't focus anywhere along the focus travel. So I took some off and tried 37.5mm of rings = 50mm of backfocus and found a slightly off focus around 10mm of focus travel out, couldn't get it sharp looking at m13, managed to image it but not happy with the focus. The camera is not the highest resolution but I wanted to test it as it's the only astro cam OSC I have. Any suggestions?

[alacant]

Hi

Yes, that's what would happen. The cc throws the focus over 5cm further out from where it was without the cc, so your observations of not being able to focus without removing spacers is correct.

So that the cc corrects, you need to leave the 75mm intact and add spacers to move the whole assembly further out.

But hey, I doubt you'll need a cc with the 224 anyway.

Cheers and HTH.

Edited March 19, 2022 by alacant

[pete_81]

Hello all,
Rather than blocking up (and losing the thread) in the "imaging with 130PDS", or starting the same topic again, thought I'd post here.
Going around in circles with what CCs are ideally suited to the 130PDS for both visual and imaging, with the primary aim of improving my images (with APS-C DSLR).

From searching around, there are several possible choices of CCs for the 130PDS, each having pros and cons:

• TV Paracorr
• ES HR CC
• TS GPU
• SW Aplanatic f/4
• Sharpstars (selection of different ones around)
• SW 2"
• SW 0.9x CC (I think this is differerent to the one above, where there is no change to the focal length)
• Baader MPCC III
• GSO Coma Corrector

The paracorr is out of the price range!
The ES is likely best suited to visual, and there's not much on using it for imaging. Also expensive to others that mean others better suited.
The TS GPU both in this thread and the 130PDS imaging one, finds itself at the top of quality ladder (and likely top of budget!)
I think the SW Aplanatic is essentially the same as the TS (same optical prescription), and the sharpstars seem to fall into this too?
The SW 2" seems to do a decent job overall, but seems like folks tend to go for the f/4 version?
The Baader seems to get lots of negativity when it comes to misalignment of any sort
The GSO sounds like quite a nice CC, quietly sitting in the background, but cheapest of the bunch.

From what I've read on reviews and here, there's the requirement to chop the focuser to avoid the pacman effect on stars, on pretty much all the CCs except the GSO.

I think I'm being directed towards the GPU (or SW aplanatic) or GSO. Earlier in this topic, @alacant did tests on some of these and found the GPU (SW f/4) the best optically, with the GSO coming close behind...
For half the price, the GSO did a very good job correcting coma optically, but at expenses of increasing focal length (+10%) and extra back-focus required, so balancing becomes harder/may induce extra tilt to stock focuser.

Now, the extra back focus (quoted as 75mm rather than ~55mm) may be attractive - I am thinking about the ZWO filter drawer, which could be used in the GSO setup as follows:
130PDS + M54 Compression adapter + GSO (uses M48 thread, which would connect directly to) + M48/M42 Filter Drawer (21mm thick) + T-Ring (11mm thick) + Canon (44mm to sensor), so I'd get 76mm back focus, which is very close to the 75mm GSO (and other reviews) recommended.
I can take or leave the filter-drawer, leaving it out with any setup is fine, but the GSO would need extra spacers to get the working distance correct (I think?)
Using the standard T-Ring is possibly too thick (11mm) for some of the correctors that say 55mm back focus, but actually perform better at ~53mm (other links within this thread direct to this), so a thin (1mm) profile T-Ring and ~8mm spacers are reportedly the way to go...

So, looking for further thoughts on the GSO and GPU in use before Christmas 2023!
Thanks!

[alacant]

6 hours ago, pete_81 said:

GSO and GPU in use before Christmas 2023

The best is the original prescription GPU. A search avoiding the term 'TS' shows it available here. There's also a used one here.

For the 130pds over aps-c we use 52mm cc to sensor. At focus, this needs 10mm removing from the inner focuser barrel to prevent its intrusion into the light cone.

That's ap. I've never looked through a PDS, with or without cc.

Cheers and HTH

 

Edited January 4, 2023 by alacant