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130PDS Coma Corrector


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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?

634191005_WOcomacorrector.thumb.jpg.173f972795d922c1aac2efbac2c3e7a1.jpg1955997081_GSOcomacorrector.thumb.jpg.0eb4d285d47b71e15e93535318fbc948.jpg

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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?

634191005_WOcomacorrector.thumb.jpg.173f972795d922c1aac2efbac2c3e7a1.jpg1955997081_GSOcomacorrector.thumb.jpg.0eb4d285d47b71e15e93535318fbc948.jpg

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

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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.

1572846525_NoCorrector_SingleExp_MG_9135.thumb.jpg.c7ce4f44e89da3804b0f0565b85bac7f.jpgGSO_SingleExp_MG_9305.thumb.jpg.3f288247a72654d4246bfe387c57fe7c.jpg

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 ⬇️

IMG_20211125_180639.thumb.jpg.928baf0e78281356bb9b054c79befa85.jpg

With CC ⬇️

IMG_20211125_180700.thumb.jpg.008471aa514b5555755d510514938e31.jpg

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.

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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.

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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 by Pitch Black Skies
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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.

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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 by alacant
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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.

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