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Bubble Nebula - C8 at full focal length


Padraic M
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Here's a 'status report' version of the Bubble Nebula as a test exercise on the C8 SCT.

 

I've been experimenting with my Celestron C8 SCT this last week to get it properly configured for AP. I know that it's not designed specifically for AP but at the same time, it's certainly implied by Celestron that with the 0.63x FF/FR fitted that results should be acceptable. There are also a number of very presentable examples on astrobin and elsewhere that claim to be C8 XLT (non-EdgeHD) images.

 

The main issue with the Celestron 0.63x SCT FF/FR is that there is no information published by Celestron that states the correct back focus distance, or the point on the reducer from which to measure back focus. The most considered information comes from some conversations on CO, and an old article in Sky at Night.

 

I've taken some measurements at various distances from the FR male threading (camera end) and graphed as below. This implies that a distance of 118mm from that point to the camera sensor provides the correct 0.63x reduction factor. I intend to test at this distance the next time we get some decent sky weather, and hopefully post a full update with those results.

 

image.png.8e46b056399cdc52c45ce77551d7d2bb.png

 

In the meantime, and following a suggestion by Brian AstronomyUkraine, I've removed the FR entirely and tested with the SCT at full FL of 2031mm. Interestingly, while the stars are not great, there's very little quality difference between this and the results so far with the FR at 90 and 105mm. 

This is:

40x60s each of RGB (2 hours)

66x300s Ha (5.5 hrs)

Total integration of 7.5 hours over two nights in Bortle 8 city skies.

 

Bubble_SCT_flat_scaled.thumb.jpg.ad30a8fccf629693e8a43adffac94dad.jpg

 

This is taken with the SCT @ 2031mm, ASI1600MM Pro with Baader Ha and RGB filters, PhD2-guided on the HEQ5 belt-mod with the OVL OAG and ASI290mm Mini guide cam.

Image is processed in APP and Gimp, and the final image is scaled to 33% (which I believe equates to 3x3 binning?) giving a pixel scale of 1.157"/px.

There's a light crop to remove stacking artefacts and to centre the bubble slightly.

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5 hours ago, Padraic M said:

Here's a 'status report' version of the Bubble Nebula as a test exercise on the C8 SCT.

 

I've been experimenting with my Celestron C8 SCT this last week to get it properly configured for AP. I know that it's not designed specifically for AP but at the same time, it's certainly implied by Celestron that with the 0.63x FF/FR fitted that results should be acceptable. There are also a number of very presentable examples on astrobin and elsewhere that claim to be C8 XLT (non-EdgeHD) images.

 

The main issue with the Celestron 0.63x SCT FF/FR is that there is no information published by Celestron that states the correct back focus distance, or the point on the reducer from which to measure back focus. The most considered information comes from some conversations on CO, and an old article in Sky at Night.

 

I've taken some measurements at various distances from the FR male threading (camera end) and graphed as below. This implies that a distance of 118mm from that point to the camera sensor provides the correct 0.63x reduction factor. I intend to test at this distance the next time we get some decent sky weather, and hopefully post a full update with those results.

 

image.png.8e46b056399cdc52c45ce77551d7d2bb.png

 

In the meantime, and following a suggestion by Brian AstronomyUkraine, I've removed the FR entirely and tested with the SCT at full FL of 2031mm. Interestingly, while the stars are not great, there's very little quality difference between this and the results so far with the FR at 90 and 105mm. 

This is:

40x60s each of RGB (2 hours)

66x300s Ha (5.5 hrs)

Total integration of 7.5 hours over two nights in Bortle 8 city skies.

 

Bubble_SCT_flat_scaled.thumb.jpg.ad30a8fccf629693e8a43adffac94dad.jpg

 

This is taken with the SCT @ 2031mm, ASI1600MM Pro with Baader Ha and RGB filters, PhD2-guided on the HEQ5 belt-mod with the OVL OAG and ASI290mm Mini guide cam.

Image is processed in APP and Gimp, and the final image is scaled to 33% (which I believe equates to 3x3 binning?) giving a pixel scale of 1.157"/px.

There's a light crop to remove stacking artefacts and to centre the bubble slightly.

 Just come across this description of the Meade 0.63 SCT reducer. The back focus for the meade reducer is 85 mm. Maybe you could try a backfocus of 85mm with your Celestron reducer, you have nothing to lose.

 

The Meade f/6.3 Focal Reducer/Field Flattener (4-element, 41mm diameter, multicoated lens elements) # 07545 threads on to the rear cell of any Meade SCT telescope or to other Schmidt-Cassegrain telescope brands. It reduces the telescopes focal ratio by a factor of 0.63. With this accessory, f/10 telescopes are converted to f/6.3 and f/ 6.3 telescopes are converted to f/4.

Simultaneously, it helps to flatten the field, significantly improving edge-of-field corrections. The f/6.3 Meade Focal Reducer enables a reduction of photographic exposure times by about 50%, while producing an actual field diameter of 1.5" at the film plane. The f/6.3 Focal Reducer duplicates the rear cell thread of the telescope, allowing you to attach all standard rear cell visual and photographic accessories. Has a standard male SCT thread (2" outside diameter x 24 TPI) on one side and a standard SCT female thread on the other.  The working distance for the Meade f/6.3 Focal Reducer/Field Flattener is 85mm.

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Thanks @AstronomyUkraine I've seen that - I think (without proof) that the Meade and Celestron reducers are the same device. The two frontrunners for distance are 85mm and 105mm, but my own measurements are showing that I'll get the advertised 0.63x reduction at a longer distance.

The other complication is that there's no consensus on where to measure from - it is possible that it's 85mm from the camera-side edge of the corrector glass, or 105mm from the scope end of the corrector. Who's to know?!!

I'm going to try 118mm next, and then if I'm still not happy I'll look to the shorter end.

I'll also post image samples from each distance. It's possible that there is another problem here such as tilt, that might be independent of the back focus.

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13 minutes ago, Padraic M said:

Thanks @AstronomyUkraine I've seen that - I think (without proof) that the Meade and Celestron reducers are the same device. The two frontrunners for distance are 85mm and 105mm, but my own measurements are showing that I'll get the advertised 0.63x reduction at a longer distance.

The other complication is that there's no consensus on where to measure from - it is possible that it's 85mm from the camera-side edge of the corrector glass, or 105mm from the scope end of the corrector. Who's to know?!!

I'm going to try 118mm next, and then if I'm still not happy I'll look to the shorter end.

I'll also post image samples from each distance. It's possible that there is another problem here such as tilt, that might be independent of the back focus.

Just found this on Bintel's website.

 

CELESTRON REDUCER/CORRECTOR F/6.3

This combination focal reducer and field corrector lens accessory works with all Celestron 5″, 6″, 8″, 9 1/4″, 11″ and 14″ Schmidt-Cassegrain telescopes. This clever accessory makes it possible to have a dual focal ratio instrument, without sacrificing image quality. The Reducer/Corrector is f/6.3 for C5, C6, C8, C9¼ and C11 telescopes and f/7 for the C14 telescope.

If offers wide fields of view with any Schmidt-Cassegrain telescope. Used for astrophotography, it reduces exposure time by a factor of 3.

Celestron doesn’t offer a f/6.3 Schmidt-Cassegrain telescope, because the design is impractical, with a large central obstruction, and resulting loss in contrast. This, added to the inability to utilize longer focal ratios for certain objects, led Celestron to design the four-element, fully multicoated f/6.3 Reducer/Corrector. Results are amazing both visually and photographically.

Produces a corrected field of 24mm diameter. The back focus is 85mm, from the reducer’s back element to the cameras sensor

THE #94175 REDUCER/CORRECTOR LENS IS NOT COMPATIBLE WITH CELESTRON EDGEHD  or MEADE ACF OPTICAL SYSTEMS.

 

https://www.bintel.com.au/product/celestron-reducercorrector-f6-3/?v=322b26af01d5

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