M27 image with Coma using c9.25

[Imd]

Hi all budding astrophotographers,

I recently acquired a C9.25 and after a few days I had my first clear night (moon rising). I bought a Celestron f6.3 reducer which I connected after removing the 2" back, then connected my Canon 1100d DLSR camera via a T-tring.

Many of the central stars seem sufficiently round in shape but I was surprised to find so many stretched stars in the corners even after the installation of the x0.63 reducer, I thought it also acted as a coma corrector. The dew shield was not perfectly sitting on the scope but I don't imagine this could be the cause.

Is this coma on the corners or something else?

Do you think I'm being unrealistic of my expectations when using the reducer to also flatten the image?

Is it because of the unrealistic field of view from a DLSR camera on the C9.25 or perhaps I have possibly a dodgy reducer or could collimation be the source of my problem?

FYI, the image is a single 5 min exposure without any processing, guided using a finder guider and using a CLS filter, EQ6 on a pier. Im pleased with the image apart from the corners due to the stretched stars.

 

Many thanks for looking, your advice is always appreciated.

 

 

[Marky1973]

Hi there - I'm no expert, but the reducer apparently does act as a field corrector IIRC. I really don't know for sure but is the effect more rotational than coma? Hopefully someone who actually knows what they are talking about will come along soon! But, regardless, that's a really nice image....I have the C9.25 as well and will hope to try some imaging with it at some point, so that gives me hope! :-)

[carastro]

Looks like field rotation to me rather than coma.  The nebula has come out well.  

I think I am right in saying that field rotation is due to inaccurate PA.

Carole 

[John78]

6 minutes ago, carastro said:

Looks like field rotation to me rather than coma.  The nebula has come out well.  

I think I am right in saying that field rotation is due to inaccurate PA.

Carole 

+1 its definitely that.

[Imd]

I agree it could look like field rotation. I  suppose that as the central stars are nice and round that the guiding was ok, I need better polar alignment. I think I may have been a bit hasty about popping it back on the pier after a star party. This is really good news, Ill check polar alignment the next chance I get.

I'm please with the finder guider results, I suppose if I have a camera with a smaller chip the guiding may be more sensitive.

Thanks both for being so helpful.

Ian

 

 

 

 

 

 

[newbie alert]

Didn't think sct suffered with coma, more than likely flexture on the guider poss? As the middle stars look round..the left stars/noise are shifting and to the right is more prominent....to guide a long focal length scope is tough..maybe oag maybe a better solution..no image shift or flexture...just some possibilities..

[Merlin66]

Probably field curvature rather than coma....

[Mick J]

'single 5 min exposure without any processing'    very nice, what a great start 

[Imd]

2 hours ago, newbie alert said:

Didn't think sct suffered with coma, more than likely flexture on the guider poss? As the middle stars look round..the left stars/noise are shifting and to the right is more prominent....to guide a long focal length scope is tough..maybe oag maybe a better solution..no image shift or flexture...just some possibilities..

I doubt it would be flexture as its a tight grip use a baader double screw shoe but I admit the focal length differences are quite significant. But if it was guiding I would have expected the whole image to be affected.

 

[Imd]

2 hours ago, Merlin66 said:

Probably field curvature rather than coma....

Please could you expand on what is meant by field curvature, is this something I can correct?

[Imd]

1 hour ago, Mick J said:

'single 5 min exposure without any processing'    very nice, what a great start 

Thank you, I hope there is room for improvement without great expense

I only managed 4 subs and 2 darks for processing later, no flats collected yet, here was the result after stacking in DSS a bit of PS tuning and a lot of cropping

 

 

[alexbb]

The Celestron 0.63 fr/ff does improve the field curvature, but will not make it perfect. With your example, I think the backfocus distance is not correct and then you have field curvature. How far is the sensor to the fr/ff? What extension tubes do you have between the fr/ff and the T-Ring? The recommended distance is 105mm, though the backfocus distance is not that critical as other ff or coma correctors.

I also don't think you would notice field rotation with 5 minutes exposures unless you had a very very bad polar alignment.

Read more here and also follow the links:

 

[steppenwolf]

Not coma or field rotation, I think that this is a spacing issue, camera to reducer.

[Imd]

1 hour ago, moise212 said:

The Celestron 0.63 fr/ff does improve the field curvature, but will not make it perfect. With your example, I think the backfocus distance is not correct and then you have field curvature. How far is the sensor to the fr/ff? What extension tubes do you have between the fr/ff and the T-Ring? The recommended distance is 105mm, though the backfocus distance is not that critical as other ff or coma correctors.

I also don't think you would notice field rotation with 5 minutes exposures unless you had a very very bad polar alignment.

Read more here and also follow the links:

 

This is a an interesting article although some of it goes above my head and to me using those graphs it looks like the optimum for a a 9.25 with a reducer would have a BF of less that 100 to achieve optimum aperture but I have heard previously 105mm is the magic number.

I have measure the distance from the collar at then back of the SCT to the front of the camera and its about 95mm and then the sensor is probably 10mm or 15mm inside the camera so it looks about right. There is not actual extension tube but  do use a celestron T-adapter to attach to the reducer. The back focus seems about right, I think.

What do you say?

[Imd]

Just wanted to say thanks already to all those who have taken the time to respond so far

[BRUN]

just wanted to post, that picture is amazing, well done

[alexbb]

I believe the spacing is correct. You should measure it from the fr/ff's glass to the sensor. Canon has ~44mm from sensor to flange, the T2 adapter is 10-11mm so you'd need a ~50mm extension tube.

You may read more about spacing, flatteners and stars elongation here:

If I look closer, your stars are out of focus also in the center. Maybe all stars will look better if you focus at ~1/3 on the diagonal, away from the center.

And, by the way, nice dumbbell, I went straight to the technical aspects. I see in the first image that the core is not blown whereas in the second you white clipped some brighter areas. Lift only the curve instead of moving the white point to the left.

[Imd]

27 minutes ago, moise212 said:

I believe the spacing is correct. You should measure it from the fr/ff's glass to the sensor. Canon has ~44mm from sensor to flange, the T2 adapter is 10-11mm so you'd need a ~50mm extension tube.

You may read more about spacing, flatteners and stars elongation here:

If I look closer, your stars are out of focus also in the center. Maybe all stars will look better if you focus at ~1/3 on the diagonal, away from the center.

And, by the way, nice dumbbell, I went straight to the technical aspects. I see in the first image that the core is not blown whereas in the second you white clipped some brighter areas. Lift only the curve instead of moving the white point to the left.

Thanks Alex,

You are of course correct, too much tweaking. Ill give it another go sometime.

The T-adapter-sc is 50mm long so in total I would have a bf of 104mm-105mm.

I will definitely try the focus again as you describe.

 

[Imd]

2 hours ago, steppenwolf said:

Not coma or field rotation, I think that this is a spacing issue, camera to reducer.

I enjoyed your book Steve, its nice to hear from you.

I think the BF is 104-105mm

[steppenwolf]

3 hours ago, Imd said:

I enjoyed your book Steve, its nice to hear from you.

I think the BF is 104-105mm

I'm delighted that you enjoyed my book!

If your spacing is between 104 and 105mm then you have indeed got it correctly spaced.

[ollypenrice]

To see if it's field rotation due to poor PA you could just take some short subs in a bright-star-rich part of the milky way. Poor PA takes time to show itself during an exposure. If the stars are not bright enough, just take the same moderately bright star located first in the centre and then in the corners over just a few seconds each time. Don't go for over-bright stars because elongation shows best in fainter ones.

Olly

[newbie alert]

1 hour ago, steppenwolf said:

I'm delighted that you enjoyed my book!

If your spacing is between 104 and 105mm then you have indeed got it correctly spaced.

It's not a book it's a astro bible!

[blinky]

It's field rotation, I get the same with my 8" SCT and F6,3 reducer when imaging with my qhy 8. The field reducer does not work over the full fog of big chip ccds

[michael8554]

In my opinion the image with the tape measure shows the spacing is only about 95mm.

The spacing is measured from the back of the reducer, at 30mm on the measure, to the focal plane mark on the dslr body. This mark is on the top of the dslr body, but the white index mark for the rotating dial is close, if not identical, at 125mm on the measure.

This means the spacing is only 95mm.

Michael

[Merlin66]

Michael,

The OP seems to be using the standard Celestron visual back and standard T2 adaptors to the DSLR.

In the Celestron manual is arrangement provides the correct spacing for the reducer......

Correction: It was taken from the Meade manual - same reducer...

http://www.astroromp.com/astronomy/focal reducer.pdf