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Is this the best I can do with a C8SE SCT?


oymd
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Posted (edited)

Hello everyone.

Have been in the hobby for 2 years, imaging with widefield refractors, mainly a WOStar71 and 294MC Pro, with good results. Image scale is 2.7"/pixel.

With galaxy seson starting a month or so ago, I decided to give it a go with my Celestron C8SE, which I had bough a couple of years ago as a full setup.

I used an OVL OAG and my 290MM Mini for guiding. I also used Celestron's 0.63x reducer.

I set back focus at 105mm

Collimation looks OK.

I platesolved and checked on Astrometry, and indeed my image scale was 0.75"/pixel, confirming my FL was 1280mm exactly.

I am well polar aligned, and PHD2 is showing RMS of 0.5-0.7, so overall good guiding. Looping on 2 seconds.

I am using a ZWO EAF for autofocus, and its been working well, except for the fact that it seems focus changes a little whenever I am slewing around. I guess that is Mirror flop? Anyways, durimg imaging, I set the focuser to AF every 10 images.

When imaging, and seeing the images come in on NINA, I can clearly see all around the periphery of the image that the stars look properly egg shaped, quite severe at the edges.

Is this an inherent property of the SCT, especially that its not an EDGE?

Since I have never imaged with an SCT before, I am not sure what to expect?

This is from yesterday, 60 L of 300s, 20 Fs, 20DFs and 20Ds. Dithering EVERY frame.

Compared to my refractor experience, images look very very soft.

I had to use a LP filter, the IDAS LPS D2, as my skies and LP is terrible.

Many thanks

M51 C8 LPS D2.png

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Edited by oymd
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Posted (edited)

Hi,

I have shared in a similar experience to you, in that I originally purchased a Celestron 8SE when I first became interested in Astronomy. However as I quickly became keen on Astrophotography I purchased a WO Star71 which made life a lot simpler, nice round stars with 25mins subs.

Moving back to the 8SE has been a challenge but if you accept that its not got a flat field and the focuser is not especially good then I think it can be worth while.

Is this an inherent property of the SCT, especially that its not an EDGE? - Yes

I get egg shaped stars on the corners which relates to coma. Cropping is basically the best way of dealing with that although its a shame to lose some resolution and image detail from doing this.
Within the centre you should get relatively round stars.

I have taken 20mins subs with the 8SE and could have gone longer. My main issue is that the OAG stars are quite distorted but PHD seems to cope with this.
Your guiding RMS looks good but don't be too taken with low RMS meaning good guiding. I have had RMS of 1.0 and have managed to get round stars within the centre of the image.

Have a look at my FlickR channel and you can see some of the results (Link within my signature) . They are not very special but I am fairly happy with them.
I do however find galaxies a lot tougher than nebula!

Good Luck.

Edited by Droogie 2001
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55 minutes ago, Droogie 2001 said:

Hi,

I have shared in a similar experience to you, in that I originally purchased a Celestron 8SE when I first became interested in Astronomy. However as I quickly became keen on Astrophotography I purchased a WO Star71 which made life a lot simpler, nice round stars with 25mins subs.

Moving back to the 8SE has been a challenge but if you accept that its not got a flat field and the focuser is not especially good then I think it can be worth while.

Is this an inherent property of the SCT, especially that its not an EDGE? - Yes

I get egg shaped stars on the corners which relates to coma. Cropping is basically the best way of dealing with that although its a shame to lose some resolution and image detail from doing this.
Within the centre you should get relatively round stars.

I have taken 20mins subs with the 8SE and could have gone longer. My main issue is that the OAG stars are quite distorted but PHD seems to cope with this.
Your guiding RMS looks good but don't be too taken with low RMS meaning good guiding. I have had RMS of 1.0 and have managed to get round stars within the centre of the image.

Have a look at my FlickR channel and you can see some of the results (Link within my signature) . They are not very special but I am fairly happy with them.
I do however find galaxies a lot tougher than nebula!

Good Luck.

Thank you for your reply.

That's exactly the same purchase route I went through!

I bought a Nexstar C8SE off Amazon in Xmas 2019, and initially was enjoying it for just visual, and learning the skies. In Feb 2020 decided to get an EQ mount, and bought an AZ-EQ6 Pro, and since then, it has been downhill!! I am a much, much poorer individual now...

:)

 

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Posted (edited)
1 hour ago, oymd said:

This is from yesterday

Hi

Not sure where you are but you may not have the astronomical darkness necessary for faint stuff. Add to that light pollution and I think you've done extremely well, certainly a lot better than we were able to get.

Not sure either which is the 8se model, but we tried everything with an old orange tube Celestron 8-something; with and without reducer, with and without oag. 105mm, 85mm, Focus lock. No focus lock... It was great for looking at Saturn and drew frequent 'wows' when looking at craters on the moon, but for deep sky photography...? We gave up.

If we have the seeing to support this amount of focal length, we use a 6" f8 Newtonian. They're cheap, don't need a corrector and about the same focal length as your 6.3 Celestron but with clean-contrasty images. Our current favourite for galaxies is a GSO 203mm f5, which is not that much more expensive but only 1000mm focal length. TBH, if you have enough data, the focal length doesn't seem to make that much difference. 

Cheers and HTH

Edited by alacant
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Posted (edited)

I think this is a great image though of course each of us may have our own criteria - and I am no astrophotography expert. I imagine you might not get as much scale and resolution on your WOStar71. The corrected image size for the 6.3 reducer is specified as 24mm so in theory it should be okay for the 4/3" sensor of your camera to the edge of the field. The soft stars are also expected because of the 0.75 vs 2.7 arcsec/pixel image scale. Do you know the reason for the coarse noise patten of the background ?

Best!

 

Edited by beka
typo
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5 minutes ago, beka said:

.I think this is a great image though of course each of us may have our own criteria - and I am no astrophotography expert. I imagine you might not get as much scale and resolution on your WOStar71. The corrected image size for the 6.3 reducer is specified as 24mm so in theory it should be okay for the 4/3" sensor of your camera to the edge of the field. The soft stars are also expected because of the 0.75 vs 2.7 arcsec/pixel image scale. Do you know the reason for the coarse noise patten of the background ?

Best!

 

Yes, I have been noticing a wishy washy ugly dirty background pattern

Its hard to give it a scientific name, or to describe it accurately, but it is like someone smudged the background.

I think my skies have been terrible recently, and might be haze? Not sure.

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Posted (edited)
35 minutes ago, alacant said:

Hi

Not sure where you are but you may not have the astronomical darkness necessary for faint stuff. Add to that light pollution and I think you've done extremely well, certainly a lot better than we were able to get.

Not sure either which is the 8se model, but we tried everything with an old orange tube Celestron 8-something; with and without reducer, with and without oag. 105mm, 85mm, Focus lock. No focus lock... It was great for looking at Saturn and drew frequent 'wows' when looking at craters on the moon, but for deep sky photography...? We gave up.

If we have the seeing to support this amount of focal length, we use a 6" f8 Newtonian. They're cheap, don't need a corrector and about the same focal length as your 6.3 Celestron but with clean-contrasty images. Our current favourite for galaxies is a GSO 203mm f5, which is not that much more expensive but only 1000mm focal length. TBH, if you have enough data, the focal length doesn't seem to make that much difference. 

Cheers and HTH

Thanks, I thought so.

I just wanted to hear: Don't worry too much, its the C8, and its not meant for AP, which is what you summarised.

I have an Edge 11HD, but I am first learning the SCT ropes with this C8. The Edge will definitely need a better OAG, and possibly a better guide camera. My current OAG is the OVL one, 8mm prism, 5mm opening in the OAG turret, which overall is a joke, and a PAIN to use. Its like threading a needle in total darkness!! I will also need a better mount!!

I am happy to keep experimenting, as long as I am doing something very wrong.

Since March there is hardly ANYTHING large to image, so the WO71 and Esprit100 are useless really, and its a good time to experiment with the1280mm FL of the C8 on the only thing out there to image these days, Galaxies.

BTW, what is the BACKFOCUS OF THE NON EDGE C8 WITHOUT the 0.63x reducer?

I'm contemplating trying 2030mm FL?

The galaxies are small anyways, so will always have to crop, and edges are not an issue?

 

:)

 

Edited by oymd
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I have tried unguided imaging with a C8, and ASI224 MC camera. At f10 most images were so soft and small-scale as to be almost unusable.  Results at f6.3 were much better. I think the seeing affects the softness of the images achieved.

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Posted (edited)
26 minutes ago, oymd said:

possibly a better guide camera

Your 290 has a bin2 in firmware which combined with a bigger prism, should get you there. Failing that, the next step up would be a 174 which begins to get €silly, so gotta be worth trying the 290 first.

Good luck.

Edited by alacant
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1 hour ago, alacant said:

Your 290 has a bin2 in firmware which combined with a bigger prism, should get you there. Failing that, the next step up would be a 174 which begins to get €silly, so gotta be worth trying the 290 first.

Good luck.

yes, im using bin2 and median3x3 in PHD2 to get ANY sort of guiding going!

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1 hour ago, Cosmic Geoff said:

I have tried unguided imaging with a C8, and ASI224 MC camera. At f10 most images were so soft and small-scale as to be almost unusable.  Results at f6.3 were much better. I think the seeing affects the softness of the images achieved.

agreed, its just an experiment i thought id give it a go!

:)

 

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I think that it's a good quality image, although it does need more subs... more data (and dithering subs) will remove the noise and color mottle.

That said, I love imaging with my C8, (same specs as the 8SE) and the 8SE, or any 8" SCT will have it's challenges and limitations. One is the coma at the edges which needs to be cropped off.

The other problem is the fact that with 2032mm (or 1280mm with a 0.63 reducer) needs very clear skies for it to perform at it's capable resolution, otherwise the images will be oversampled and soft with bloated stars.
Personally I always BIN 2x2, and if my seeing is worse than 2" I just don't bother, or image with my refractor at 500mm.

But when the seeing is good, 1" or better, the details that the 8SE is capable of capturing is fantastic.

So to answer your question... NO the 8SE is capable of considerable better quality images.

 

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I love my non edge XLT 8 for imaging..I'm enjoying it so much that it's not been off the mount for over a year

A heavy dither will sort out you colour mottle on the background sky

2nd image shows a flare off the star, could be that the tube wasn't cooled to ambient temperature, or collimation.. have a scroll around and if the brighter stars all have flares then certainly could be collimation, is this a full image or a crop? 

Also I don't get your guiding, the RMS figures look really good but doesn't correspond with the star profile target and  peak to peak rms

 

 

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On 12/05/2022 at 08:40, newbie alert said:

I love my non edge XLT 8 for imaging..I'm enjoying it so much that it's not been off the mount for over a year

A heavy dither will sort out you colour mottle on the background sky

2nd image shows a flare off the star, could be that the tube wasn't cooled to ambient temperature, or collimation.. have a scroll around and if the brighter stars all have flares then certainly could be collimation, is this a full image or a crop? 

Also I don't get your guiding, the RMS figures look really good but doesn't correspond with the star profile target and  peak to peak rms

 

 

I am dithering every frame, but I guess I need to increase my dithering value.

Yes, guiding is always between 0.5 to 0.7, which would get me FANTASTIC stars on my refractor setup, but for some reason here, it does not match the result I see on my images.

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Since this is my first experience with an OAG, I would appreciate some advice regarding setting the prism location PARALLEL to the long axis of the CMOS sensor?

I must be missing a trick?

Since I have quite a few T2 extensions between the OAG and the camera, the position of the prism is dictated by the turns of those extensions when I am screwing them all together. I then have a few degrees of rotation adjustment on the OAG by those three adjustment thumbscrews, but I seem to NEVER get lucky and get the prism to end up on top part of the long axis of the sensor?

No matter how many times I unscrew everything then screw them back together, I never managed to get the prism where I want? Is there a trick I am missing?

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Posted (edited)
10 hours ago, oymd said:

a trick I am missing

Hi

Mark the long edge of the sensor orientation along the back of the camera; using the writing can be misleading. Set the prism roughly parallel and take a flat frame. Push the prism stalk inwards a mm or so at a time taking flat frames as you go. Stop when you see the shadow of the prism. It's now easy to rotate the stalk to make the shadow parallel to the edge of the frame. 

Finally move the prism away from the edge until the shadow just disappears without introducing further rotation. Get it somewhere close, but don't stress over it as you may need to rotate it again to find guide stars.

HTH

Edited by alacant
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Posted (edited)
2 hours ago, alacant said:

Hi

Mark the long edge of the sensor orientation along the back of the camera; using the writing can be misleading. Set the prism roughly parallel and take a flat frame. Push the prism stalk inwards a mm or so at a time taking flat frames as you go. Stop when you see the shadow of the prism. It's now easy to rotate the stalk to make the shadow parallel to the edge of the frame. 

Finally move the prism away from the edge until the shadow just disappears without introducing further rotation. Get it somewhere close, but don't stress over it as you may need to rotate it again to find guide stars.

HTH

I've never had to rotate or search for guidestars with my 290..

Edited by newbie alert
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9 hours ago, alacant said:

Hi

Mark the long edge of the sensor orientation along the back of the camera; using the writing can be misleading. Set the prism roughly parallel and take a flat frame. Push the prism stalk inwards a mm or so at a time taking flat frames as you go. Stop when you see the shadow of the prism. It's now easy to rotate the stalk to make the shadow parallel to the edge of the frame. 

Finally move the prism away from the edge until the shadow just disappears without introducing further rotation. Get it somewhere close, but don't stress over it as you may need to rotate it again to find guide stars.

HTH

I think you misunderstood me.

My issue is not the prism showing up in my images.

My issue is getting the prism aligned with the long axis of the sensor.

Using thin spacers was mentioned above, but that will alter the backfocus.

 

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Posted (edited)

I keep the long edge of the imaging camera and the guide cam and the prism long edge the same

So my imaging cam is set to 90 degrees( in my plate solve software it says 270)  the long edge of the 290 set to match the long edge of the prism...  

As in the top image and not the bottom

Edited by newbie alert
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16 hours ago, oymd said:

My issue is getting the prism aligned with the long axis of the sensor.

Then I must have misunderstood. My flat-frame method is used to align the prism with the long axis of the sensor. English is very much my second language. Sorry!

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23 hours ago, newbie alert said:

I keep the long edge of the imaging camera and the guide cam and the prism long edge the same

So my imaging cam is set to 90 degrees( in my plate solve software it says 270)  the long edge of the 290 set to match the long edge of the prism...  

As in the top image and not the bottom

 

22 hours ago, alacant said:

Then I must have misunderstood. My flat-frame method is used to align the prism with the long axis of the sensor. English is very much my second language. Sorry!

Thank you gents for your replies

Let me clarify what my problem is:

My image train consists of Camera ===> T2 spacers ===> OAG ===> T2 spacers ===> Celestron 0.63x reducer ===> Celestron C8

As you can see, on BOTH sides of the OAG there are T2 spacers.

When I am assembling the imaging train, these spacers are SCREWED together, so the FINAL orientation of the OAG prism to the Camera Sensor is RANDOM, and it is a matter of LUCK where the final position of the OAG prism ends in relation to the Sensor.

Since the imaging train all relies on SCREWING adaptors together, I can never get the OAG prim to end up where I want it to be, in relation to the sensor?

The OAG has some adjustment, using those 3 thumbscrews, but the adjustment radium is not enough to rotate the prism enough to lie PARALLEL to the long axis of the sensor.

I have undone and redone the train countless times, and it always comes back to the same orientation.

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Posted (edited)
35 minutes ago, oymd said:

on BOTH sides of the OAG there are T2 spacers

OK. So, as the 0.63's 105mm isn't critical, I'd go for plumber's teflon thread tape. Or, as it seem to be working OK, just leave it?

HTH

Edited by alacant
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