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Full-Frame Results on M42 with 6" RC and CCDT67 Telecompressor

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#1 nwinston

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Posted 25 January 2020 - 04:11 PM

About a year ago I picked up a GSO 6" Ritchey-Chretien and Astro-Physics CCDT67 telecompressor. I had been interested in the 6" RC scopes for a while, and when paired with the telecompressor, make for a relatively fast imaging system (I'm not very interested in deep space imaging at f/9). Only problem is that I have Canon 6D (unmodified), and was unsure about scope/compressor compatibility with the full frame sensor. 

 

I worked out the spacing between the CCDT67 and sensor to target approximately 0.75x compression, which AP states will provide a fully illuminated image circle of 34mm. This brings the 6" RC from a focal length of 1370mm at f/9 to 1027mm at f/6.75. 

 

Most recently I went after imaging the Orion Nebula with this gear. The image below is a single raw subframe from the 6D at ISO 1600. Significant light fall-off is noticeable on the corners as well as some aberration. The mirror shadow is also clearly visible along the bottom. It's apparent that the final image will need some cropping, as flat frames will not effectively correct the very dark corners. Also, a large internal reflection is visible, which I suspect is from Rigel. Not sure what can help mitigate these reflections, maybe a large lens hood/dew shield? This will also require cropping

 

M42-900s.jpg

 

This next image is a stack of 14 x 900s exposures, calibrated with light and dark frames in DSS, stretched slightly. Illumination is much more flat across the image, however it's not perfect and will still need cropping. The annoying reflection is still visible. 

 

M42(3hr30min)(someprocessing)small.jpg

 

Finally, here is the cropped and fully processed result. It's a nearly 50% crop from the previous image, driven much by my desire to get rid of the reflection. Core details have also been blended in.

 

M42(3hr30min)Final.jpg

 

 

This setup is definitely not optimal for a full frame sensor, but it can be made to work. I've also used this gear to image Pleiades, Double Cluster, Andromeda, and the Lagoon Nebula. Results have been better than I expected, and I look forward to going after more difficult targets.

 

 

Regards,

 

Neil


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#2 ilias200400

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Posted 25 January 2020 - 04:24 PM

Excellent and promising results!

Just an idea: are you using an OAG on your setup? I have seen such reflections caused from the OAG prism when a bright star is in the FOV. If that is the case, maybe you could reduce or avoid such reflections by changing the distance of the OAG from the imaging sensor or by simply changing the framing of the subject.

Clear skies and all the best to you!

 



#3 nwinston

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Posted 25 January 2020 - 06:29 PM

Excellent and promising results!
Just an idea: are you using an OAG on your setup? I have seen such reflections caused from the OAG prism when a bright star is in the FOV. If that is the case, maybe you could reduce or avoid such reflections by changing the distance of the OAG from the imaging sensor or by simply changing the framing of the subject.
Clear skies and all the best to you!


Thank you! I am not using an OAG, I have a separate guide scope. I find this reflection to be a little weird. I haven’t seen anything else like it in other images with the same setup, and I’ve directly imaged bright stars. I’m not 100% sure if it’s from Rigel, but the reflection appears to be from that direction. Moving around the framing had little effect. The source could be one of the spider vanes or tube baffles.


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#4 JayScope

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Posted 19 May 2020 - 04:08 PM

Did you ever figure this out? I'm experiencing the exact same problem and it happens when I point my RC a certain direction in my backyard. I though I was catching light from a street lamp and shaded it from the scope but still experienced the same problem. Mine looks almost exactly like your example except bottom right corner. Same looking arch.



#5 JayScope

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Posted 19 May 2020 - 04:19 PM

Here's my example

 

NGC3718_051220_integration_clone.jpg?dl=https://www.dropbox...._clone.jpg?dl=0

 

 


Edited by JayScope, 19 May 2020 - 04:23 PM.


#6 nwinston

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Posted 19 May 2020 - 06:44 PM

Here's my example

NGC3718_051220_integration_clone.jpg?dl=https://www.dropbox...._clone.jpg?dl=0


Your image looks very similar. At you using the CCDT67? Also, are you full frame? I don’t know what the cause is, but haven’t investigated it much. This setup with a full frame sensor is already sub-optimal, so I expect to see some weird things. I’m fairly convinced that it is some type of internal reflection from nearby bright stars. I’ve seen similar reflections in different parts of the sky, sometimes multiple reflections from different directions.


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#7 JayScope

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Posted 20 May 2020 - 07:50 AM

Yes, I'm using the CCDT67 with the recommended 20mm backfocus extension, with a crop sensor Canon.

I'm pretty sure mine is being caused by extraneous light from a neighbor's backyard from one direction and a sodium street lamp from an opposite direction. I hang a car shade on a pole to block the neighbor's light - I'll experiment with that placement and also hang another shade to block the sodium light. I've got a dew shield coming in, hopefully that will help too. I'll fashion a makeshift dew shield and try that in the meantime as well.



#8 JayScope

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Posted 21 May 2020 - 08:10 AM

I found light leaks last night where the focuser slides in/out and where the reducer is inserted into the 2" receiver (see arrows) -- pretty sure that's contributing to my issues:

 

https://www.dropbox....tLeak2.jpg?dl=0


 

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  • LightLeak2.jpg


#9 sharkmelley

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Posted 21 May 2020 - 09:11 AM

I found light leaks last night where the focuser slides in/out and where the reducer is inserted into the 2" receiver (see arrows) -- pretty sure that's contributing to my issues:

 

https://www.dropbox....tLeak2.jpg?dl=0

 

 

That arc looks like the internal reflection from a bright star outside the field of view.  It's a common problem. Try drawing a straight line from the centre of the image cutting through the arc perpendicularly.  Is there a bright star in that direction?

However, the light leak could certainly be contributing to the image gradients.

Mark


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#10 JayScope

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Posted 21 May 2020 - 09:52 AM

That arc looks like the internal reflection from a bright star outside the field of view.  It's a common problem. Try drawing a straight line from the centre of the image cutting through the arc perpendicularly.  Is there a bright star in that direction?

However, the light leak could certainly be contributing to the image gradients.

Mark

Cool thanks! -- it could be star HR 4427, here it is just out of my frame - perpendicular to the arc...

 

IMG_1167.jpg



#11 sharkmelley

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Posted 21 May 2020 - 10:17 AM

Cool thanks! -- it could be star HR 4427, here it is just out of my frame - perpendicular to the arc...

 

attachicon.gifIMG_1167.jpg

No, HR 4427 is far too close.  You want to draw the complete circle that would form the arc and the star causing it will be very approximately at the very far side of that circle and slightly beyond.  It will be a long way outside the FOV of your sensor.

 

Mark


Edited by sharkmelley, 21 May 2020 - 10:31 AM.


#12 JayScope

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Posted 21 May 2020 - 10:34 AM

So around in this vicinity? Sorry OP to hijack this thread

 

IMG_1167 circle.jpg



#13 sharkmelley

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Posted 21 May 2020 - 10:44 AM

So around in this vicinity? Sorry OP to hijack this thread

 

attachicon.gifIMG_1167 circle.jpg

Yes, that's exactly what I mean.

 

Is there an obvious bright star near there?

 

The same exercise can be performed on the OP's image - I guess it's Rigel causing it.

 

Mark


Edited by sharkmelley, 21 May 2020 - 10:47 AM.


#14 JayScope

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Posted 21 May 2020 - 10:53 AM

Yes, that's exactly what I mean.

Is there an obvious bright star near there?

The same exercise can be performed on the OP's image - I guess it's Rigel causing it.

Mark



#15 JayScope

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Posted 21 May 2020 - 10:55 AM

could be Dubhe in my case

 

 

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#16 nwinston

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Posted 21 May 2020 - 10:56 AM

So around in this vicinity? Sorry OP to hijack this thread

attachicon.gifIMG_1167 circle.jpg


No worries, it’s all relevant!

If you draw a line along the radial direction of the curved reflection in my M42 image, it almost directly intersects Rigel.

Light from these bright stars outside the camera’s FOV still strike the primary telescope mirror, albeit at an undesirable angle. That reflection makes its way through the rest of the optical train and to the camera sensor. Not exactly sure how to mitigate this effect with my setup, a lens hood may help shield light in some cases.

#17 JayScope

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Posted 21 May 2020 - 11:08 AM

No worries, it’s all relevant!

If you draw a line along the radial direction of the curved reflection in my M42 image, it almost directly intersects Rigel.

Light from these bright stars outside the camera’s FOV still strike the primary telescope mirror, albeit at an undesirable angle. That reflection makes its way through the rest of the optical train and to the camera sensor. Not exactly sure how to mitigate this effect with my setup, a lens hood may help shield light in some cases.

Now not sure at all how to mitigate - hoping also that since my collimation is 10x better (hopefully) since that session, that might help. My primary wasn't centered and has been since corrected (still not star tested though). That along with shielding from the external lights leaks ...



#18 the Elf

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Posted 21 May 2020 - 03:29 PM

I have had exactly the same arc in my images taken with the RC6 and the reducer. After I upgraded to the GSO RC8 carbon, keeping the same reducer, I have never seen such arcs again.


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#19 sharkmelley

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Posted 21 May 2020 - 06:27 PM

could be Dubhe in my case

Check the orientation of your image - it's not as you have shown it.  I therefore think the arc is caused by Merak which is approx 5.5 degrees from the centre of your image.  Coincidentally Rigel is just slightly further from M42, 5.8 degrees.

 

Although I am not sure of the precise mechanism for the formation of the arc, if stars at 5 degrees or more are causing the problem then this could probably be prevented with an appropriate light shield.

 

Markl


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#20 JayScope

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Posted 21 May 2020 - 07:24 PM

Thanks Markl -- that makes better sense.

 

I'm waiting for an astrozap so hope that will help. In the meantime I'll experiment with some blackwrap.

 

Interesting how just a few degrees makes a difference.

I don't see this reflection in my pic of Owl but it's there on M101. I only see this whenever I'm imaging North in this area, so far with a just couple months of owning this scope.

owland101.jpg



#21 the Elf

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Posted 22 May 2020 - 02:00 AM

I dug out the old data. Very similar, but a double arc. Note: this is an APS-C sized sensor!

 

arc.jpg

 

While I was collimating it I found from a certain angle it was possible to look in at the rear and over the edge of the rear tube, straight out so to say. Possibly this is the problem. When I look into the RC8's rear no matter from which position I can only see the secondary mirror or the inner of the tubes. Either it is a bad construction or longer tubes would cause vignetting at this focal length.

I encourage you to update to the RC8, it has a lot more resolution.


Edited by the Elf, 22 May 2020 - 02:02 AM.

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#22 sharkmelley

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Posted 22 May 2020 - 10:08 AM

 

I don't see this reflection in my pic of Owl but it's there on M101. I only see this whenever I'm imaging North in this area, attachicon.gifowland101.jpgso far with a just couple months of owning this scope.

attachicon.gifowland101.jpg

The bright arc in the M101 image appears to come from the direction of the star Alkaid which is 5.5 degrees away.

 

In this thread we now have images of 3 different targets, all with arcs apparently caused by bright stars 5.5 to 6 degrees away.

 

Interesting - there seems to be a pattern emerging.

 

To diagnose the cause, point the scope 5-6 degrees away from a bright light, remove the camera and look into the back end of the scope to see what is reflecting the light.

 

Mark


Edited by sharkmelley, 22 May 2020 - 10:14 AM.

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#23 JayScope

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Posted 26 May 2020 - 09:11 AM

Just an update -- I got a chance to do a little testing this weekend.

 

I attached a makeshift shield that looks like it had an effect of the reflection and somewhat mitigated. The material I used is called Blackwrap (we use in the video production industry), it's not totally flat though as you can see in the pic below. Top row pics are before the blackwrap was added (for reference), bottom pics are after.

Interesting that the rings changed position in NGC3718; the rings on M101 are in the same position but the reflection is not quite as defined. I'm hoping that the astrozap dew shield is more flat than the blackwrap.

 

ReflectionsTest052620.jpg


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#24 sharkmelley

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Posted 26 May 2020 - 09:23 AM

Interesting that the rings changed position in NGC3718

It's expected. 

 

You changed the framing slightly, therefore the position of the rings will change slightly.

 

Mark


Edited by sharkmelley, 26 May 2020 - 09:25 AM.


#25 the Elf

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Posted 26 May 2020 - 10:59 AM

Difficult to tell from the photo, but the shield might be too short to change anything. My RC8 came with a shield that was as long as the telescope.




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