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M57 – The Ring Nebula in Lyra – Astro-Tech RC8 First Light #2 of 5

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#1 jgraham

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Posted 03 July 2020 - 12:25 PM

Telescope: Astro-Tech 8” f/8 Ritchey-Cretien, Orion Atlas EQ-G
Camera: Full Spectrum Modified Nikon D5300
Filter: Orion Imaging Skyglow Filter
Guide scope: Astro-Tech 60mm, Meade DSI Pro II, PHD
Exposure: 13x180sec, ISO 200, saved as RAW
Darks: Internal (Long Exposure Noise Reduction On)
Flats: 32x2sec, Tee shirt flats taken at dawn
Average Light Pollution: Red zone, Bortle 8, poor transparency, bright moonlight
Lensed Sky Quality Meter: 18.2 mag/arc-sec^2
Stacking: Mean with a 2-sigma clip.
White Balance: Nebulosity Automatic
Software: Backyard Nikon, Deep Sky Stacker, Nebulosity, Photoshop

 

M57 (7-1-2020)-1j.jpg

 

This was the 2nd of 5 first-light images taken with my new to me Astro-Tech 8” f/8 Ritchey-Cretien. The sky conditions were pretty rough with haze and bright moonlight, but I was still able to conduct some basic tests with this new system. This was my first test of the RC8’s narrow-field capability and I cropped the source image to zoom in a bit closer on the nebula to highlight some of the fine detail. Narrow-field imaging of deepsky object can be influenced by the seeing conditions and this limits the usefulness of using larger telescopes for this purpose. The RC8 at f/8 seems to be well suited for my typical sky conditions providing a nice balance of speed and field of view. Also, this was my first opportunity to test the performance of my Nikon 5300a on a warm, muggy summer evening and it performed flawlessly. My cooled ZWO ASI071MC Pro is off being repaired (faulty temperature sensor on the TEC), but so far I’m not missing it (much). :)

 

Visually, The Ring appears as a ghostly smoke ring about the size of Jupiter. The nebula itself is not actually a ring, but more likely a torus or a short hour-glass, we just happen to the looking down the long axis, giving the nebula its characteristic shape.


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#2 chanrobi

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Posted 03 July 2020 - 01:44 PM

What FL was this shot at? And this is cropped?



#3 sunnyday

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Posted 03 July 2020 - 01:57 PM

very nice shot , thx .



#4 jgraham

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Posted 03 July 2020 - 02:13 PM

Thanks!

 

This was taken using the RC8's native f/8 focal length; 1600mm. The field was cropped about 70% to zoom in on the Ring while still maintaining reasonably tight stars. The uncropped field is shown below.

 

M57 (7-1-2020)-2j.jpg

 

 


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#5 chrysalis

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Posted 03 July 2020 - 06:17 PM

Gorgeous picture, the nebula seems so lonely floating in space like that...



#6 the Elf

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Posted 04 July 2020 - 03:18 AM

Nice shot. Congratulations to the new scope. Have you got the carbon tube or the steel tube?

That reminds me M57 is next on my list but I can't get rid of the clouds. And if I could it is full moon now. :-(



#7 Starman27

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Posted 04 July 2020 - 12:27 PM

See, I was right. Here's another great image.



#8 jgraham

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Posted 04 July 2020 - 03:39 PM

Steel tube...

 

Atlas RC8 (7-1-2020)-1.jpg

 

I was a little disappointed with the carbon fiber tube of my ES Comet Hunter. I was expecting to see a very low thermal expansion coefficient, but it turns out the tube is a carbon/epoxy composite, not carbon/carbon, so the CTE is that of the epoxy resin which doesn't see to be much better than steel.

 

Anywho, my RC8 has the steel tube and so far it seems to be working okay. Over 2 nights I have had to make only tiny adjustments to the focus and none to the collimation.

 

Very nice!


Edited by jgraham, 04 July 2020 - 03:39 PM.


#9 chanrobi

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Posted 04 July 2020 - 07:55 PM

Steel tube...

 

attachicon.gifAtlas RC8 (7-1-2020)-1.jpg

 

I was a little disappointed with the carbon fiber tube of my ES Comet Hunter. I was expecting to see a very low thermal expansion coefficient, but it turns out the tube is a carbon/epoxy composite, not carbon/carbon, so the CTE is that of the epoxy resin which doesn't see to be much better than steel.

 

Anywho, my RC8 has the steel tube and so far it seems to be working okay. Over 2 nights I have had to make only tiny adjustments to the focus and none to the collimation.

 

Very nice!

... carbon is always made with epoxy as a filler, not sure what you mean by that.



#10 jgraham

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Posted 04 July 2020 - 09:39 PM

I work with all sorts of composites. Carbon/epoxy looks pretty, but it's not low expansion. Carbon/carbon composites are extremely hard, strong, lightweight, and almost zero expansion. The truss structure of the Hubble Space Telescope is made from a carbon/carbon composite. Much of the Boeing 787 is carbon/carbon. Carbon/epoxy may make a lightweight telescope tube, I'm not convinced it helps much with thermal expansion. I'll check my references when I get to work on Monday.
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#11 chanrobi

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Posted 04 July 2020 - 11:41 PM

I work with all sorts of composites. Carbon/epoxy looks pretty, but it's not low expansion. Carbon/carbon composites are extremely hard, strong, lightweight, and almost zero expansion. The truss structure of the Hubble Space Telescope is made from a carbon/carbon composite. Much of the Boeing 787 is carbon/carbon. Carbon/epoxy may make a lightweight telescope tube, I'm not convinced it helps much with thermal expansion. I'll check my references when I get to work on Monday.

Interesting, most of my experience is with high end bicycle frames/parts. They are all CRP.

 

I thought it looked pretty gimmicky on a telescope (as is some stuff on a bicycle), but thought it was mainly for weight savings/cool factor.



#12 the Elf

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Posted 05 July 2020 - 07:13 AM

There is one thing I do not understand: my RC 8 comes with a carbon tube but the front ring that holds the secondary is screwed to the carbon tube and the the aluminum dovetails, a Vixen style on one side and a losmandy on the other. The holes in the carbon tube are large and somewhat crumbling and the screws are tiny. Only the dove tail bars are really stable and offer a solid connection. To me this looks like the tube is either show or weight saver only but the temperature behavior is manly determined by the aluminum bars. Am I missing something?



#13 jgraham

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Posted 05 July 2020 - 12:28 PM

That's a good question. The proof is always in the final result; does experience show that the focus is temperature sensitive and by how much? The biggest challenge that I have face when imaging with an SCT at f/10 has been the temperature sensitivity of the focus. Tests using Reflectix insulation to slow down the rate of temperature change of the tube were encouraging. Tests with the stock, steel tubes of my 8" GSO f/12 CC and the AT RC8 were also encouraging, each showing only a slight change in focus over the course of a summer night.

 

It's always something...



#14 jgraham

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Posted 05 July 2020 - 12:28 PM

Whooops! I discovered last night that I suffered a major brain cloud and spent my first two nights testing my RC8 with my Baader Mk III MPCC still mounted on my camera. Ugh. Soooo, I'll need to start my testing all over again. Unfortunately, that will have to wait a bit as I'm going to switch over to planetary imaging for a bit. However, I did have chance to do a bit of testing last night through thin spots in the clouds and it looks like no real harm was done. The collimation held beautifully, though the focus shifted in about a centimeter or so and the field of view increased just a smidge. I couldn't do too much because of the clouds, but a few quick test images of Stephenson I showed good star shapes across the field.

 

So much to do, so few clear nights! :)



#15 the Elf

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Posted 05 July 2020 - 10:23 PM

I never recorded the values in a scientific way, but roughly speaking within 5°C I do not attempt refocus. The FWHM curves go up and down due to high clouds, seeing, whatever. Every now and then there is a FWHM curve with a strong trend from low to high. This is when I decide to stay in bed for this work thingy, kind of social clouds in my hobby. This is when I drag the rig in early in the morning and find temperature has dropped way more than 5°C. In some nights temperature does not change much and I still have in focus images in the morning.

Now how much temperature change does the steel version tolerate? My bet is 4.8°C.

I want to keep my setup simple, without a computer. That is why I have no auto focus. There is a stand alone auto focus by lacerta, called M-FOC. It is based on a temperature sensor only, not on image analysis. As I loose far more subs to clouds and seeing than to bad focus I decided not to do or buy anything at all. I hardly dare saying, but thanks to the pandemic all of a sudden home office is the way to go which saves me 2 hours of driving a day and I no longer need to get up that early. Usually I do so to avoid the traffic jams after 6am. This gives me the opportunity to refocus after midnight, stay in bed until 7, do my work and have great subs all over the night. Of course I wish there was no pandemic, but making the best of it.



#16 jgraham

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Posted 06 July 2020 - 09:13 AM

Good to know. So far I'm very happy with the focus stability of my RC8. Across 3 nights of imaging I have only had to make very small tweaks between image sets. Collimation also looks very stable. The RC8 is emerging as a fantastic imaging platform and will make good use of the APS-c sensor in my DSLR.

 

Neat stuff.




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