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Jim Smith

Ultrastar C First Light

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These images are from my first attempts at video astronomy. I'm using an SX Ultrastar C, Starlight Live, Celestron C6 with f6.3 reducer, SkyWatcher AZEQ5 in AZ mode.

I have to admit to being very impressed at how much detail I can see with a modest telescope and short exposures. The live viewing experience compares favourably with "eyeball" observing through the 14.5" Dobsonian that I owned until recently.

The M13 capture has odd, crescent shaped stars which I think was a result of dew forming on the SCT corrector plate. (I have a dew shield...I just forgot to use it!). The M17 capture was taken on the following night, with the dew shield in place. The stars still look a bit elliptical to me. I think I need to do a more careful mount alignment to get the tracking better. I'm not sure why the stars are yellowish. I used no dark frames.

I'm very happy with the set-up so far and I hope to improve on these images. Please comment if you have any handy tips for me. I'm keen to learn.

M13.jpg

m17.jpg

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Hi Jim,

Excellent first light!

I think your focus might be off a bit and that's why the stars are odd shaped.   Many find a Bahtinov mask helpful.

The 825 sensor has a yellow tinge to it with default settings.  You can correct for it by going to the Channel Selection in Display Processing and uncheck the Modify All box.  Then check the blue channel and increase the contrast in Image Modifiers.  You will see the yellow disappear, but don't go too far because it will turn blue.  Make sure to recheck the Modify All box again, so you can use the contrast for all channels.

Hope this helps.  You're off to a good start.

Don

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58 minutes ago, HiloDon said:

I think your focus might be off a bit and that's why the stars are odd shaped.   Many find a Bahtinov mask helpful.

The 825 sensor has a yellow tinge to it with default settings.  You can correct for it by going to the Channel Selection in Display Processing and uncheck the Modify All box.  Then check the blue channel and increase the contrast in Image Modifiers.  You will see the yellow disappear, but don't go too far because it will turn blue.  Make sure to recheck the Modify All box again, so you can use the contrast for all channels.

Hope this helps.  You're off to a good start.

Yes. I fiddled with the focus for about 20 minutes trying to minimise the FWHM reading. I wasn't convinced that I had got it right. I've just ordered a Bahtinov Mask to see if that makes it easier. I will also adjust the colour balance as you suggest next time. Thanks Don.

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Very nice first light for you setup (and a very nice setup it is too). A Bahtinov mask will reduce your time to focus from 20 minutes to 20 seconds, it's that good. Looking forward to seeing some more. 

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Hi Jim

Welcome to the wonderful world of EAA - great first light.

I also suggest a Bahtinov Mask for focus - easiest way to focus, I use mine at the start of my alignment routine on the first alignment star.

Clear skies and keep on posting your experiences.

Paul

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Hello and welcome from me....also  good luck and clear skies !...Alan.

 

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Some new captures from last night attached. I'm impressed at what you can see in midsummer with a nearly full moon out.

I repeated the mount's 2-star alignment until it seemed to be tracking well. It doesn't seem to always be perfect first time.

I managed to tweak the colour balance more to my liking.

It was much easier to focus using the Bahtinov mask. The crossing diffraction spikes showed up well in the Starlight Live alignment/focussing window.

All images still look a bit blurry to me though. I still seem to be getting curious crescent shaped stars...particularly the slightly dimmer ones. Some of them look a bit like soap bubbles. I checked collimation and it looked OK to me.

Sorry about the weird Target Name on the images. When I try to select the default contents to delete it something odd happens and it won't let me type anything in!

M13.jpg

m27.jpg

m57.jpg

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Hi Jim

I don't have a SCT, but that still looks like a focus problem to me (and perhaps some minor coma issues leading to the eggy shapes). If it is, then (i) are you using a mask designed for your scope? (ii) are you ensuring that all 3 spikes cross at the mid-point? You need to use a sufficiently bright star, or an integration time long enough for the spikes to show up well. The former is preferable so that you can see the middle spike moving in real time as you adjust focus.

cheers

Martin

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4 minutes ago, Martin Meredith said:

I don't have a SCT, but that still looks like a focus problem to me (and perhaps some minor coma issues leading to the eggy shapes). If it is, then (i) are you using a mask designed for your scope? (ii) are you ensuring that all 3 spikes cross at the mid-point? You need to use a sufficiently bright star, or an integration time long enough for the spikes to show up well. The former is preferable so that you can see the middle spike moving in real time as you adjust focus.

Hi Martin,

The mask is the Celestron C6 one from FLO. Yes, as I adjusted the focus I could clearly see the three spikes moving relative to each other. I got them to cross at the mid-point.

I've had a thought! I'm using the Celestron f6.3 reducer. Does the changed focal length make a difference to the mask?

Also I'm using the camera through a Baader T2 Amici prism diagonal. Perhaps I should lose that from the optical train when I next try?

Regards, Jim

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Hi Jim

I honestly don't know the answers to those questions, so I hope someone more knowledgeable will come along... but I wouldn't have thought that either of the things you suggest would have made such a difference. If it is focus, it is some way off. One thing I do is zoom in on the StarlightLive display while adjusting focus, so that the only thing that fits is the star and its diffraction spikes. You could try lifting the mask off briefly when you have achieved focus and check if the donut shapes are still there.

Are you able to focus without the mask to achieve smaller stars? If so, then there is a definite discrepancy between the mask and the non-mask focus.

Martin

Edited by Martin Meredith
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1 hour ago, Jim Smith said:

Also I'm using the camera through a Baader T2 Amici prism diagonal. Perhaps I should lose that from the optical train when I next try?

Hi
I am using baader t2 prism for visual and it is perfect at f9 but at f6 or lower it gives optical aberrations. I think it would be good idea not to use it with camera, especially near and below f6. Worth a try for sure.

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Definitely coma in images - they should 'point' to middle of frame - prior to their correction/minimisation. Try system without the star diagonal - unless the camera hits the mount without ;-(

Nytecam

Edited by nytecam

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Last night I decided to try my 80mm f/6 refractor instead of the C6 to see if I got the same problem. This telescope also has a Baader Amici prism diagonal but it's a 2" one. I decided to leave the diagonal on for this test. I do not have a Bahtinov mask for this scope so I focussed used the FWHM reading from StarLight Live. Three images are attached, all taken with the 80mm scope regardless of what the caption says!

On brighter stars there is a horizontal spike which also appears visually when using this diagonal. Other than that though, I reckon the stars appear in better shape than when using the SCT.

Next time out, my plan is to recollimate the SCT as accurately as I can and try it without the diagonal. Perhaps I should try it without the focal reducer too?

m13.jpg

m27.jpg

m101.jpg

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Hi Jim,

I agree with Martin that your focus with the C6 is still not right.  It appears to be uneven across the field as well, indicating that something is not square in the optical train.  I would get rid of the diagonal if you can.  

I do the same as Martin and make close inspection of the stars when focusing.  I don't use a mask.  Your 80mm captures look pretty good.  Color is better, too. You are making great progress.

Don

 

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On 7/18/2016 at 05:28, Jim Smith said:

Sorry about the weird Target Name on the images. When I try to select the default contents to delete it something odd happens and it won't let me type anything in!

That happens to me as well.  Instead of trying to highlight everything and delete it, just click until you get cursor and use the back button to delete the contents.  If I try to select all of it, it locks and won't allow me to change it again until I restart the program.

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Agree with the others - you are out of focus with the C6, significantly.  The frac looked much closer.

I use a Bahatinov mask (Farpoint is the maker, purchased on Amazon) with my C8 and 0.63 FR and it nails perfect focus in moments.  The next time you are using it, I would suggest saving a screen grab of what the pattern looks like and post it here.  You also have coma, which I think is interacting with the out-of-focus problem to make the crescent shaped stars.  What brand of reducer are you using?  Have you checked the spacing from reducer to chip (including the path through the diagonal?  You may have it too long, which could exacerbate coma.  If you can lose the diagonal without running into mount clearance problems with the camera, I would definitely suggest it.  If I had an alt-az celestron I would put it on a wedge just to avoid the clearance issue (or get a GEM).  Oh, looking back, I see you have an AZEQ mount.  Definitely use it in EQ mode and lose the diagonal.

Welcome to the EAA club - keep trying stuff and posting the results here, and you will be a pro before you know it!

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6 hours ago, aparker said:

What brand of reducer are you using?  Have you checked the spacing from reducer to chip (including the path through the diagonal?  You may have it too long, which could exacerbate coma.  If you can lose the diagonal without running into mount clearance problems with the camera, I would definitely suggest it.  If I had an alt-az celestron I would put it on a wedge just to avoid the clearance issue (or get a GEM).  Oh, looking back, I see you have an AZEQ mount.  Definitely use it in EQ mode and lose the diagonal.

I use the Celestron reducer. I haven't checked the spacing...I didn't know it was significant. My next test is to remove the diagonal. After that I might try the mount in EQ mode. Thanks.

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16 hours ago, Jim Smith said:

I use the Celestron reducer. I haven't checked the spacing...I didn't know it was significant. My next test is to remove the diagonal. After that I might try the mount in EQ mode. Thanks.

I don't have that brand of reducer, but according to this source: http://www.morrell.ws/index.php?option=com_content&view=article&id=49:focus-reducer-spacing&catid=7:best-practice&Itemid=42  

The required spacing is 85mm from FR lens to image plane (CCD surface).

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7 hours ago, aparker said:

The required spacing is 85mm from FR lens to image plane (CCD surface)

If I just use the Celestron 1.25" back, I think I might be below that figure. If I also insert a 1.25" mirror diagonall, that might be too much. Anyway, I can experiment and see what happens. Thanks.

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Presumably the telescope side of the reducer is SCT-threaded, so it can screw on to the back of the scope.  From what I can see of pictures on line the back of the reducer is also SCT-thread.  The most robust connection you can make is to use an SCT-thread to T-thread adapter, then appropriately sized T-thread spacers to get the right reducer-to-chip distance.  With the Ultrastar, the last piece in the chain has to be a T-thread to C-thread adapter, putting a male C-thread on the back.  The front of the camera is female C-threaded, so it will screw right on.  The T-adapters come in many different lengths, so you can mix/match to get the spacing you need.

Here are example products that might be part of the solution:

http://agenaastro.com/blue-fireball-sct-female-t-t2-male-adapter-adjustable-t-02.html

https://www.firstlightoptics.com/adapters/baader-t2-extension-tube.html

http://agenaastro.com/blue-fireball-t-thread-female-c-mount-male-adapter.html

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6 hours ago, aparker said:

The most robust connection you can make is to use an SCT-thread to T-thread adapter, then appropriately sized T-thread spacers to get the right reducer-to-chip distance.  With the Ultrastar, the last piece in the chain has to be a T-thread to C-thread adapter, putting a male C-thread on the back.  The front of the camera is female C-threaded, so it will screw right on.  The T-adapters come in many different lengths, so you can mix/match to get the spacing you need.

Thanks for the information. As all of the connections are threaded, won't I lose the ability to rotate the camera to frame the shot?

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Yes.  Unless you get a Baader rotating T adapter.  For EAA purposes I generally don't bother with camera rotation - I'm looking to capture a view of the object and move on.  But the rotating T adapter is the thing if you want to be able to do this.

the biggest advantage of all-threaded is rigidity - stuff stays where you put it without flexing, so no worries about focus changing when the scope moves etc.

This is not a huge issue with the Ultrastar because it is so light, but if you get a heavier camera, filter wheel, etc in the future it becomes critical.

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Great start, Jim! With the assistance of this wonderful community you will get the residual issues sorted out easily.

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So last night I tried M13 using my C6 @ f/6.3 again. I removed the amici prism diagonal. I couldn't find anyway to space the camera sensor about 85mm from the reducer, except by using a mirror diagonal, so I used that. I think the spacing was somewhere between 75mm to 100mm.  I used the Bahtinow mask for focus.

The telescope name is wrong on the caption...I keep forgetting to press the "Set" button after entering the details.

To me, the result is much better. It sinply looks more in focus.  Some of the stars still look a bit odd, but I think they are better than before. I may invest in some method of holding the camera more rigidly, with the correct spacing as aparker suggests. The camera wasn't entirely secure in the diagonal.

m13.jpg

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