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A first step back to good images?

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#1 dcaponeii

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Posted 08 August 2020 - 12:44 PM

Many of you have been helping me improve my imaging for the past couple of months in multiple threads I've posted and I continue to appreciate all the excellent input I've been receiving.  Most recently I noticed that Firecapture was reporting back my optics as f/24 which is well about the f/15 I would have expected of my f/8 12" LX-600 with a 2x Barlow.  It appears to be the result of a long projection distance to the camera from the Barlow lens.  Yesterday afternoon, I took the step of inserting my f/6.3 reducer/flattener to see if this would bring my optics into the f/15 range needed for my camera pixel size (2.9 microns)

 

Step 1 of my efforts was immediately rewarded by better results.  I first shot a quick sequence with the existing optics for comparison.  The I pulled everything off the back of the scope, including the focuser.  I then installed the f/6.3 flattener, the eyepeice adapter, 2x Barlow and the ASI-290MC camera.  No attempt to change the scope collimation was made (it was left in the final condition from the other day's efforts to collimate.

 

Both images are 5% stacks of about 5000 frames.  I was just trying to get a quick look at whether this exercise was worth the time it would take.

 

The initial image is here:  Firecapture reports this as f/24.

 

2020-08-08-0142_2-U-L-Jup_AP48_3xAS_56120w.jpg

 

The image with the reducer is here:  Firecapture reports this as f/16.

 

2020-08-08-0145_4-U-L-Jup_AP48_3xAS_56104w.jpg

 

I was very pleased with the result.  Round moons and moon shadows are not my usual result!!  I reassembled the entire optical train based on this image with the JMI focuser, the f/6.3 flattener, the 2x Barlow (threaded into the ADC barrel), the camera, with the IR/UV Cut filter threaded into the camera barrel.  I'm still processing these images and will post them at a later time.  It took a bit of fiddling to get everything on and in a order that still allows for proper focus to be achieved.  Perhaps these challenges will assist others just getting started (at the very least it might explain what NOT to do)


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#2 phileefan

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Posted 08 August 2020 - 01:31 PM

dcaponeii,

 

Here is a response on how to check your focal length and focal ratio. The numbers may be close in Firecapture but WinJupos will be more accurate. Hope this helps..... smile.gif

 

 

Focal Length.jpg


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#3 RedLionNJ

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Posted 08 August 2020 - 05:59 PM

You can lead a horse to water, but you can't make it drink.....   all this has been suggested before, but the addition of yet another application doesn't make it appealing to a beginner.

 

 

So we have now ascertained the addition of a focal reducer (BAD, bad, bad) into the optical train seems to produce better results. And we didn't take anything out.

 

There's really no reason the data gathered at f/24 shouldn't be just as viable as that at f/16, other than being a little dimmer and requiring resizing under most circumstances. And all the components which were present at f/24 are still there at f/16 - and we added an additional one!

 

I don't know what more to suggest at this point. John B's suggestion regarding the IR-cut filter seems to be moot, as it's explicitly mentioned it again in this thread.

 

If your'e finding the collimation screws are indeed nearly all the way down, that may be unduly warping the secondary. Normal practice when you get into this situation is to back off all three screws a few turns and try to recollimate from that point. I still think there's something very suspect with the 'half image' recorded during the collimation process. 

 

Can we maybe get an aligned, processed stack of an in-focus star, showing the Airy disk and first couple sets of diffraction rings?


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#4 dcaponeii

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Posted 09 August 2020 - 12:27 PM

You can lead a horse to water, but you can't make it drink.....   all this has been suggested before, but the addition of yet another application doesn't make it appealing to a beginner.

 

 

So we have now ascertained the addition of a focal reducer (BAD, bad, bad) into the optical train seems to produce better results. And we didn't take anything out.

 

There's really no reason the data gathered at f/24 shouldn't be just as viable as that at f/16, other than being a little dimmer and requiring resizing under most circumstances. And all the components which were present at f/24 are still there at f/16 - and we added an additional one!

 

I don't know what more to suggest at this point. John B's suggestion regarding the IR-cut filter seems to be moot, as it's explicitly mentioned it again in this thread.

 

If your'e finding the collimation screws are indeed nearly all the way down, that may be unduly warping the secondary. Normal practice when you get into this situation is to back off all three screws a few turns and try to recollimate from that point. I still think there's something very suspect with the 'half image' recorded during the collimation process. 

 

Can we maybe get an aligned, processed stack of an in-focus star, showing the Airy disk and first couple sets of diffraction rings?

You are ABSOLUTELY correct and I have been listening.  I got some great images last night that I have not had time to finish processing.  Still not perfect and then change in symmetry between in and out of focus was still there.  Collimating using MetaGuide minimzed the problem by addressing the image at very near focus with their technique.  Nontheless the change in symmetry going through focus means there is still something wrong and all of you have said so and I've not been ignoring it.  So...

 

I just spent two hours this morning outside using an artificial star (a clean lightbulb on a stick that the sun was shining on).  After collimating and placing the defocused image on camera with image centered on the chip, the Poisson point was NOT in the center of the donut.  Shifting the image to the upper right hand corner of the chip yielded a good donut with the Poisson point in the center.  So what the heck did that mean??  Then it hit me...my JMI focuser was not aligned with the optical axis of the telescope.  Turns out the JMI housing was in too far on it's bracket and one of the set screws was hitting the shoulder cocking it sideways.   I did some captures of the artificial star and am working on processing them now.  I'll put something up later after getting them all organized.  I'm fairly confident that this correction of how the JMI was mounted is actually the root cause of the problem.  Sure looke that way with the Artificial star but the proof will be the next time I can do some star imaging.  Wish me luck.

 

On a related note, I also realized that the threaded extensions that came with the f/6.3 reducer allowed me to remove 1.25 barrel from the ADC and thread it directly to the JMI instead of using the eyepiece adapter from Meade that only has a single set screw.  In addition the 2x barlow is now threaded in front of the camera where it belongs with the IR/UV Cut filter threaded into the Camera 1.25 tube from the backside so it is between the chip and the Barlow.  All of this reduced the length of stuff hanging off the back of the scope by almost 1 1/2"



#5 Peregrinatum

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Posted 09 August 2020 - 01:57 PM

I'm going through the same process right now as you are in learning SCT collimation.

 

I used the Robyn Casady Daytime Collimation and it worked fantastically to get me about 90% there, with a centered donut on a defocused blob using no magnification.

 

I tried next to get Metaguide going, setting up the guider, autocentering etc and was not successful last night... woke up, licked my wounds, researched some and found the error of my ways... going to try again tonight with a 2x barlow and planetary camera to see if I can get the red dot centered on the apex...

 

I see the iteration is to turn the collimation screws so the star moves in the same direction as the red dot, wasn't doing that last nigh, trying again tonight.

 

Have you been successful getting Metaguide going, if so any tips on how to get it functioning?


Edited by Peregrinatum, 09 August 2020 - 02:05 PM.


#6 dcaponeii

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Posted 09 August 2020 - 02:14 PM

I'm going through the same process right now as you are in learning SCT collimation.

 

I used the Robyn Casady Daytime Collimation and it worked fantastically to get me about 90% there, with a centered donut on a defocused blob using no magnification.

 

I tried next to get Metaguide going, setting up the guider, autocentering etc and was not successful last night... woke up, licked my wounds, researched some and found the error of my ways... going to try again tonight with a 2x barlow and planetary camera to see if I can get the red dot centered on the apex...

 

I see the iteration is to turn the collimation screws so the star moves in the same direction as the red dot, wasn't doing that last nigh, trying again tonight.

 

Have you been successful getting Metaguide going, if so any tips on how to get it functioning?

I just did about 2 hours of work this morning using the sun glinting off a light bulb as an artificial start.  I've added the info to my other post.  Net result of the two hours work is that my JMI Motorized Crayford focuser was not in good alignment with the optical axis.  Metaguide is a bear using the red dot.  I find that unless the seeing is really good the dot jutmps around all over the place.  I end up setting the averaging to 1.0 and sometimes 1.5 to reduces the scintillation of the image.  I'm still learning Metaguide as well but I think I'm getting the hang of it.




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