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CCDInspector One-Star Collimation & Polaris

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#1 Salacious B Crumb

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Posted 15 March 2019 - 08:20 AM

Trying to kill some time here while waiting for the CP4 to come back.

 

I bought the CCDInspector and I’m planning to use it to collimate my RC8. Has anyone done this by using the Polaris, meaning is it bright enough for one-star collimation and I could do this now without slewing or tracking? Has anyone done this and any thoughts if the QSI or the Lodestar would be better suited for this purpose? I briefly tried the multi-star collimation option the other night and the program kept telling me that my FOV is not large enough (or something similar). Further, any other tips before I start to rotate those pesky screws of the secondary mirror (my plan is not to touch the primary mirror unless absolutely necessary).

 

 

Thanks,

 

Mikko



#2 drmikevt

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Posted 15 March 2019 - 08:53 AM

Your  RC8  plus QSI has a plenty large enough FOV.  The problem, most likely, was there there were not enough stars in your FOV.  The program likes a very full star field for collimation.  It will definitely be better for you to first polar align your mount and start tracking.  I normally take 30sec images through an L filter to feed into the program for collimation.  Tends to work well.  

 

Collimating your RC should be 3 distinct steps (in my opinion - others have other opinions)

 - First, use a laser to make sure your camera is properly aligned with the central spot on the secondary

 - Next, use a perfectly centered out of focus star to collimate the primary to the camera (described here: http://www.deepskyin...re_Ver_1.0.pdf)

 - Finally, use CCDI to collimate the secondary.  There are other ways to collimate the secondary and some people don't fully trust CCDI, but it seems to give me good results.  I think that using a full star field will give better results than a single star. 



#3 Salacious B Crumb

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Posted 15 March 2019 - 09:17 AM

Thanks Mike. I did already the laser alignment and I think it's OK. I say this as I find it difficult to see exactly where the laser is pointing behind the secondary or where it hits back to end of the tube. I guess using a single star collimation AND tracking would be difficult when I have to defocus the star so much. This of course unless I refocus the Guide Camera after doing this. Not sure if one method is better than another (single versus many stars).

 

 

- Mikko



#4 Salacious B Crumb

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Posted 21 March 2019 - 09:33 AM

The Mach1 is back in business and I used yesterday evening trying to collimate the RC until the clouds rolled in at midnight. I said trying as I'm not sure how I succeed. I used the multi-star option taking 5 - 10 sec exposures and averaging out three of them every time. That seemed to have given me enough stars and wide enough FOV for most of the time. Sometimes the CCDInspector was still complaining about the spread of stars.

 

What I don't understand is the changing collimation error values without touching anything. I'm not talking about a few pixels due seeing or something but 20 pix swings from one direction to another between sets where I have not adjusted anything. Has anyone else experienced this and what causes that? At some point when I got tired of this I returned all the knobs to their original position (I smartly marked them before starting this) and then adjusted one knob at the time both ways about 45 degrees and watched the FWHM's and star shapes. When I found a combination where the stars looked good and the FWHM was the smallest, I then fine tuned again each knob ever so slightly both ways to see if it improves or worsens the situation. At the end the combination below where the knob at 8 o'clock is in its original position and the two other knobs are slightly adjusted gave me best results,. This also collimation pix error wise which was constantly under 2. Do these adjustments seem reasonable as I'm still struggling to understand how small is a small adjustment (the photo below is not the best one but shows the original positions and where like said I ended up)?

 

The last thing I did was looked at the live collimation window which showed only some Blue colors, no Black at all. This worries me a bit but hopefully it was just one of the many images it was inspecting. I plan to spend some more time with this today as it's supposed to be clear again.

 

IMG_2497.JPG

 

- Mikko


Edited by Salacious B Crumb, 21 March 2019 - 10:00 AM.


#5 RogeZ

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Posted 21 March 2019 - 09:38 AM

Mikko:

You need to use an artificial star and Hotech SCT collimator, collimating with a star live is asking for seeing to distort the image. Thats why you see change without actually changing anything.
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#6 Salacious B Crumb

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Posted 21 March 2019 - 11:39 AM

Roger I have one of those but find it hard to use. I also bought a Howie Glatter collimator but without a Concentric Circle attachment it's useless (should be back in stock in April again). Now could I use an artificial star and the CCDInspector? It would be awesome to collimate inside and during the daytime without wasting valuable imaging time for that.

 

 

- Mikko



#7 Salacious B Crumb

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Posted 21 March 2019 - 09:18 PM

Here is a couple of screen shots from the CCDInspector. They are from today, shooting M67, 3 min subs with a red filter. I think they are not perfect but with my small sensor, the stars should be adequate. What I don't like is the softness I still see on individual images and the fact that I don't know from where it's coming from. The HFR's are around 4.5 and the autofocus gave me a nice V-curve. What gives?

 

 

- Mikko

 

Capture1.PNG

 

Capture2.PNG



#8 rockstarbill

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Posted 22 March 2019 - 03:11 AM

Hey Mikko,

 

I have not attempted to collimate an Astro-Tech RC telescope, but I have worked on an RCOS RC and of course an ONTC Newt. I can tell you that in my experience, CCD Inspector is not a good tool for critical collimation of optical systems such as the RC. It will give wildly different results, because it is software that is trying to fight against the many elements in the sky that can skew results from one image to another.

 

What I have found to be useful for the RC, is to leverage the DSI method of collimation. You can use whatever rough collimation tools you desire (I personally use the Takahashi Collimation Scope) then finish up the job with star testing via the DSI method. 

 

You can read more about the method here: http://www.deepskyin...ure_Ver_1.0.pdf

 

Eliminating on-axis error, then off-axis error is critical to getting the RC well aligned. If your secondary mirror is spotted, the usage of the Takahashi Collimation Scope is quite desirable. If it is not spotted, then the scope really has no true value against the laser tools you are using now. At any rate, if you look through the DSI method you can make far more sense of what you are trying to correct, and how to correct it appropriately. I read through the guide a good 5 times or so trying to soak up all of the info they make available. 

 

Be sure to download the balanced and unbalanced sample fits they provide from a 16803 chip. It really helps put things into perspective to see the actual data results. 



#9 Salacious B Crumb

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Posted 22 March 2019 - 11:36 AM

Thanks Bill. I'm starting to sense that the CCDInspector might in deed be too dependable of the seeing conditions. I have your document in question and if I remember correct, I didn't like it because you need to rotate the camera with this method. But I guess if it gives better results, I should try it anyway. Now the big question is, do you know / have experience if I can use an artificial star with this method? 

 

I compared my 5 best and worst FWHM subs from last night to the ones I took in January before I touched the collimating screws. It seems that I went at least to the right direction even if the collimation is not yet great. The ones on the right are from January in case it's not clear.

 

 

- Mikko

 

Capture13.JPG

 

 

 



#10 choward94002

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Posted 22 March 2019 - 11:58 AM

CCD Inspector does it's best work when it's in a starfield, not so much on an individual star.  For what it's worth, here's my protocol:

 

At the beginning of each session, slew to a known good star at the same altitude as my target for the evening and use Frank's MetaGuide for a collimation check: it will tell me if I'm good or not pretty quickly.  SharpCap can also work, I like MetaGuide

 

If things look off, I will bring the OTA in and use a Hotech SCT collimator (with the OTA at the same angle as the general altitude) to get it pretty well dialed in as well as ensuring the secondary is centered on the optical center.  You can also use a Hubble artificial star, just use a couple of convex mirrors to reduce the "dot" down to a few microns (I use that with my 8's and 6's as the Hotech doesn't work lower than 9.25")

 

At night I'll find a nice cluster somewhere with at least 20 good stars (again at the same general altitude) and use CCD Inspector to dial it in.  You want to get it so that the "red" corners are all uniform and balanced

 

A few notes ...

 

- Make sure that you collimate at the angle that you will take pictures at ... many people collimate when the OTA is horizontal, then wonder why it looks different when the OTA is nearly vertical in use ...

- The C14's (and C11's to a lesser extent) suffer from mirror flop which can throw off the collimation.  Once I have finished collimating I use some blue painters tape to mark the "down" position of the collimation and simply don't let the OTA go vertical or inverted from that mark (so, no meridian flips) ... problem solved.  If you don't do this, be aware that your collimation may get goofy as you go across the zenith

- Any kind of impact on the OTA can also throw off the collimation

 

Clear skies!



#11 drmikevt

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Posted 22 March 2019 - 02:25 PM

- Make sure that you collimate at the angle that you will take pictures at ... many people collimate when the OTA is horizontal, then wonder why it looks different when the OTA is nearly vertical in use ...

This is only necessary if you are using a non-locking primary, like in a standard SCT.  It  should not be an issue with an RC where the mirrors should not move once adjusted.  


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#12 rockstarbill

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Posted 22 March 2019 - 02:26 PM

Thanks Bill. I'm starting to sense that the CCDInspector might in deed be too dependable of the seeing conditions. I have your document in question and if I remember correct, I didn't like it because you need to rotate the camera with this method. But I guess if it gives better results, I should try it anyway. Now the big question is, do you know / have experience if I can use an artificial star with this method? 

 

I compared my 5 best and worst FWHM subs from last night to the ones I took in January before I touched the collimating screws. It seems that I went at least to the right direction even if the collimation is not yet great. The ones on the right are from January in case it's not clear.

 

 

- Mikko

 

Yeah getting the camera aligned correctly really does help. The artificial star is not likely to be of much use, since you need to look at the whole field of stars to judge on-axis vs off-axis error. I suppose you could move the artificial star in the FOV to different spots on the chip but that seems very tedious to me.  Do you have shots of your star field outside of focus? Seeing that would speak volumes about the current state of your collimation. 



#13 WadeH237

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Posted 23 March 2019 - 08:34 AM

This is only necessary if you are using a non-locking primary, like in a standard SCT.  It  should not be an issue with an RC where the mirrors should not move once adjusted.  

On a typical RC, where the focuser is attached to the back of the telescope, this is correct.

 

On the solid tube (and older truss tube) GSOs, the focuser is attached to the primary mirror cell.  If you have a heavy imaging package, then it's weight is supported completely by that mirror cell.  Also, these scopes have a lot of back focus, so that heavy imaging package is often attached to extenders that give a strong mechanical advantage.



#14 Salacious B Crumb

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Posted 26 March 2019 - 11:32 AM

I re-read the DSI collimation instructions and they didn't look any easier than the previous time. I keep wondering how difficult it is to see the brighter sides of the defocused stars and if I can tell the difference, how do I translate that to adjusting the collimation screws other than with trial and error (like now). Anyhow, I need to forget my collimation issue for now as I have bigger fishes to fry. Namely the latest subs I've taken show still significant image softness and I need to understand whether this is a focusing issue or something else. More here... https://www.cloudyni...irst-with-a-rc/

 

 

Cheers,

 

Mikko




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