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optical path alignment

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#1 steve girard

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Posted 20 June 2013 - 02:39 PM

Hi Everyone
I’m no optical engineer but I do have some common sense (I think). I have a 10” LX200GPS mounted on a pier in my observatory. I noticed early on that when I use a focal reducer to widen my field I of course get some vegnetting. This did not bother me except when I would focus I noticed the stars in the upper left and lower left would not focus the same place and the rest of the field (coma maybe?). It also caught my attention the flat field appeared not to be centered in the camera frame. That is to say the lighter center was not in the center and the darker corners were larger in the upper left and lower left than on the other side. I was using a DSI Pro at the time and did not think about it because I assumed the chip my not have been center correctly in the camera. Then I updated cameras to a SBig ST2000XM and found the same problem. I check collimation several times and found it OK. I have rotated the camera around to different positions and the problem stays orientated to the scope and not moving with the camera. I feel the main mirror in not aligned properly with the rear cell but don’t know how to prove it or what to do about it if it is.
Does anyone know what I am talking about and what I can do about it. It is not a deal breaker but is bothers me because I know something is not right.
Thanks
Steve

#2 Cliff Hipsher

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Posted 20 June 2013 - 03:30 PM

You wouldn't be using a diagonal when you collimate, would you? If so, try it using a visual back...

#3 steve girard

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Posted 20 June 2013 - 04:42 PM

Hi Cliff

NO - I do not use a diagonal - not sure what you mean by "visual back". The light comes off the secondary straight out the back and into the reducer and then into the camera. I then de-focus a star until I can see the secondary shadow and measure (on the screen)all the way around the image to insure the secondary is centered.

Steve

#4 Christopher Erickson

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Posted 20 June 2013 - 05:28 PM

It DOES sound like a collimation issue. Possibly the threaded back port isn't square with the optical path or the baffle tube is not square. Sort of hard to tell without putting the entire scope on an optical bench and going through it.

A barlowed-laser collimator might show something interesting but also might not.

Another possibility might be the Hotech SCT laser collimator.

http://www.hotechusa...tegory-s/23.htm

I'll invite David Ho to come over and join this thread, if he has time.

#5 Cliff Hipsher

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Posted 20 June 2013 - 07:12 PM

A visual back is nothing more than an accessory holder/eyepiece holder that threads onto the rear port of an SCT. I have a Blue Fireball brand that will hold 2" accessories/eyepieces. It also has a 2" to 1.25" adapter so I can use smaller EPs/accessories.

With that being said, I'm not sure I'd trust an image on a laptop screen over what I can see with my own eye(s)...

#6 steve girard

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Posted 20 June 2013 - 09:24 PM

Thanks for responding everyone
Like I said, I’m no optical expert but I will add this. On occasion I have produced a ghost image on the screen. I played around with that one time and found when I centered the star the ghost was off to one side and slightly low. I kept on moving the star around and got the ghost closer and closer to the star until I finally was able to hide the ghost directly behind the star. I would think this would only happen when the star was on the center axis of the optical path, but my star was not in the center of the field. This leads me to believe the center of my optical path is not the center of my CCD chip. But it could mean nothing, I’m not sure. I may be all excited over nothing.
Steve

#7 Markigno

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Posted 21 June 2013 - 07:53 AM

Hello, it seems a problem of collimation, but it could also be the focal reducer, or the sensor is not perpendicular to the optical axis of the primary mirror. However, without a picture it is very difficult to make a diagnosis. In general line, to solve any problems, we need to understand the cause. If it was just the collimation, the problem is easily solved. Point the telescope in a portion of the sky full of stars in the Milky Way of magnitude 7 or 8. Set the live view to 1 second continuous and de-focus the image slightly and look at the shape of the stars. If you have only some stars not collimated and all facing the same direction, then you can collimating slightly the secondary mirror towards them. If you do not solve the problem, then you need the tools to analyze more deeply the problem. As a first step download CCD Inspector (it's free for 30 days) and analyzes the image (the one with many stars). Among the many information, look at the data of the inclination of the image. If the percentage values deviate much from zero, then the sensor is not perpendicular to the optical axis. In this case it is necessary to bring the telescope at a specialized center that uses an optical bench, or, alternatively, obtain a Hotech Advanced CT laser collimator. This tool is not a simple laser collimator, but it is a real optical bench that allows a perfect alignment of optical / mechanical. In addition, David is a very nice person and always willing to help.
Marco

#8 HOTECH

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Posted 25 June 2013 - 05:18 AM

As Marco mentioned, this issue can be compounded from the orthogonality of the focal plane and the primary optical axis or centration of the focal reducer in the optical train.

A. Try the image analyzer s/w tool to verify the sensor squarness first
- This step helps to confirm the orthogonality of the sensor to the final optical train
B. Identify the centration of final optical axis at sensor.
- From your description, it is possible that your primary optical axis is not orthogonal to the focal plane due to mirror flop or offset between the mechanical and optical axis of the primary mirror. The offset ghost image you see is a good indication of some where in the optical train is not lined up. You will need to verify this with the help of the Advanced CT Laser Collimator in the following sequence.

1. Co-align the collimator and remove the secondary mirror first to see if both primary mirror reflection and focal plane reflection (from the reflector mirror) overlaps. If not, you will need a tilt capable focuser to correct this. This step will help you line up the focal plane to the primary mirror.

2. Verify secondary mirror optical centering in the primary mirror’s optical axis. Drop in the secondary mirror and observe the center shadow on the target to see if it cast a symmetrical ring correspond to the target ring. If not, you will need to nudge the secondary mirror to center the shadow. Only 1mm correction will provide great improvement in optical efficiency.

3. Collimate secondary mirror tilt with the three laser reference on the collimator target.

4. Verify if the focal reducer is centered by observing the converged lasers behind the reflector mirror. If the lasers are all in the center, this means the final optical train is concentric in the focuser. If not, you will have to iterate step 2 & 3 to line all up.

Can you rotate the reducer in the scope?

Clear Skies!
David

#9 Christopher Erickson

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Posted 25 June 2013 - 10:22 PM

Here is a link to an excellent thread on SCT collimation issues:

http://www.cloudynig...5711542/page...

#10 steve girard

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Posted 27 June 2013 - 10:48 PM

Hi David

Boy! You got over my head in a hurry. I will have to read your post several times and try to stay with you. I will add that I have sent my ST2000 in for maintenance. They are going to check everything out and clean the chip. I made the mistake of thinking I could clean the chip but was I wrong. So I don't expect to get it back for another week to 10 days. I assume most of the checks you are talking about would require the Hotech SCT laser collimator talked about in a previous post. I do not have one of those but have not ruled out getting one. I also don't know what the image analyzer s/w tool is. Is it something you can download or do you have to buy it. Sorry for the lack of knowledge but I would like to learn. You asked about rotating my reducer. Everything from the scope back is screwed on or held in place my set screws. It goes like this, Meade zero shift focuser-6.3 focal reducer-spacer-sbig filer wheel-sbig st2000xm. I have it figured that I am shooting at f5. So, if you want to try to get me understanding your ideas better, that's great, but if you don't I understand that also. Sometimes its like pushing a log chain to get my "cork to bobbin"

Thanks for responding

Steve

#11 Markigno

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Posted 28 June 2013 - 06:14 AM

Hi David

Boy! You got over my head in a hurry. I will have to read your post several times and try to stay with you. I will add that I have sent my ST2000 in for maintenance. They are going to check everything out and clean the chip. I made the mistake of thinking I could clean the chip but was I wrong.

Hi Steve,
I do not understand why you did not clean the sensor yourself. Also I have a St2000XM and I run the sensor cleaning 2 times with great success. This is a delicate operation, but not difficult to do, just follow the correct procedure and use the right tools for cleaning the sensor, that's all.
Marco

#12 steve girard

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Posted 28 June 2013 - 11:16 AM

Hi Marco
Like I said in my post, I did try to clean the chip myself and made a mess of it. The more I cleaned the worse it got. I could move the dust donuts around but could not get rid of them. I must not have the proper tools. I used alcohol, q-tips, compressed canned air, cleaning cloth for my glasses wrapped around a q-tip, and could not get it clean. I gave up and decided to let someone do it correctly before I screwed something up. Maybe being a heavy equipment diesel mechanic all my life I’m to sledge hammer orientated. But you know mechanics, we think we can fix anything.
Steve

#13 Markigno

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Posted 29 June 2013 - 02:53 AM

Hi Steve, from your description you did not use appropriate tools. You must use a specific cleaning kit for cameras sensors, by cutting the spatula to right size of Kai 2020 sensor. Especially do not use compressed air (spray), but only use a manual pump. On many websites you will find several tutorials on how to properly execute the sensor cleaning. However, cleaning the sensor in a SBIG camera, is very seldom needed. Donuts of dust present in the images are normally caused by dust on the external filters or at the most by dust on the window outside of the camera, then removable without disassembling the camera. Dust and dirt that are placed directly on the sensor do not look like donuts blurry, but look as small black spots almost in focus.
Marco






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