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c8 edge collimation problems

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#1 iwols

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Posted 04 May 2021 - 02:19 PM

Hi all just trying to collimate my c8 edge with a 414ex ccd ,but i cant seem to get the image large enough to adjust collimation any advice please ,is it the 414ex is no good with the c8 for this?



#2 pyrasanth

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Posted 04 May 2021 - 02:33 PM

Can I assume that you can bring a star to a focus of course accepting that it may not be as sharp as it could be if not well collimated?

 

If this is so then as a first step pick a star that is fairly bright and defocus so the star looks like a ring with a black central spot. Try and adjust that spot the best you can by centering the spot by eye and bringing the image back to the center of the field after each very tiny screw turn- evaluating after each screw turn- it is very easy to move a screw too far. Never fully loosen more than 2 screws at once.

 

You then need to move toward progressively higher magnification eyepices as you bring the star closer to focus but able to see the rings and the center spot.

 

It is only when you reach a magnification of 35-40 per inch of aperture and have good steady seeing that you might be able to do the final minute turns of the screws to bring the black spot central. This is iterative- so take your time and remeber to move the star back to the middle of the field as you make those tiny but important adjustments.

 

A well collimated SCT is light and day with regard to fine details- especially on the moon and planets that can be resolved.

 

Good luck


Edited by pyrasanth, 04 May 2021 - 02:35 PM.

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#3 jrschmidt2

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Posted 04 May 2021 - 02:36 PM

I presume you are asking about in-focus collimation. Can't you use add a 3x barlow to increase the Airy pattern size?  The only limitation would be the pixel size (6.5 micron, it seems) with this camera.  But a substantial Barlow should do the trick, I would think.


Edited by jrschmidt2, 04 May 2021 - 02:37 PM.

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#4 iwols

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Posted 04 May 2021 - 02:38 PM

thanks but the s

 

I presume you are asking about in-focus collimation. Can't you use add a 3x barlow to increase the Airy pattern size?  The only limitation would be the pixel size (6.5 micron, it seems) with this camera.  But a substantial Barlow should do the trick, I would think.

that might be possible but wont that effect my ccd backfocus



#5 iwols

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Posted 04 May 2021 - 02:42 PM

Can I assume that you can bring a star to a focus of course accepting that it may not be as sharp as it could be if not well collimated?

 

If this is so then as a first step pick a star that is fairly bright and defocus so the star looks like a ring with a black central spot. Try and adjust that spot the best you can by centering the spot by eye and bringing the image back to the center of the field after each very tiny screw turn- evaluating after each screw turn- it is very easy to move a screw too far. Never fully loosen more than 2 screws at once.

 

You then need to move toward progressively higher magnification eyepices as you bring the star closer to focus but able to see the rings and the center spot.

 

It is only when you reach a magnification of 35-40 per inch of aperture and have good steady seeing that you might be able to do the final minute turns of the screws to bring the black spot central. This is iterative- so take your time and remeber to move the star back to the middle of the field as you make those tiny but important adjustments.

 

A well collimated SCT is light and day with regard to fine details- especially on the moon and planets that can be resolved.

 

Good luck

thanks but im using my ccd and not an eyepiece



#6 jrschmidt2

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Posted 05 May 2021 - 09:29 AM

thanks but the s

 

that might be possible but wont that effect my ccd backfocus

Some Barlows have a "screw off" 1.25" threaded lens at the bottom.  It won't give you as large a magnification since the spacing is presumably less, but it works.  Alternatively if the CCD comes with a 1.25" nosepiece that screws on (as do many CMOS cameras) you can use that as well.  I use a Barlow with my ASI224MC for collimation, as do many others with similar cameras.


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#7 iwols

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Posted 05 May 2021 - 10:38 AM

Some Barlows have a "screw off" 1.25" threaded lens at the bottom.  It won't give you as large a magnification since the spacing is presumably less, but it works.  Alternatively if the CCD comes with a 1.25" nosepiece that screws on (as do many CMOS cameras) you can use that as well.  I use a Barlow with my ASI224MC for collimation, as do many others with similar cameras.

thanks thats great advice is the back focus not effected or does it not matter thanks



#8 jrschmidt2

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Posted 05 May 2021 - 11:20 AM

thanks thats great advice is the back focus not effected or does it not matter thanks

Well the "back focus" (in this case, the distance between the sensor and the barlow) will certainly effect the resulting magnification and focus, but should "work" with whatever choice.  The same case with reducers:  if you get the backfocus "wrong", the reduction factor and focus changes but it still functions.


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#9 cuzimthedad

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Posted 05 May 2021 - 11:31 AM

Moving to Cats & Casses


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#10 dcaponeii

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Posted 05 May 2021 - 02:25 PM

I recommend using Metaguide for your collimation.  It works at focus and provides an easy to read indicator as to which direction to move the star as you are focusing.  An IR filter is very helpful as well in suppressing atmospheric distortions.


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#11 iwols

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Posted 06 May 2021 - 06:19 AM

Well i managed to collimate with a x2.5 and eyepiece ok,but when i put my ccd back in and managed to defocus all the stars looked like it needed collimating again,maybe because the length of the 2.5 and eyepiece pulled the centre off(is that possible),anyway image added and would welcome your thoughts again,still need to recollimate with just the ccd ,the stars still look a bit big although it was autofocused well  ,maybe its because ive been using a scope with focal length of 400mm

 

 

new image

test Cigar Galaxysmall.png


Edited by iwols, 06 May 2021 - 06:36 AM.


#12 iwols

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Posted 06 May 2021 - 06:28 AM

and original

 

test2.png


Edited by iwols, 06 May 2021 - 06:28 AM.


#13 pyrasanth

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Posted 10 May 2021 - 02:28 PM

A defining but not neccessarily the best characteristic of the SCT class of instrument is large blobby stars. This is not the fault of the instrument- just the laws of optics. Long focal length instruments have big stars falling on those tiny sensor pixels, at least when the sensor size is ill matched. Some of the finest images with large SCT's come from sensors such as the 16803 with big 9 um pixels- often binned to 2x2 to try and desperately get an image scale on earth that does not belong on the surface of the moon.

 

As you noted, good focus can help as well as shorter exposures so your not spreading that light over too many pixels.

 

Let's do the maths to see what your working with

 

8" SCT has a focal length of about 2000 mm. The 414EX sensor has a pixel sixe of 6.45 um so at F10 we get an image scales of (6.45/2000) x 206.3 = 0.665 arc secs per pixel.  If your seeing is average I would bin the sensor to 2x2 to give you a much nicer 1.33 arc sec per pixel. This will give you nicer stars, easier to guide and a higher SNR. You will not notice the collimation issues as much.

 

Collimation can be difficult if seeing is bad- you will just have to keep at it. Always small amounts with the turn of each screw and carefully evaluating the results. What your eye sees may very well be different from the camera- always try and be as simple as possible with what sits between your eye and the telescope when collimating. Sometimes even a diagonal can be produce very different results by eye to what you finally get with the camera.

 

Nice test images of the cigar by the way- good detail showing. Don't worry about the stars- gentle adjustment is all that is required- just take your time. A tool like CCD inspector can help with fine tuning however it is not cheap.


Edited by pyrasanth, 10 May 2021 - 02:41 PM.

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