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I hit a brick wall collimating my Cat

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#26 WadeH237

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Posted 23 June 2019 - 07:34 AM

Sorry.  It's not my opinion.

 

It is well established.

That was a bit short.

 

Your discussion of software vs hardware zoom is unrelated.  A telescope (or any lens, really) creates an image in space at the focal plane.  The size of that image is determined by the focal length and only the focal length.  When you put a sensor at the focal plane to capture the image, you can express the characteristic of the image in terms of scale, in arc seconds per pixel.  That's what we talk about in astrophotography.

 

To talk about "magnification", you need to have some kind of reference.  In the case of visual use, you can compare the angular size of the rendered image to the angular size that your eye would see.  The focal length of the telescope, divided by the focal length of the eyepiece determines this.  This is what we talk about when we use the term "magnification" with a telescope.

 

With imaging (except afocal), there is no eyepiece and no eye to use as a reference for magnification.  You just have the image at the focal plane and the sensor's pixels.

 

In terrestrial photography, there is a term "magnification".  I'm not a terrestrial photographer, but I believe that the photographical use of the term represents the size of the field of view versus some reference, which is probably a 35mm frame at some specific focal length.

 

So while you may not think that you can dispose of the term "magnification" in astronomical imaging, you'll find that you'll have more effective communication if you don't try to toss out the established terminology and science in exchange for you opinion.

 

Just my two cents,

-Wade


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#27 psandelle

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Posted 23 June 2019 - 10:37 AM

I agree with Wade about jargon. It is important to a given field for clarity of communication in both distance and time. People think lawyers have "lawyer speak" just to confuse people and bill more hours (which, might also be true), but if the field of law did not have an agreed-upon jargon, then making a contract in the 1970's in California, would not be able to be read and enforced in Massachusetts in 2019, as the meaning of the words would be in question. But because of the jargon, certain words mean exact things. It's also like how common-speak has taken the world "theory" from science and turned it into what the word "hypothesis" means in science...which is a huge problem.

 

I was going to add a little more, but found a good thread from the past about this which is worth a read: https://www.cloudyni...-magnification/

 

Also, if I may (if not, moderators, I'll gladly delete), repost something Jared Willson wrote:

 

As others have mentioned, you can't quite do this. A camera doesn't correspond to a particular magnification. Think of it this way... Assume your camera's image is displayed on your computer screen. The amount of magnification would depend on how big the screen was, whether you were viewing at 100% size, and how far away the screen was. A photographic print would work the same way. How big is the print? Is the picture cropped? What's the viewing distance.

Would you say a billboard has a heavily magnified view? Sure seems like it if you are up close, but not if you are looking at it from 1,000 meters. The camera itself has no predetermined magnification.

 

It seems counter-intuitive. You think of a billboard, for example, as being magnified since you can say that the objects on the billboard are 10x larger than life or 3x larger than life or whatever. But that's only the case if you are taking a picture of a small object--like a person or a cell phone or something like that. But if you take a picture of a mountain and put it up on the billboard are you going to say the billboard is 5,000x smaller than life?

 

I know a lot of people try to convert the camera field of view to a particular eyepiece magnification, but I think that's a bit misleading. For example, my camera/scope combination provides roughly the same field of view as a 35mm Plossl which corresponds to about 60x magnification (rounding up slightly). Now let's assume I crop the image to show just half the field. Am I suddenly at 120x just because I cropped it? It's the same camera and the same scope--did I gain magnification? Just by cropping? Going the other way, if I show the image on my computer as a tiny thumbnail, is the magnification reduced? It's still the same image, after all, how can I say the magnification changed?

 

I think a better approach if people ask you how magnified the picture is is just to tell them how much of the sky they are seeing. Tell them they are seeing an amount of sky equivalent to a postage stamp held at arms length (about right for my scope/camera combination) and you are telling them something much more meaningful than a 60x magnification number that is dependent on cropping, etc.. That let's people visualize, for example, why Orion's belt isn't in the picture when I photograph the Orion Nebula.

 

Paul



#28 Ballyhoo

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Posted 23 June 2019 - 11:35 AM

\

 

I think a better approach if people ask you how magnified the picture is is just to tell them how much of the sky they are seeing. Tell them they are seeing an amount of sky equivalent to a postage stamp held at arms length (about right for my scope/camera combination) and you are telling them something much more meaningful than a 60x magnification number that is dependent on cropping, etc.. That let's people visualize, for example, why Orion's belt isn't in the picture when I photograph the Orion Nebula.

 

Paul

Well as I think I said i my post, I need to devote more time to the intricacies here. It is my weak-point among other things ) But the analogy about how much sky they are seeing, not all FOV's have the same detail. More image scale means more detail, or resolution, no?  You can have two images of the moon, but one will have more detail.  In my laypersons thinking, I think of increased resolution is, you can get closer to the object, and at the same time, not lose as much detail.

 

 

Edit, but

 

I know a lot of people try to convert the camera field of view to a particular eyepiece magnification, but I think that's a bit misleading. For example, my camera/scope combination provides roughly the same field of view as a 35mm Plossl which corresponds to about 60x magnification (rounding up slightly). Now let's assume I crop the image to show just half the field. Am I suddenly at 120x just because I cropped it? It's the same camera and the same scope--did I gain magnification?

 

Exactly the point I am trying to make.  But I understand terminology is very important. But what I am trying to show, is that some concepts yo cannot get rid of. I am in no position to debate this because I know people here know more about this stuff than I do.

 

edit, but what does increased magnification do? it increased detail per unit area. That is what increased image scale does, right? so that is why I am saying the two are analogous.   I am not trying to be a heretic to terminology, but show how we express same concepts, in different contexts


Edited by Ballyhoo, 23 June 2019 - 11:49 AM.


#29 Ballyhoo

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Posted 23 June 2019 - 11:55 AM

That was a bit short.

 

 

You mean, a long methodical beating is better than  drop-kick?  lol.gif uhhoh. here it comes....


Edited by Ballyhoo, 23 June 2019 - 11:55 AM.


#30 psandelle

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Posted 23 June 2019 - 11:57 AM

Well as I think I said i my post, I need to devote more time to the intricacies here. It is my weak-point among other things ) But the analogy about how much sky they are seeing, not all FOV's have the same detail. More image scale means more detail, or resolution, no?  You can have two images of the moon, but one will have more detail.  In my laypersons thinking, I think of increased resolution is, you can get closer to the object, and at the same time, not lose as much detail.

 

 

Edit, but

 

I know a lot of people try to convert the camera field of view to a particular eyepiece magnification, but I think that's a bit misleading. For example, my camera/scope combination provides roughly the same field of view as a 35mm Plossl which corresponds to about 60x magnification (rounding up slightly). Now let's assume I crop the image to show just half the field. Am I suddenly at 120x just because I cropped it? It's the same camera and the same scope--did I gain magnification?

 

Exactly the point I am trying to make.  But I understand terminology is very important. But what I am trying to show, is that some concepts yo cannot get rid of. I am in no position to debate this because I know people here know more about this stuff than I do.

 

edit, but what does increased magnification do? it increased detail per unit area. That is what increased image scale does, right? so that is why I am saying the two are analogous.   I am not trying to be a heretic to terminology, but show how we express same concepts, in different contexts

Again: two different things, though, and, although analogous metaphorically, are not analogous literally (which is what counts, for clarity's sake and scientific communication). "Theory" is a great word. Many people use it in common-speak to me "an unproven idea" and therefore say things like "the Theory of Relativity is not proven, and therefore my theory of Bozo Continuum Nachos is just as valid, because Relativity is just a 'theory.'" In fact, in science, the word "theory" is the closest thing to a fact science has (since science always leaves the door open for new data which changes things). What the common-speaker means is "hypothesis," which is an unproven idea to scientists. Same here. Although magnification may "seem" the same, to add it to a discussion about imaging is to confuse the issue.

 

Paul


Edited by psandelle, 23 June 2019 - 12:01 PM.


#31 Ballyhoo

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Posted 23 June 2019 - 12:01 PM

Again: two different things, though, and, although analogous metaphorically, are not analogous literally (which is what counts, for clarity's sake and scientific communication). "Theory" is a great word. Many people use it in common-speak to me "an unproved idea" and therefore say things like "the Theory of Relativity is not proven, and therefore my theory of Bozo Continuum Nachos is just as valid, because Relativity is just a 'theory.'" In fact, in science, the word "theory" is the closest thing to a fact science has (since science always leaves the door open for new data which changes things). What the common-speaker means is "hypothesis," which is an unproven idea to scientists. Same here. Although magnification may "seem" the same, to add it to a discussion about imaging is to confuse the issue.

 

Paul

okay, there not the same thing.... Then in imaging, how do we express two FOV's of the moon, but one instrument (camera and telescope,etc) yields more detail? Is that simply an expression of image scale? 



#32 psandelle

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Posted 23 June 2019 - 12:06 PM

Are you talking actual "resolution" of the image when you say "detail," or just the changing of the FOV, but the resolution stays the same, or gets less (but the image we're looking at is on our computer screen differently? The link I sent from all those years ago has a lot of what you're asking explained well. Take a look and see if that answers your question better.

 

Also, when you say "one instrument (camera and telescope,etc)" what changes then, if all is the same?

 

Paul


Edited by psandelle, 23 June 2019 - 12:07 PM.


#33 Ballyhoo

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Posted 23 June 2019 - 12:11 PM

Are you talking actual "resolution" of the image when you say "detail," or just the changing of the FOV, but the resolution stays the same, or gets less (but the image we're looking at is on our computer screen differently? The link I sent from all those years ago has a lot of what you're asking explained well. Take a look and see if that answers your question better.

 

Also, when you say "one instrument (camera and telescope,etc)" what changes then, if all is the same?

 

Paul

I think you are right, I need to read up. then I will pick this up. 


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#34 WadeH237

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Posted 23 June 2019 - 02:27 PM

You mean, a long methodical beating is better than  drop-kick?  lol.gif uhhoh. here it comes....

Hardly :)

 

I just thought better of my first response.  It was accurate, but did nothing to educate or even open it to more discussion.  After a bit, I thought that it would be better to offer a brief explanation.



#35 Magnetic Field

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Posted 24 June 2019 - 02:30 PM

I think I understand the general concept, but no matter what I did last night, I could not get the secondary shadow to move from the left side of the de-focused star.  I put a pencil at the thin side, and I used the opposite two knobs to loosen, so I could tighten up the knob on the thin side. And I tried the reverse. But really, no matter what I did, I could not change where the secondary shadow was, except to make it worse.  What happens when we hit a brick wall collimating these?  I should have been able to solve this last night but I could not. 

I have never done a SCT but my Vixen VMC 110L is a pain in the **** and I wager a bet it is more difficult than a SCT.

 

Would it be possible for you to a buy a cheap Chesire collimating eyepiece (I paid £15 on ebay).

 

You can start over again in the living room by fully loosening the collimation screws. Of course the Chesire will trick you into believing the scope is collimated but it will not be.

 

However, you are close  and can finish off with a star in focus and nudge the collimation screws ever so slightly to find perfect collimation. I don't care how the intra- and extra focal pattern looks like. All that counts is an airy disc surrounded by concentric airy rings.



#36 Mongo75

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Posted 09 July 2019 - 11:34 AM

Ok, so I'm a newb, and having a real tough time getting my collimation. I have the Celestron 8" XLT OTA. I'm switching between my DSLR and a 25mm EP. I'm using a DIY artificial star, a milk bottle cap painted flat black with a pinhole from a very tiny sewing needle fitted very tightly over a LED flashlight set about 49-50 feet away. The DIY is not optimal, I know, but it was fast and cheap, and meant to be used as a reference only.

 

Here's my problem. When I unfocus all I see is a crescent, not the donut shape. I've done just about everything I can think of, from reading this thread and others, but no matter what I do, I can't get the donut shape. I've tried loosening all three screws all the way as a starting point, then tightening all three all the way. Nothing! I see the mirror moving as I make adjustments and recenter, but the shape never changes. It's always the crescent shape.

 

HELP!!!

 

     - tony -

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#37 WadeH237

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Posted 09 July 2019 - 01:48 PM

Before jumping to any conclusions, I would recommend pointing the telescope at a real star and seeing what it looks like.

 

If you want a sanity check, go and read post number 22 in this thread.  In particular look at the "do this in the daytime" part.  That's a good way to verify that the collimation is not ridiculously far out of whack.



#38 carolinaskies

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Posted 09 July 2019 - 05:58 PM

Ok, so I'm a newb, and having a real tough time getting my collimation. I have the Celestron 8" XLT OTA. I'm switching between my DSLR and a 25mm EP. I'm using a DIY artificial star, a milk bottle cap painted flat black with a pinhole from a very tiny sewing needle fitted very tightly over a LED flashlight set about 49-50 feet away. The DIY is not optimal, I know, but it was fast and cheap, and meant to be used as a reference only.

 

Here's my problem. When I unfocus all I see is a crescent, not the donut shape. I've done just about everything I can think of, from reading this thread and others, but no matter what I do, I can't get the donut shape. I've tried loosening all three screws all the way as a starting point, then tightening all three all the way. Nothing! I see the mirror moving as I make adjustments and recenter, but the shape never changes. It's always the crescent shape.

 

HELP!!!

 

     - tony -

Mishapen hole of your point light source pinhole is likely your problem here along with bad light source. 

Use a thinner material and spin the pin as you move it back and forth through the material.  Also, you'll want a diffuser between the light source and the hole. Without this the light waves are inconsistent.    The Hubble Optics artificial star man buy can be made simply using a DIY $3 flashlight with a diffuser material and multiple size pinholes in a sheet of plastic.  

5star-6.jpg

 


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#39 Mongo75

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Posted 10 July 2019 - 09:29 AM

Thank you for the replies :)

 

I'm going to use a real star in the very near future, and forego the DIY method. The adjustment screws on the secondary are so tight, and it requires so much pressure to turn them, they're already starting to round out, so I ordered a set of Bob's Knobs yesterday. As soon as they get here, it's out to the back yard to get'er done the right way.



#40 munirocks

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Posted 13 July 2019 - 12:32 AM

There are two basic types of secondary mirror mount: one with a central pivot, and one without. 

 

If yours has a central pivot, you must loosen one or two of the three screws when tightening the third one. 

 

If yours has no central pivot then you can adjust one screw at a time. These usually have springs on the screws to remove slop. If one or more of the screws is tightened so much that the spring (or anything else) is bottomed out so you can't turn it anymore, this would give you the brick wall mentioned in the OP, and which I experienced the first time I tried (and failed) to adjust collimation. In this case you'll need to back off all three screws one complete turn and start again. 

 

Figure out which of the above systems you've got, and tighten/loosen as appropriate. Collimation screws shouldn't need to be that tight. 


Edited by munirocks, 13 July 2019 - 12:38 AM.

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#41 Mongo75

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Posted 16 July 2019 - 02:41 AM

Thanks again, to all!

 

Lessons learned...use a real star!!!!!
Order collimation knobs from Bob's Knobs!!!!!
Don't look at the moon without a moon filter!!!!!

 

I received my Bob's Knobs last Saturday, and replaced the factory screws. Following the instructions, I got the collimation in the ballpark looking through the end of my scope and eyeballing it. I could tell it wasn't going to take much to get it done once I got it outside at night. It's been hovering around 100 degrees during the day, and by the time it gets dark enough, I'm drained, so I've been procrastinating, lol.

 

Last night at 21:30, I got a spurt of energy and headed out to the back yard with all my gear, set up, and got a poor but usable polar alignment. Still fearing the worst, I decided to have a look at the Moon and Saturn, being as they were so close together. OMG! That dang ole moon is bright! I ordered a moon filter, the Orion 5560 1.25-Inch Variable Polarizing Filter while waiting to get my night vision back.

 

Night vision back, I took a look at Antares. A bit out of focus, but not too bad. Time to start tweaking those knobs. After about 10 minutes, I had it really close. Time to have a look at Arcturus. WOW! looking good. Just a couple more tweaks and there it was. Due to my poor eyesight, I can't get a real sharp focus, but now, the fuzziness is concentric, not flaring off to one side like it was when I started.

 

Again, thanks to everyone for the input. It really helped!


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