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Barlow hitting bottom of focuser

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#1 Jimceez

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Posted 11 August 2020 - 07:34 AM

Hi everyone,

 

I’m new to the hobby and just bought a used Orion DOB XT8.  I have some questions that maybe someone can help.

 

1. I tried using the Barlow that came with the scope with the 10 mm & 25 mm eyepieces but it seems I’m hitting the bottom of the focuser.

 

2. I was looking at Jupiter last night with the 10mm and it just doesn’t seem crisp at that magnification.  Saturn looked better so I’m wondering about atmospheric distortion.

 

I’ve tried to collimating the scope as best I can.  I’ve done it 4 times in the last 4 days thinking it may be off.  I have a bit of OCD so a miss is as good as a mile to me. I made my own. Cap and have a Celestron Cheshire piece.

 

any help would be appreciated.  Thanks.



#2 rhetfield

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Posted 11 August 2020 - 07:56 AM

On collimation, take a look at the following writeups:

 

https://garyseronik....to-collimation/

 

https://garyseronik....pe-collimation/

 

The scope was originally sold with only the 25mm.  The rest were added later by the prior owner.

 

I like to focus on Saturn to get the rings crisp, then go to Jupiter.  Jupiter sometimes suffers from too much brightness.  I sometimes add a variable polarizer to cut it back a little.  If you are too much past bedtime, Jupiter will be lower in the atmosphere than Saturn and may see a little more intererence.

 

I would take a picture of the barlow and of the focuser and post it.  Maybe somebody will see something out of wack with it.

 

Edit:  Another thing to note - the telescope originally sold with a 2" focuser and a 2"-1.25" adapter.  Make sure everything is kosher with that combination.  If the focuser is not a 2", then you know it was swapped out at some point.


Edited by rhetfield, 11 August 2020 - 08:10 AM.

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#3 jiblet65

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Posted 11 August 2020 - 07:57 AM

I'm sure others will know much more but from looking at the specs your highest useful mag should be 300x so even the 10mm EP with the Barlow should be well within range. Not sure what's going on with the Barlow but you'd think a stock one would be designed for it. Is it flush or does it ride a little high when seated in the focuser?



#4 sg6

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Posted 11 August 2020 - 08:14 AM

Odd what is said about the barlow and focuser as I would have expected the barlow to have an insertion length the same as an eyepiece, so guessing something is wrong. Suppose the obvious is the presence or absence of a 1.25" adaptor in the focuser.

 

I suppose the barlow can only go in one way.

 

The 10mm and the scope will be I guess 120x, think the scope is f/6. Scope should deliver that but I suggest you have a mental limit of 200x and that could be a case of luck as much as anything. Usuing a max of the aperture in mm is likely a better idea, then anything more is a bonus.

 

Low altitude of the target(s) will be a problem. Also what are the eyepieces.



#5 Jimceez

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Posted 11 August 2020 - 08:31 AM

Thanks for the advice.

 

on the collimating process, I maybe overthinking it. As the article stated, I love to tinker so much so that I might be messIng it up.  I’ll post pictures of the scope to show where I am.

 

i went to relook at the Barlow and it’s not the stock Barlow for the Orion.  It’s a Meade.  I’ll have look at the length of the Meade and Orion Barlows to see if there’s a length issue.  With the Meade I have to take focuser all the way down to get a 10 mm close to focus. It does sit all the way down in focuser. Is it possible to just make it sit higher by tightening?

 

thanks for all the help.  It’s been really fun looking at things at night.  Last night as I was watching Jupiter a satellite passed thru the viewer.  I didn’t get a chance to review which one it was since Im

eft my iPad in the House.


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#6 Tony Flanders

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Posted 11 August 2020 - 09:10 AM

I went to relook at the Barlow and it’s not the stock Barlow for the Orion.  It’s a Meade.  I’ll have look at the length of the Meade and Orion Barlows to see if there’s a length issue.  With the Meade I have to take focuser all the way down to get a 10 mm close to focus. It does sit all the way down in focuser. Is it possible to just make it sit higher by tightening?


Okay, that makes it crystal-clear what is happening. When you said that you had problems in the original post, it wasn't with actually inserting the Barlow, but with getting it to come to focus, right?

 

It sounds as though your stock 10-mm eyepiece just barely comes to focus; you have to crank the focuser knob until hardly any focuser tube is showing, right? You are close to running out of in-focus; you can barely get the eyepiece close enough to the mirror. It's also possible to run out of out-focus, where the maximal amount of focuser tube is showing, the view is getting sharper as you turn the focuser knob, but it's still not sharp enough.

 

In such a situation, the last thing you want to do is seat the eyepiece higher in the focuser tube. Yes, you can do it just by sliding the eyepiece halfway out of the focuser and then tightening the lock screw. But that makes it farther from the mirror, not closer. It's a fix for running out of out-focus, not for running out of in-focus.

 

A Barlow often needs more in-focus than the eyepiece that sits in it. So when you added the Barlow to your 10-mm eyepiece, you did indeed run out of in-focus before the image was sharp.

 

The only reasonably easy fix is to advance the primary mirror on its collimation screws. Instead of bringing the eyepiece closer to the mirror, you're bringing the mirror closer to the eyepiece. Turn all three screws the same amount in the same direction and watch what happens to the mirror.

 

People have a tendency to collimate by turning the screws in one particular direction. So if you do that blindly using all three screws, the mirror tends to creep gradually toward or away from the back of the tube. The fix for that is to scrupulously collimate using only two of the three screws. Then the third screw anchors the mirror at the best possible position -- once you've figured that out.

 

If advancing the mirror on its collimation screws doesn't work either, you have a choice between performing major surgery on the telescope or giving up on the eyepiece that doesn't come to focus.


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#7 Tony Flanders

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Posted 11 August 2020 - 09:17 AM

Hi everyone,

 

I’m new to the hobby and just bought a used Orion DOB XT8.  I have some questions that maybe someone can help.

 

1. I tried using the Barlow that came with the scope with the 10 mm & 25 mm eyepieces but it seems I’m hitting the bottom of the focuser.

 

2. I was looking at Jupiter last night with the 10mm and it just doesn’t seem crisp at that magnification.  Saturn looked better so I’m wondering about atmospheric distortion.

 

I’ve tried to collimating the scope as best I can.  I’ve done it 4 times in the last 4 days thinking it may be off.  I have a bit of OCD so a miss is as good as a mile to me. I made my own. Cap and have a Celestron Cheshire piece.

 

any help would be appreciated.  Thanks.

Here's my bet. First off, 120X is a respectably high magnification for objects that are low in the sky -- which Jupiter and Saturn are, as seen from Pennsylvania. So your view probably is in fact somewhat fuzzy due to atmospheric distortion.

 

Add onto that the fact that Jupiter and Saturn are in fact fuzzy -- they're not called "gas giants" for nothing! You are looking at cloud tops, not a solid surface. It's fuzzy, just like clouds on Earth. And the features on Jupiter are whole big climate systems. Viewing the North Equatorial Belt is like viewing the north trade-wind zone on Earth -- a whole big mess of fuzzy clouds stretching fuzzily over a huge distance.

 

With Saturn, I betcha that what made it look sharp isn't the disk, but the rings, which are indeed very sharply defined, white against black with razor-sharp edges. Your eye seeks out sharpness, so you see those high-contrast ring edges as sharper than a photo would reveal, and the planets' disks as fuzzier than a photo would reveal.



#8 rhetfield

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Posted 11 August 2020 - 09:24 AM

Okay, that makes it crystal-clear what is happening. When you said that you had problems in the original post, it wasn't with actually inserting the Barlow, but with getting it to come to focus, right?

 

It sounds as though your stock 10-mm eyepiece just barely comes to focus; you have to crank the focuser knob until hardly any focuser tube is showing, right? You are close to running out of in-focus; you can barely get the eyepiece close enough to the mirror. It's also possible to run out of out-focus, where the maximal amount of focuser tube is showing, the view is getting sharper as you turn the focuser knob, but it's still not sharp enough.

 

In such a situation, the last thing you want to do is seat the eyepiece higher in the focuser tube. Yes, you can do it just by sliding the eyepiece halfway out of the focuser and then tightening the lock screw. But that makes it farther from the mirror, not closer. It's a fix for running out of out-focus, not for running out of in-focus.

 

A Barlow often needs more in-focus than the eyepiece that sits in it. So when you added the Barlow to your 10-mm eyepiece, you did indeed run out of in-focus before the image was sharp.

 

The only reasonably easy fix is to advance the primary mirror on its collimation screws. Instead of bringing the eyepiece closer to the mirror, you're bringing the mirror closer to the eyepiece. Turn all three screws the same amount in the same direction and watch what happens to the mirror.

 

People have a tendency to collimate by turning the screws in one particular direction. So if you do that blindly using all three screws, the mirror tends to creep gradually toward or away from the back of the tube. The fix for that is to scrupulously collimate using only two of the three screws. Then the third screw anchors the mirror at the best possible position -- once you've figured that out.

 

If advancing the mirror on its collimation screws doesn't work either, you have a choice between performing major surgery on the telescope or giving up on the eyepiece that doesn't come to focus.

In a worse case scenario, you may have to loosen the primary mirror clips to get more play in the collimation screws or put a spacer underneath the focuser assembly if running the primary screws back is not enough.  Neither should be too difficult if you are mechanically inclined.


Edited by rhetfield, 11 August 2020 - 09:26 AM.


#9 SteveG

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Posted 11 August 2020 - 03:20 PM

In a worse case scenario, you may have to loosen the primary mirror clips to get more play in the collimation screws or put a spacer underneath the focuser assembly if running the primary screws back is not enough.  Neither should be too difficult if you are mechanically inclined.

The clips have nothing to do with the position of the primary mirror. Additionally the OP needs more inward focus, so if resorting to moving the mirror it would be loosening the primary screws - not tightening. Shimming the focuser would only make his problem worse.

 

I suggest the OP confirms that he is getting enough inward travel to focus with the barlow. Most likely, the blurry view is due to the planets sitting so low on the horizon.



#10 Ulmer Spatz

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Posted 11 August 2020 - 03:38 PM

I'm not sure if my thinking is correct on this. "Infocus and "outfocus" always confuse me. But some years ago, I bought the Orion 80 mm table top GoScope and the Orion "Shorty" 2x barlow. You focus the scope by moving the objective lens in and out with a rotating knob. With the supplied eyepieces, all was good. But using the barlow, you'd hit the mechanical focus stop just before you got crisp image focus.

 

The solution was to shorten the barlow by taking it apart, grinding down the aluminum spacer about 3 millimeter and putting it back together. In other words, the "Shorty" needed just a smidgeon of post-sale shortening. I suppose it's no longer an exact 2x barlow, but the change was too slight for me to notice.  


Edited by Ulmer Spatz, 11 August 2020 - 03:43 PM.


#11 Jimceez

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Posted 12 August 2020 - 06:38 AM

Thanks for all the tips. I do get what is being said about the primary mirror.  It’s almost just human nature to screw inward.  I’ll check it out.

 

as for Jupiter, I was thinking it was just it being “the gas giant”. Lots of atmospheric disturbance from Earth and Jupiter.

 

later today I’ll post some pics of the views in the scope to show where I am i. Regards to collimating.  Thanks for all the tips!



#12 rhetfield

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Posted 12 August 2020 - 07:58 AM

The clips have nothing to do with the position of the primary mirror. Additionally the OP needs more inward focus, so if resorting to moving the mirror it would be loosening the primary screws - not tightening. Shimming the focuser would only make his problem worse.

 

I suggest the OP confirms that he is getting enough inward travel to focus with the barlow. Most likely, the blurry view is due to the planets sitting so low on the horizon.

Good point - I am thinking the other direction.  Probably also need to take another look at the mirror cell again.  I am guessing that you are saying the screws move the entire cell - which would make sense.  



#13 Jimceez

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Posted 13 August 2020 - 01:12 PM

Here’s pics after my attempts at collimating.

 

1. thru a homemade cap

2. thru a Cheshire tool



#14 gnowellsct

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Posted 13 August 2020 - 01:29 PM

I'm sure others will know much more but from looking at the specs your highest useful mag should be 300x so even the 10mm EP with the Barlow should be well within range. Not sure what's going on with the Barlow but you'd think a stock one would be designed for it. Is it flush or does it ride a little high when seated in the focuser?

The best way to test this barlow situation is to take the scope out in daytime, identify a tree bush or mountain or telephone pole a mile distant, and see if you can reach focus using the barlow.  If you can't then it may be that you need to exchange for a shorty barlow.  I only hesitate to bring this up because if this is the Barlow that shipped with the scope then in theory it should work fine.  But these days you never know.

 

Except for unusually good nights, the seeing cap is usually around 200x to 300x.  I often try 280x and usually end up dialing back to 230x or even 190x.  

 

At 190x, and certainly below that,  I say it's not a good night for planets and move on to other things.  In other words this is not a Barlow issue and it's not a scope issue, or so we hope.  It's a ceiling seeing issue.  People have to let the newcomers know that 200-250x is usual 300x is pretty durn good and anything beyond that is gravy.  I think we'd all move to Florida if they didn't have bad transparency.

 

That said I think the comes-with oculars and barlows are better than they used to be but there's room to improve.  But not much of that improvement will be visible on planets that are low to the horizon.

 

The seeing cap is generalized most places most of the time, you can do better in certain places that the astronomers have identified.  Like the top of Hawaii or the Andes.  So yes you could in theory get 400x on the 8" Newt or 712x on my C14 or 800x on a 16" Newt but don't bet the farm on it.  And this is assuming the optics are perfect.

 

The other side of this discussion is exit pupil and aperture newcomers tend to think that the higher the magnification the more they will see.  But the truth is you see more in a 14 inch aperture at 100x than you do at 100x in a 92 mm.  And I am referring to planetary detail.  When you make that jump on a star field it is amazing how many more stars there can be.  That's just a way of saying the larger aperture brings in more information, and it does that on the planets too.   

 

Magnification is vitally important but in the usual way of thinking exaggerated, newcomers tend to want the extra barlow or the high power ocular when they should be concentrating on getting out of town for better views, or evaluating their immediate surroundings for good and bad places to set up (hot cement or asphalt = bad, grass = good; looking over the roof is bad, etc.).  

 

Greg N


Edited by gnowellsct, 13 August 2020 - 01:35 PM.

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#15 Jimceez

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Posted 14 August 2020 - 06:21 AM

Greg,

 

Thanks for the comments.  I have backed out the primary and redid the collimation.  I put pictures up of what I’m seeing. I’ll test the Barlow soon.  I injured my arm so it’s tough moving the DOB right now.  Hopefully, I can do it the testing this weekend.


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#16 jiblet65

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Posted 14 August 2020 - 07:16 AM

Greg I found out quickly from this place that bigger doesn't always equate to better. With my little Tasco I can use my 20mm EP with the 2.25x Barlow and get a pretty amazing image of the Moon. It takes up almost the whole FOV and for what I'm working with the clarity is pretty nice. I've found with the eye relief you can take in the whole view and if you close in you can get the image to focus in on a particular area or region of the craters or seas. With the Pleiades I can get close to the whole field of stars in view with just the 32mm but any more magnification than that and I'd have to scan around to see all of them. I still have a library worth of knowledge to learn but I feel like I've come down the road a bit since last April when I got my Oberwerk binos. And much of it is largely thanks to posts like yours and many others on here. 



#17 gnowellsct

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Posted 14 August 2020 - 08:35 AM

With my little Tasco I can use my 20mm EP with the 2.25x Barlow and get a pretty amazing image of the Moon.


I suppose any telescope can be tested on the moon and a lot of people do that but the results are not terribly meaningful. That is because it takes an extremely sophisticated observer to do a telescope evaluation on the moon. The moon offers so many bewildering details in any aperture that the view always looks good and the contrasts are so stark that details are always easy to see.

Furthermore picking out details requires a depth of familiarity with the moon that most of us, including myself, don't have. So for example there are very subtle lunar domes that are only visible at certain times of the month. There are certain shades of gray and blue that you only see in certain places and at certain times. And finally in order to ascertain the difference you have to look at a rock next to a rock next to a rock next to a rock where the sequence of rocks gets smaller. And the better scope with more aperture will see that last small rock. If you eliminate all these last small rocks from the view you still have a bewilderingly complex image of the moon.

The benefits of increased aperture are fairly easy to see if you're looking at how the population of stars increases in a familiar field as you up the aperture. The star field around m57 or the Perseus double cluster is an example. The differences aperture makes are quite apparent. But on the moon The increase in granular detail that comes with aperture is camouflaged by the distracting level of details That are already there.

I actually think one might be better off, if one like d to spend hours and hours studying the moon, by beginning the project with a small refractor under 100 mm. It would take a couple of years to get to know the features well and then when you used a larger aperture you would have the familiarity needed to see all the extra stuff that was there to see.

#18 Asbytec

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Posted 14 August 2020 - 09:39 AM

"1. I tried using the Barlow that came with the scope with the 10 mm & 25 mm eyepieces but it seems I’m hitting the bottom of the focuser."

Many focusers come with an extension tube. You may try removing it when using a Barlow so the focuser can bring the Barlow deeper into the focuser travel. The focuser will not bottom out. I had the same problem, except my Barlow came to focus just before the focuser bottomed out. I got lucky. But, with the 2" extension installed, I cannot focus the Barlow with downward focuser movement.

Edited by Asbytec, 14 August 2020 - 09:41 AM.


#19 Jimceez

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Posted 14 August 2020 - 01:33 PM

I tried the Barlow with the 10mm eyepiece looking at a power line tower a mile or so away after backing out the primary mirror. I had plenty of adjustment left in the focuser. I could see the heat waves coming off the beams.  A close one I saw crystal clear.
 

Next test will be with Saturn and Jupiter. I imagine Jupiter will be a touch blurry as it’s atmosphere will effect my view.

 

i posted eyepiece views after the collimation.  They are above.  If someone could comment I’d appreciate it.

 

thanks to all!



#20 Sky Muse

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Posted 14 August 2020 - 02:24 PM

This is an image of the collimation-scene of my 6" f/5 Newtonian...

 

collimation1ab.jpg

 

I'm seeing where at least one of the spider-vanes within your image is not taut, straight, there at the right.  Also, the centering of the secondary's shadow is off.  Make certain that the secondary mirror is centered directly under the focusser.  The cross-hairs of the Cheshire can help with that...

 

secondary coll.2.jpg

 

You will then aim the center of the primary-mirror towards the center of the secondary-mirror with the adjustment-screws of each; a bit of orchestration, but not too terribly difficult.  This tutorial should be of help...

 

http://www.schlatter...y/collimate.htm


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#21 Jimceez

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Posted 14 August 2020 - 04:23 PM

This is an image of the collimation-scene of my 6" f/5 Newtonian...

 

attachicon.gifcollimation1ab.jpg

 

I'm seeing where at least one of the spider-vanes within your image is not taut, straight, there at the right.  Also, the centering of the secondary's shadow is off.  Make certain that the secondary mirror is centered directly under the focusser.  The cross-hairs of the Cheshire can help with that...

 

attachicon.gifsecondary coll.2.jpg

 

You will then aim the center of the primary-mirror towards the center of the secondary-mirror with the adjustment-screws of each; a bit of orchestration, but not too terribly difficult.  This tutorial should be of help...

 

http://www.schlatter...y/collimate.htm

Thanks for advise.  It seems it was off as I tightened the vanes and then measured the distance.  I was afraid to tighten the vanes too much.

 

I’ll work on the vanes and start over.  As a newbie, I’ve read 5 different methods and some contradict each other.  I’ll work with Cheshire exclusively.  That seems to give me the best results.



#22 SteveG

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Posted 14 August 2020 - 04:41 PM

Thanks for advise.  It seems it was off as I tightened the vanes and then measured the distance.  I was afraid to tighten the vanes too much.

 

I’ll work on the vanes and start over.  As a newbie, I’ve read 5 different methods and some contradict each other.  I’ll work with Cheshire exclusively.  That seems to give me the best results.

The 2 pictures you posted again don’t tell us anything. There’s no central disk showing in the collimating cap picture, and there are no crosshairs or any other references in the Cheshire tool picture.

 

Yes your spider vanes are bent. You need to make sure the center screw in the secondary hub is exactly centered in the tube opening. Next make sure the vanes are super tight. That should be the last time you ever have to touch the vanes.



#23 Sky Muse

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Posted 14 August 2020 - 04:51 PM

Thanks for advise.  It seems it was off as I tightened the vanes and then measured the distance.  I was afraid to tighten the vanes too much.

 

I’ll work on the vanes and start over.  As a newbie, I’ve read 5 different methods and some contradict each other.  I’ll work with Cheshire exclusively.  That seems to give me the best results.

When I collimated this telescope, essentially a 5" f/4, my sight-tube with cross-hairs worked best...

 

sight tube - 081819e.jpg

 

...although I did use a collimation-cap to check as well.

 

Secure the Cheshire in place, and just as an eyepiece would be.  Never shim the Cheshire to fit tightly in the center of the eyepiece-holder.  I've done that before, and it's not as accurate.  


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#24 Asbytec

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Posted 14 August 2020 - 06:41 PM

"Make certain that the secondary mirror is centered directly under the focusser. The cross-hairs of the Cheshire can help with that..."

The spider (secondary mount) does not have to be "exactly" centered in the telescope tube. A good attempt to measure and put it as close to center as you dare is good enough. We're not after "perfect" mechanical alignment, just good enough to make precise "optical" alignment easier.

Try this after getting a nice looking centered secondary under the focuser as Sky Muse shows a few posts above. Remove the paper blocking the secondary. You likely need to (or should) check and refine your secondary rotation before you start tilting the secondary. It may be good enough, maybe not.

It can be difficult to eyeball how circular the secondary appears to be under the focuser. When you remove the paper, the primary reflection might be seen almost anywhere in the secondary. The closer it is to the secondary center, the easier collimation will be.

Just because the secondary looks circular doesn't mean it's pointing or sufficiently rotated relative to the primary and the center marker. For that, you might want to actually see the primary reflection because accurate axial alignment is more important than a circular secondary placement. Good refinement of your secondary rotation will save some effort during secondary axial tilt.

If the primary reflection is off the secondary center line, loosen one of the secondary adjustment screws (in-line with the focuser) so the secondary can rotate freely enough. Not sloppy loose, leave some light friction to hold it. Rotate the secondary so the primary reflection is centered on the secondary's major axis. The horizontal cross hair can help guide you as shown in Sky Muse's image, then tighten that screw. Don't worry about centering on the minor axis, tilt will take care of that later.

Now you're *more* ready to tilt the secondary and align the focuser axis with the primary center. Doing so will bring the primary toward center of the secondary pretty much (mostly) along the horizontal cross hair until the primary center hits the (vertical) cross hair. You will save a little confusion and avoid potential difficulty during collimation.

"As a newbie, I’ve read 5 different methods and some contradict each other. I’ll work with Cheshire exclusively. That seems to give me the best results."

Yes, the site tube/Cheshire will better help center the secondary. And yea, many of us have developed techniques to do the exact same tried and true collimation steps. We develop slightly different methods to skin the same cat, to do the same steps and arrive at the same place. I offer mine if you understand and care to try it. Saved me a lot of additional posts on CN.

Edited by Asbytec, 14 August 2020 - 07:20 PM.


#25 SteveG

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Posted 15 August 2020 - 01:49 PM

Make certain that the secondary mirror is centered directly under the focusser.  The cross-hairs of the Cheshire can help with that...

 

 

It is the outline of the Cheshire tube that is used to center the secondary under the focuser, not the crosshairs. You should set the tube so that you can see the outline of the primary (as reflected through by the secondary) just inside the circular opening of the site tube. Actual site tubes do not have crosshairs.




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