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First Scope On The Way! - Couple of Questions

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#1 tmossman01

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Posted 28 October 2020 - 06:51 PM

  • Adding some edits to include answers to questions in original post.... (bullet pointed items)

 

I have a 12''/F5 Dobsonian on the way...  I have a couple of questions/clarifications as I have not purchased a full range of accessories.  Guidance would be appreciated.

 

  • I should have mentioned that this was for an Explore Scientific Truss Dobsonian

 

I have a decent range of eyepieces (for a fast reflector) and am replacing the RDF with a RACI finder.  (arthritis in neck).

 

Collimation:  I am intending to get a catseye set, but do not know if there are any hidden "gotcha's" in that selection.  (AKA, are laser collimation or other things needed?  That is, a cheshire, sight tube, and AC are all that are needed and border on overkill for a beginner (and the AC is realistically optional)... Or am I missing something?)

Note: My brother has a 90mm Newtonian without collimation tools which I have "hand" collimated, so any tools will probably be "super" to me!)

 

  • Added:  All of that is overkill for a new person with no equipment. Get a collimation cap and learn to use it.  It is inexpensive and generally useful no matter the situation.  Once you have that you can know your weak points in collimation skills and get tools specifically to address those needs.  (Also, using your highest power eyepiece and a star test. Polaris is probably your friend for a newbie with a non-tracking mount for this.)

 

Barlow/Focal Extender: I have not purchased these at this point.  I know that they work somewhat differently in the optic behavior, but want to verify whether or not a focal extender disqualifies "Barlowed laser collimation".  This is not something which I have seen answered clearly in my research.

It seems to me that the main advantage of a Barlow is for those with removable optics which can be screwed on "like a filter" but with reduced magnification.  (Slight extenson --- If going focal extender, then Glatter+Tublug is the optimal choice if "laser collimation" is desired?)

 

  • See above, but if Barlowed laser collimation is in play, if you need/want to do this, you can get away with just getting a laser if you already have a Barlow lens.  Better results would be expected if you get a specialized tool for this (e.g. HG laser + TuBlug)

 

Also, are there any good rules for what to evaluate as far as the quality of the scope are? I have found some articles on evaluating optics and such, but don't find a clear checklist to make sure that I am not missing something.

 

  • Careful evaluation of the diffraction pattern for the star test should allow for gross level evaluation of the optics for faults

 

Thanks, all.

Moss.


Edited by tmossman01, 29 October 2020 - 12:11 PM.


#2 Bean614

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Posted 28 October 2020 - 07:29 PM

You haven't stated whether you bought a Truss, Strut, or Solid tube.  Nor have you stated the brand---- these things would have told us what your scope DOES come with!



#3 SeattleScott

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Posted 28 October 2020 - 07:52 PM

Yes you should get a laser collimator of some sort. Lots of options.

Barlow can be good for getting double duty out of some eyepieces. Handy when you are starting out and don’t have a lot. Could also use it to fine tune collimation. As I got more eyepieces I use my barlow less and less.

Scott
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#4 brentknight

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Posted 28 October 2020 - 08:02 PM

Are you confusing barlow with barlowed laser?  They are the same type of lens, but are really two different things.

 

I have a collimation tool called a Tublug that works with a laser collimator.  The barlow in this case diffuses the laser so that the shadow of the center spot can be used to collimate the primary mirror.

 

A barlow just by itself is used to multiply the magnification of an eyepiece.  They normally come in 2X, 3X up to 5X.  Some of these allow you to unscrew the lens from the barrel and then attach that to an eyepiece.

 

Barlowed lasers are very easy to use for collimation, but a sight tube is more general purpose.  Without knowing which eyepieces you have, it's hard to recommend a barlow for eyepiece magnification...


Edited by brentknight, 28 October 2020 - 08:02 PM.


#5 tmossman01

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Posted 28 October 2020 - 08:21 PM

Ok,  I did forget the make/model...  It is an ES Truss, as such it does not really come with much in the range of accessories -  In particular, no collimation tools. I am willing to go with just a collimation cap for the start, it would certainly be better than just the Mk1 Eyeball.

 

As such, I am aware of the fact that moving it around is likely to collimation errors during a session.

 

I have one low power eyepiece and several to give magnifications in the ~100-~200 range. 

 

The main question about the Barlow/Focal Extender is whether a focal extender cannot be used for Barlowed laser collimation.

 

I would be fine with ... get a collimation cap and a (HoTech or other) laser with a decent Barlow with a removable lens assembly (aka, maybe triple the fun).

 

I mainly want to make sure that I am not missing anything......

 

I have scope and eyepieces on the way (and a RACI finder)... (actually coming sooner than expected- supposed to arrive Monday)

 

I know that I need ... I want to make sure that I am not missing anything...

 

1. Collimation tools

2. Eyepiece case

3. Red Light

4. maybe a chair

5. maybe a carpet/padding if I base on concrete/driveway. Dropping something will happen sooner or later.

 

Coma correction and other sorts of things can come later.  I want to learn how to aim at things before I worry about getting more things to refine views.

 

(It is hard as a newbie to digest all of what might be good.... maybe we can put together a good checklist.  I did not find a (reasonably) good sort of checklist in the Useful Links for Beginners, but I might have missed something.)

 

Thanks, all.

 

 


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#6 tmossman01

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Posted 28 October 2020 - 08:33 PM

Are you confusing barlow with barlowed laser?  They are the same type of lens, but are really two different things.

 

I have a collimation tool called a Tublug that works with a laser collimator.  The barlow in this case diffuses the laser so that the shadow of the center spot can be used to collimate the primary mirror.

 

A barlow just by itself is used to multiply the magnification of an eyepiece.  They normally come in 2X, 3X up to 5X.  Some of these allow you to unscrew the lens from the barrel and then attach that to an eyepiece.

 

Barlowed lasers are very easy to use for collimation, but a sight tube is more general purpose.  Without knowing which eyepieces you have, it's hard to recommend a barlow for eyepiece magnification...

Perhaps I am mistaken, that is why I am asking... I was under the impression that one could use a standard Barlow to augment a laser collimator.

 

My only short term reason for getting a Barlow (in my potentially errant belief) was as a collimation aid.  I was trying to ask if what a good set of collimation tools was (or rather, if I got a Catseye set, would I regret missing something like a Glatter+TuBlug --or-- if a lesser alternative might be better.)
 


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#7 brentknight

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Posted 28 October 2020 - 09:26 PM

I've normally used a cheshire/sight tube and then a star test in the field.  The Glatter laser + Tublug is a very recent purchase, and even though it was pretty darn expensive - it makes collimation simple and easy.  With that combo, I check collimation whenever I suspect it might be off - it takes about a minute.

 

There are cheaper setups than mine and others could recommend them, and you can use a standard barlow with a standard laser, but the tools I have work really well when you can't easily see inside the focuser tube.

 

If you need to adjust the centering of the secondary in the tube, then the sight tube would be a better tool, but this adjustment isn't usually a problem and not something you need to do every session.  Also, the autocollimators (like the Catseye tools) can do everything the barlow laser can do - I just thought they were a little more complicated.


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#8 brentknight

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Posted 28 October 2020 - 09:34 PM

Looking at your list, I would recommend you put the chair a good bit higher up...definitely above the eyepiece case.

 

And if your on a budget, I'd probably go with a good sight tube instead of the HoTech.  Actually there are a couple topics currently about the best collimation tools to get first.  Lots of recommendations there.



#9 vtornado

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Posted 28 October 2020 - 10:09 PM

You can use a standard barlow and a reasonable laser collimator.

It is not as  good as what Brent is talking about (tublug) but it is a lot cheaper.

 

http://www.smartavtweaks.com/RVBL.html

 

I have a skywatcher 12 inch dob, and the primary springs are not great.

As a result of bouncing  the scope on a cart while going over lumpy ground, the primary mirror always has to be

collimated.  The laser works well in the dark, it is a  one person job.

It takes about a minute.  Sometimes 5 if the primary really got jostled.

This happens when I can't see the dougnut on window at all and I have to do

several iterations of non-barlow, barlowed to get things right.


Edited by vtornado, 28 October 2020 - 10:14 PM.

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#10 Cames

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Posted 28 October 2020 - 10:29 PM

Congratulations on your telescope choice.  Its power is considerable and will amaze you.  Best wishes for your astronomy adventures.

 

If your experience will be like mine, the more you use your new telescope, the greater will be your enjoyment.  I know that I spent countless hours familiarizing myself with the dob's 'personality' and learned to fine-tune it until we could work together effectively as one.  It required mastering a pretty steep learning curve but it was certainly worth the effort - very rewarding. 

 

Aligning the optics was my first challenge.  Getting all the optical axes to coincide made my observing sessions more productive. I gradually began to understand the particulars of optical alignment. Truth be told, I'm still learning and experimenting with collimation years later.

 

My first breakthrough was replacing the secondary mirror Allen-screws with long knobs that I could twist by hand. Made process of perfecting secondary mirror tilt much easier.  I could never go back to the Allen screws.

 

Next issue I noticed was that my eyepiece adapter had a sloppy fit.  I got two collimating tools right away - (1) a laser collimator with the open side viewport and (2) a Cheshire/reticle/sight tube combination tool.  I highly recommend getting both. They complement each other.  Both tools showed inconsistent mounting alignment each time I inserted them into the adapter.  It took practice just to be able to mount my tools consistently.

 

Another alignment issue involved my RACI finderscope.  It's important to have the optical axis of the finder parallel to that of the primary mirror.  Both the finder and the telescope have to be looking at the exact same spot in the sky.  It took me a while to realize that tilting my primary mirror to collimate, changes the point in the sky where the telescope is looking relative to the external telescope surfaces and the finder has to be realigned after collimating the secondary and primary mirrors.

 

I also found it was helpful to use both the red-dot reflex finder alongside the RACI.  They also complement each other.  I'm glad that you will have both available to you.

-----------

C

 

 


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#11 SeattleScott

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Posted 28 October 2020 - 11:43 PM

Looking at your list, I would recommend you put the chair a good bit higher up...definitely above the eyepiece case.

And if your on a budget, I'd probably go with a good sight tube instead of the HoTech. Actually there are a couple topics currently about the best collimation tools to get first. Lots of recommendations there.

For a 12” F5 the viewing position is high enough that standing works pretty well unless you are quite tall. Certainly worth experimenting with a lawn chair first. Some people are sitters and some people are standers. Depending on where you live, it can get cold this time of year, and standing helps keep body heat up if nothing else.

Scott

#12 Tony Flanders

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Posted 29 October 2020 - 05:39 AM

My main (or at least biggest) scope is a 12.5-inch f/5 Dob. I collimate it exclusively with a sight tube and Cheshire eyepiece. I do not use a chair, except for viewing objects very low in the sky.

 

As far as evaluating optical quality is concerned, a star test reveals all.

 

 


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#13 tmossman01

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Posted 29 October 2020 - 06:44 AM

First, thank you all for your responses, (I have not yet figured out how to respond to multiple posts at the same time, so please bear with me on that.)

 

I've normally used a cheshire/sight tube and then a star test in the field.  The Glatter laser + Tublug is a very recent purchase, and even though it was pretty darn expensive - it makes collimation simple and easy.  With that combo, I check collimation whenever I suspect it might be off - it takes about a minute.

 

There are cheaper setups than mine and others could recommend them, and you can use a standard barlow with a standard laser, but the tools I have work really well when you can't easily see inside the focuser tube.

 

If you need to adjust the centering of the secondary in the tube, then the sight tube would be a better tool, but this adjustment isn't usually a problem and not something you need to do every session.  Also, the autocollimators (like the Catseye tools) can do everything the barlow laser can do - I just thought they were a little more complicated.

I know that you have asked about budget... I have been saving up for a while, so I have "budget".  I am mainly trying to avoid buying something which would not be used.  I should have focused my question a bit better last night, but it would probably have been better stated....

 

"If I get a Catseye collimation suite, do I need anything else to do collimation?" 

then

"If yes, Barlowed lasers seem to be a common preference, if I don't go with something like HG+TuBlug, then if I use a laser with a Barlow, does it need to be a (2 element) Barlow or will a 4 element focal extender also work?"

 

1.  I am still leaning toward getting the Catseye suite (initially).  This may be a bit overkill, but if I get into AP in the future the AC component would be useful.  (aka, meets need plus extended function in the future which I may want.  Additional complexity is not an issue for me.)

 

2. A Barlowed laser makes (spot) collimation in the field easier (assuming a good coarse collimation as a start point).  This is an important consideration for Truss Dobsonians, as the collimation will/may wander as the telescope is slewed.  I am thinking that I can defer the decision on getting this until I get some more field experience.

 

3.  The answer to the second question, "Can a 4 element focal extender be used for Barlowed laser collimation?" is not something for which I have seen a clear answer.  Given the different strategy for achieving the magnification, I suspect the answer is "No".



#14 MellonLake

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Posted 29 October 2020 - 07:03 AM

Get a Telrad, I really prefer a non-magnifying finder for a Dob.  An RACI is also nice but it only helps once you are in the correct patch of sky.

 

 

If you are going to go the cheapo laser and barlow technique, I have done this.  It is not easy to make work perfectly but is cheap.

1) Buy a very cheap barlow, nothing special is needed.  I would just get a Celestron,SVbony etc. 2X barlow to use just for collimation. Any focal extender or barlow should work.

2) The cheap lasers often come incorrectly collimated themselves.  The are often off by enough to cause issues for collimation.  Read up in CN on how to collimate the cheap laser collimators (I put mine in my 2" to 1.25" reducer clamp it in a vice and aim it 10 ft. away.  I then rotate the laser and adjust it until it does not travel in a circle.)

3) The other problem with the cheap lasers is that they do not register well in the focuser (their 1.25" diameter is a little too small.  To address this I bought a Parallizer 2" to 1.25" adapter which I really like (it is a useful tool).  

4) Do star tests to confirm collimation.

 

You will still need the collimation cap to ensure the secondary is centred and rounded.  A laser cannot help with this first step of collimation (which only needs to be done very rarely).  

 

I would suggest buying a UHC filter (Lumicon brand).  These really really help with viewing nebula.

 

Rob 


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#15 rhetfield

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Posted 29 October 2020 - 07:37 AM

For collimation, read these if you haven't yet:

 

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

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

 

For finding stuff in the sky:

https://www.cloudyni...degree-circles/



#16 SloMoe

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Posted 29 October 2020 - 07:38 AM

I might suggest that you go to Youtube and search telescope collimation before you buy any tools, that might give you a better idea of what you need.

 

A better understanding of collimation is free, buying tools and guessing isn't.

 

The Catseye tools are nice, their web site has lots of good tutorials, also Vic Menard's book is very good, $25 well spent.

 

Once you've learned what collimation is all about, then cancel your order for the ES dob and buy a refractor.


Edited by SloMoe, 29 October 2020 - 07:39 AM.

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#17 SloMoe

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Posted 29 October 2020 - 08:15 AM

OK, all seriousness aside,

 

Collimation isn't hard once all the ducks are in a row, you'll find that having the Barlow's reflection target on the bottom of a Barlow recessed in a long draw tube difficult, a 2X Barlow will yield a rather large blob of laser light to find the shadow of your center spot in, but it also makes that shadow rather large, I use my coma corrector as my Barlow and a laser with a "side window" or external target.

 

The Hotec self centering system is a great concept, but in use it's basically a failure, the laser never registers the same every time it's installed, the rubber compression rings leave to much inconsistency in seating into the eye piece holder.

 

Laser registry is a term used meaning you can drop the laser into the eyepiece holder and every time it hits the exact same spot, that's checked by simply noting where the dot is on your primary mirror and remove the laser and put it back in, if it hits the same spot, or you can rotate the laser in the eye piece holder and the laser dot doesn't walk around on the primary mirror.

 

Astrosystems has a magnetic target that sticks to the bottom of their laser, again, nice idea but if you accidentally knock it off while removing the laser it will go all the way down and hit your primary mirror.

 

Farpoint also sells a darn good laser, 

 

I would recommend that while you are learning how to collimate that you do it with the scope almost level so tools don't drop into the scope.

 

Collimation has several steps, first step is to check if the draw tube is "square" to the optic axis, install your laser and run the focus  knob both ways, watch the laser dot on the primary mirror, if it moves back & forth then you have to adjust the draw tube so it doesn't.

 

Next the secondary mirror has to be aligned and rotated so has the reflection of the primary mirror centered and slightly towards the bottom of the secondary mirror, 

 

Now we aim the primary mirror using the Barlow'd laser method of centering the reflection of the center spot shadow on the target.

 

Once that's done then you drop the auto colimator in and start to stack the center spot reflections, when they are as close to a perfect stack of 4 then your done, most likely won't have re-adjust anything for the rest of the night.

 

Save for steps 1 & 2, you'll have to do this every time you set up.

 

If your truss is loose it will change every time you move the scope around or tracking a target, so make sure it's secure, also the primary mirror shouldn't shift in the Dob at all when moving the scope up or down.


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#18 SloMoe

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Posted 29 October 2020 - 08:23 AM

Here's another thing that throws a lot of collimations off, when adjusting the secondary, the mirror will rotate around if you keep loosening and tightening in the same order, so what I did on mine was to get the scope completely collimated, then removed my secondary mirror assembly and then separated the secondary from the holder and dimpled the backing plate where the adjustment screws touched, I then rounded the touching end of the adjustment screws so they only contact at one point instead of the entire surface of the screws diameter.

 

I did re check my work with a depth gauge in the draw tube on both sides.


Edited by SloMoe, 29 October 2020 - 08:26 AM.


#19 SloMoe

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Posted 29 October 2020 - 08:40 AM

Next comes the boundary layer on the primary mirror, that's the layer of air on top of the mirror where the thermal exchange is going on and why every body uses cooling fans on their mirrors, the idea is to get the mirror to the same temperature as the ambient air around it.

 

Extreme example is when you're looking across a heated surface and can see the wavy light, like on a hot road at a distance.

 

There is a bit of a problem with cooling mirror fans, well two problems really, first is that you have no way of knowing if the mirror is cooled down to ambient without some way of measuring the temp of the face of the mirror, some infrared meters are used, but something else to fiddle with in the dark.

Second is if the fan vibrates, so I would suspend it rather than bolt it directly to the mirror cell.

 

An alternative is to install a boundary layer fan to just blow that thermal layer of air off the face of the mirror, doesn't matter how warn or cold the mirror is, once that layer is gone you've got good optical clarity.

 

The really big advantage of a proper boundary layer fan is that there is no guessing for one and two by the time you're done collimating your views can be what you've worked for with out a half hour wait.

 

EDIT:

 

On the topic of fan vibration, you're magnifying that fans minute vibration 200X, so just because you can't feel it doesn't mean there isn't any.


Edited by SloMoe, 29 October 2020 - 09:18 AM.


#20 tmossman01

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Posted 29 October 2020 - 12:42 PM

Get a Telrad, I really prefer a non-magnifying finder for a Dob.  An RACI is also nice but it only helps once you are in the correct patch of sky.

 

 

If you are going to go the cheapo laser and barlow technique, I have done this.  It is not easy to make work perfectly but is cheap.

1) Buy a very cheap barlow, nothing special is needed.  I would just get a Celestron,SVbony etc. 2X barlow to use just for collimation. Any focal extender or barlow should work.

2) The cheap lasers often come incorrectly collimated themselves.  The are often off by enough to cause issues for collimation.  Read up in CN on how to collimate the cheap laser collimators (I put mine in my 2" to 1.25" reducer clamp it in a vice and aim it 10 ft. away.  I then rotate the laser and adjust it until it does not travel in a circle.)

3) The other problem with the cheap lasers is that they do not register well in the focuser (their 1.25" diameter is a little too small.  To address this I bought a Parallizer 2" to 1.25" adapter which I really like (it is a useful tool).  

4) Do star tests to confirm collimation.

 

You will still need the collimation cap to ensure the secondary is centred and rounded.  A laser cannot help with this first step of collimation (which only needs to be done very rarely).  

 

I would suggest buying a UHC filter (Lumicon brand).  These really really help with viewing nebula.

 

Rob 

Thanks, I may yet get a Telrad, but I wanted something that did not require me to hold my neck at a weird angle.  That is on my "will purchase with more experience" list.

 

For collimation, read these if you haven't yet:

 

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

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

 

For finding stuff in the sky:

https://www.cloudyni...degree-circles/

Yes, I have read up an that.  I did not mention that I intend to get an inclinometer and such. In retrospect, I should have asked, "If I have no collimation tools, do I need to buy anything more than a collimation cap to get started?"....  It is real easy to chase the "next better thing" in this hobby.

 

I might suggest that you go to Youtube and search telescope collimation before you buy any tools, that might give you a better idea of what you need.

 

A better understanding of collimation is free, buying tools and guessing isn't.

 

The Catseye tools are nice, their web site has lots of good tutorials, also Vic Menard's book is very good, $25 well spent.

 

Once you've learned what collimation is all about, then cancel your order for the ES dob and buy a refractor.

I agree... I may just get his book on that.  I have gone through some of the old threads on the theory behind the AC and such (but have not fully digested the info.)  I am also thinking about getting a refractor, but I kept going through the "upgrade this, upgrade that" cycle that I put my foot down and ordered a Dobsonian mount.

 

Next comes the boundary layer on the primary mirror, that's the layer of air on top of the mirror where the thermal exchange is going on and why every body uses cooling fans on their mirrors, the idea is to get the mirror to the same temperature as the ambient air around it.

 

Extreme example is when you're looking across a heated surface and can see the wavy light, like on a hot road at a distance.

 

There is a bit of a problem with cooling mirror fans, well two problems really, first is that you have no way of knowing if the mirror is cooled down to ambient without some way of measuring the temp of the face of the mirror, some infrared meters are used, but something else to fiddle with in the dark.

Second is if the fan vibrates, so I would suspend it rather than bolt it directly to the mirror cell.

 

An alternative is to install a boundary layer fan to just blow that thermal layer of air off the face of the mirror, doesn't matter how warn or cold the mirror is, once that layer is gone you've got good optical clarity.

 

The really big advantage of a proper boundary layer fan is that there is no guessing for one and two by the time you're done collimating your views can be what you've worked for with out a half hour wait.

 

EDIT:

 

On the topic of fan vibration, you're magnifying that fans minute vibration 200X, so just because you can't feel it doesn't mean there isn't any.

Well, the ES mounts have boundary layer fans... (I may have some 80mm anti-vibration gaskets from PC builds that might be useful for these.)

 

I have some ideas for this and for "truss settling" that I may try.


All, thank you for your responses. 

 

I have gotten more than I hoped for and thank you for your wisdom and assistance on keeping me sane.  I think the basic intended answers that I wanted.... "What to get for collimation tools to start with" and "what should I do to check for optical deficiencies - thanks Tony Flanders" have been sufficiently answered and don't want this topic to drift too much...

 

I can start new topics if more questions and/or feedback are needed.
 

Thanks again,

Moss.



#21 SloMoe

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Posted 29 October 2020 - 04:57 PM

Get & read the book before you buy any tools.


  • tmossman01 likes this

#22 tmossman01

tmossman01

    Sputnik

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Posted 29 October 2020 - 06:45 PM

I am on the site.  I ordered a collimation cap.  It should give me, at least, the basics and will also allow me to better collimate my brother's 90mm Newtonian.  (I am considering giving him an "offer" for it, as he does not really use it so that I have a "guilt free" grab and go.

 

Heck, I have been going through alignment scenarios in my mind and considering the axial and lateral errors along with the corresponding gradients with respect to focal ratio (along with the crazy considerations of circles, ellipses and parabolas as the closure of the family of ellipsoid curves (|| f2 - f1 || -> infinity).)

 

The calculations in the book will be fun to see if my thought experiments are reasonable and good!



#23 brentknight

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Posted 29 October 2020 - 06:50 PM

Collimation of a Newtonian is impossible -- until you do it.


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