Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

SCT collimation question: different methods give different results?

  • Please log in to reply
80 replies to this topic

#26 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 11,220
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 06 November 2021 - 07:20 PM

If you are using an external focuser then make sure the primary focus knob has been turned counter clockwise while the telescope is pointing up.  That will load the mirror against gravity in a well defined way.  If you have mirror locks then lock them at that point.

 

If you aren't consistent in the way you load the primary mirror then your collimation will vary.

 

The various collimation methods based on looking in from the front of the scope won't be reliable because they are more based on the physical appearance of the optics than what the wavefront is actually doing.  It may be fine to have something physically offset if it ends up better aligned in terms of the surface figures.

 

No matter what - what matters is how the star looks at focus.  It's ok if the donut looks a bit off as you go in or out from focus.

 

The key is to do initial collimation based on the appearance of the out of focus donut - but for final collimation you can ignore the donut and focus on the star itself - in-focus.

 

Frank


  • LukaszLu likes this

#27 Kokatha man

Kokatha man

    James Webb Space Telescope

  • *****
  • Posts: 16,146
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 07 November 2021 - 07:32 AM

I don't want to start up an argument between Frank & myself but it is in that slightly defocused star that you will be doing all the leg-work for your collimation Lukas...& I've already dismissed donuts as nothing more than very rough indicators.

 

You simply cannot enable good collimation from a focused star with an SCT in-camera in anything but absolutely great seeing & I stand by that with C11's & C14's...but like the example I posted in my last reply here, when those inner rings light up & begin to display a very complete & even illumination you are virtually there, insofar as at that degree of tuning, the star then focused will display an Airy Disk...or as close to what you are going to see with consideration to seeing, scope equilibrium & optical peccadilloes as is possible. This might not be a full, even circle but rather surrounding, even arcs to the central star...

 

I also mention the appearance of strobing that is another precursor of approaching collimation which begins before those inner rings approach the aforementioned state - not mentioned in any reading I have ever made upon this subject but an undeniable fact also...but I claim no credit for that as it was my co-imager who first observed & remarked upon said phenomenon.

 

I have literally dozens & dozens of very high resolution images on our website to back up these assertions & these also involved a lot of situations where the conditions were not conducive to easy collimating - similarly, I don't doubt Frank's Metaguide program's ability to do the job & can see its benefits for many folkd, perhaps a better job in those less-than-ideal situations for most people...but as far as in-camera collimating, the points I've outlined are as it is. wink.gif

 

An after-market focuser is just about mandatory also btw! wink.gif


  • LukaszLu likes this

#28 LukaszLu

LukaszLu

    Viking 1

  • -----
  • topic starter
  • Posts: 823
  • Joined: 28 Sep 2020
  • Loc: Poland

Posted 07 November 2021 - 12:45 PM

Thank you for all your invaluable comments. I read them carefully and I will try to test the methods you suggest. I must admit, however, that as a long-term user of refractors, I am a bit devastated by the scale of the problems that have to be overcome in order to obtain elementary image quality in the SCT telescope.

 

I wonder if every SCT user is struggling with such problems? What about those who have limited themselves to the manufacturer's recommendations? Shouldn't following these recommendations give you at least a good picture? Here are Meade's instructions for this telescope:

 

kolimacja-instrukcja.jpg

 

P.S. @Kokatha man: where can I find more info about your "extreme (but highly effective) method of bringing the scope to ambient" ?



#29 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 11,220
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 07 November 2021 - 05:32 PM

I don't want to start up an argument between Frank & myself but it is in that slightly defocused star that you will be doing all the leg-work for your collimation Lukas...& I've already dismissed donuts as nothing more than very rough indicators.

 

You simply cannot enable good collimation from a focused star with an SCT in-camera in anything but absolutely great seeing & I stand by that with C11's & C14's...but like the example I posted in my last reply here, when those inner rings light up & begin to display a very complete & even illumination you are virtually there, insofar as at that degree of tuning, the star then focused will display an Airy Disk...or as close to what you are going to see with consideration to seeing, scope equilibrium & optical peccadilloes as is possible. This might not be a full, even circle but rather surrounding, even arcs to the central star...

 

I also mention the appearance of strobing that is another precursor of approaching collimation which begins before those inner rings approach the aforementioned state - not mentioned in any reading I have ever made upon this subject but an undeniable fact also...but I claim no credit for that as it was my co-imager who first observed & remarked upon said phenomenon.

 

I have literally dozens & dozens of very high resolution images on our website to back up these assertions & these also involved a lot of situations where the conditions were not conducive to easy collimating - similarly, I don't doubt Frank's Metaguide program's ability to do the job & can see its benefits for many folkd, perhaps a better job in those less-than-ideal situations for most people...but as far as in-camera collimating, the points I've outlined are as it is. wink.gif

 

An after-market focuser is just about mandatory also btw! wink.gif

No need to argue - people post images of Airy pattern's all the time in multiple forums in CN.  It just requires good seeing - nothing spectacular.  I've been doing it all around the world for years now, with scopes from small refractors to cdk20.  It's been made much more possible in recent years as the pixels get smaller.

 

If the seeing doesn't allow you to see the Airy pattern at all then you may get hints of miscollimation from a star slightly out of focus - but the sensitivity will decrease as you move out of focus.  So a general guide would be to do the final tweaks as close to focus as possible, while still seeing the impact of those tweaks.  The ultimate goal is to have a uniform first ring on the Airy pattern - and such images are often posted in CN.

 

Skilled people will use a variety of methods, one example being to look at the fuzz around the moons of jupiter - again, in-focus.

 

The Meade instructions are ok in that they start with a low power view and centering the donut.  But at the bottom they talk about higher power.  At that point you are only defocusing slightly and looking at the star spot highly magnified.  They are also talking about doing it visually, but video has many advantages.

 

Frank



#30 Kokatha man

Kokatha man

    James Webb Space Telescope

  • *****
  • Posts: 16,146
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 07 November 2021 - 07:11 PM

...I've seen a lot of "in camera" images of AD's on CN Frank, nearly all of them at very short f/l's when compared to planetary image scales...but very few on the planetary imaging forum except the occasional one from ourselves or someone else rarely - although there are those that are not actual AD's, such as those this member posts with his images: https://www.cloudyni...s#entry11341763  which might confuse some people. (Post #8)

 

That's the critical difference, that & the other most important aspect (including our own)...they are always stacked images - neither small refractors nor CDK20's are used very often at 5-10 metre f/ls!

 

Your "higher power" comments are of course right on the money...& in should be said that seeing AD's visually though an ep is much easier - & common - but I am completely against imaging without collimating "in camera" for a number of obvious reasons. wink.gif

 

I appreciate the comments about folks using <"the fuzz around the moons of jupiter"> but I'm not too sure about the quality of many of those images tbh...I see it as akin to  Bahtinov mask focusing, or those that claim their SCT's maintain collimation for months or years on end.....like anything, the combinations of "adequate collimation" coupled with very good seeing can produce outstanding images from time to time - but to obtain consistency over years & years of imaging you cannot get away with an approach like this.

 

Pat & myself rather conceitedly claim this sort of consistency over many years, with hundreds & hundreds of high-resolution images. (not dozens & dozens as I stated earlier lol.gif )

 

And, surprising or not to you Frank, we have never once seen an Airy Disk manifest itself onscreen as a complete, consistent entity in all our imaging years..! This includes 2017 when we beat the Keck to capture images of EQ storms on Neptune.....mind you, as I've said frequently since, the seeing was so darn good that we could clearly see the difference in focus using the blue filter with a shift of 12 microns - that is some seeing! bigshock.gif No complete AD however...except in the stacked image.

 

That said, FireCapture's live stacking (or a similar feature in Sharpcap as I understand it) should prove invaluable wrt these points...although we do not utilise that function personally.

 

In the words of the great man himself (Torsten Edelmann) <"In FireCapture select "Stack" from the pre-processing drop-down list and from the number of frames you want to stack from the slider below. It will stack those in the preview and in the captured data.">

 

None of the preceding is any any way meant to depreciate Frank's general comments.....& these excellent capture programs' live-stacking options appear to remove the main issues in my preceding comments (perhaps even explain some of Frank's comments about seeing AD's wrt planetary imaging)...& I encourage people to adopt his Metaguide software also, especially if they find commentary like mine too confusing, which is often, I suspect grin.gif  - we're "Old School" & it took a long time to get to where we are, requiring the sort of OCD fairly common amongst planetary imagers of our ilk lol.gif

 

Lukas, along with our "rapid cooling" technique I've also installed over-sized knurled brass adjusting "wheels" on the secondary screws...actually brass sheet cut into disks with knurling filed into the edges...drilled & tapped to accept machine threads longer than & in lieu of the original phillips head screws. (3 or 4mm off the top of my head)

 

This gives me considerable advantage in applying those final tweaks in very finite amounts to the extent that at the finish of collimating I will apply only pressure on any of them without actually detecting any movement in the screw-wheel itself - but manifesting itself in the resultant star-pattern. (again, if the seeing allows so)

 

As to the cooling method, we use 2 large orange, heavy duty garbage bags (I think they are a universal product) with  the outer one a "fail safe" protector. The inner one we carefully fill with 5kgm of "party ice" & sprinkle 1kgm of common salt onto it & mix in quickly, applying the bags to the C14 facing straight down & using a cloth face washer to protect the Moonlite focuser & some pointy bits on the back casting first.

 

We will also carefully wrap a doona around the scope & over the ice bags to assist further. (it is already on the mount of course) 

 

When the temperature is about 2°C below the anticipated air temp of the start of our imaging session we remove all this paraphernalia & allow at least 20-30 minutes for the primary mirror to relax before collimating at the start of every imaging session.  (I should mention that we installed a dual thermometer to measure primary & air/ambient temperatures - easy with any of the ventilated Celestrons but a bit more complex on those that are not, such as our old C11)

 

Being a degree or so above or below the ambient does not seem to matter terribly, but too warm & heat flares & image degradation occur.....at colder temperatures the DR's take on an elliptical appearance but don't seem to affect the image as badly as too warm a primary...



#31 LukaszLu

LukaszLu

    Viking 1

  • -----
  • topic starter
  • Posts: 823
  • Joined: 28 Sep 2020
  • Loc: Poland

Posted 07 November 2021 - 09:01 PM

Why did I sell my APO ????? !!!!!! :-)


  • DouglasPaul likes this

#32 jkmccarthy

jkmccarthy

    Mariner 2

  • *****
  • Posts: 253
  • Joined: 10 Mar 2021
  • Loc: Los Angeles, California U.S.A.

Posted 08 November 2021 - 12:47 AM

LukaszLu --

 

Please add this on-line resources to the web sites others have shared with you above.   This is the one that I swear by when collimating my MCTs ... note that it ends (in the author's Third Step ...) by tweaking final collimation on a star at focus (which does require above average seeing for larger aperture OTAs ...).   Anyway, FWIW ....

 

http://www.astrophoto.fr/collim.html

 

(Also interesting reading is the author's web page regarding central obstruction:   http://www.astrophot...bstruction.html ... and still more may be found starting here:  http://www.astrophot.../technique.html : - )

 

Lastly, FIGURE 96 found here:  https://www.telescop...aberrations.htm  ... [particularly the simulated in-focus images for unobstructed (left) and 30% obstructed (right) telescopes] can be a helpful visual reference when trying to diagnose common optical aberrations from a star test.

 

Best wishes,

 

        -- Jim


Edited by jkmccarthy, 08 November 2021 - 12:50 AM.

  • LukaszLu likes this

#33 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 11,220
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 08 November 2021 - 03:59 AM

...I've seen a lot of "in camera" images of AD's on CN Frank, nearly all of them at very short f/l's when compared to planetary image scales...but very few on the planetary imaging forum except the occasional one from ourselves or someone else rarely - although there are those that are not actual AD's, such as those this member posts with his images: https://www.cloudyni...s#entry11341763  which might confuse some people. (Post #8)

 

That's the critical difference, that & the other most important aspect (including our own)...they are always stacked images - neither small refractors nor CDK20's are used very often at 5-10 metre f/ls!

 

Your "higher power" comments are of course right on the money...& in should be said that seeing AD's visually though an ep is much easier - & common - but I am completely against imaging without collimating "in camera" for a number of obvious reasons. wink.gif

 

Well it is true that when I use "video" to collimate - I am doing stacking - but it's realtime so it's all interactive.  If you combine that with a red or IR filter - it makes it very easy to see the Airy pattern.  I think there is a variety of software out there that can do realtime stacking that would basically work - but there is one software that I would recommend since it is designed specifically for stacking speckly and turbulent star images in realtime.

 

So as long as you can do realtime stacking I would keep the star as in-focus as you can for the final tweaks.

 

The reason you get a nice and steady view when you go out of focus is precisely because you have lost sensitivity to aberrations - and you start getting misleading information just like with a donut that works best when it is slightly off center.

 

But I sure encourage people not to fear collimation.  If you end up making it worse you can always use the donut to get in the ballpark - and then fine tune it.

 

A lot of the guides on the web are fairly old and based on visual collimation.  That's fine if you don't want to use a camera or don't have one - but if you do have a camera with small pixels - and if you have a red or IR filter - I would give it a try.  And be sure to play with gain, exposure and contrast to find out what works best for you to see a slight flaring or coma shape to the star, if you don't see the Airy pattern.

 

And for my EdgeHD11 I have wingnuts on the collimation screws so I can just reach up and tweak them precisely.

 

Frank


  • LukaszLu likes this

#34 Kokatha man

Kokatha man

    James Webb Space Telescope

  • *****
  • Posts: 16,146
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 08 November 2021 - 04:00 AM

...apologies Lukas, I must be getting old! (then again, I am 72..! rofl2.gif ) I do this sort of thing regularly - when I speak of "AD" or Airy Disk I of course mean Airy Pattern: the (false) disk & surrounding first diffraction ring...not sure whether I'm downright senile or if it is because I frequently use the term "AP's" to reference "Alignment Points" as in Stack processings. gramps.gif

 

I'm not too keen on simulated images compared to actual onscreen appearances as there is a lot of difference between them, but the principle & steps are the same whether it is my commentaries or that of the reference Jim has provided.

 

Apologies if I've confused you in any way - a C8 would probably be far more amenable to seeing Airy Patterns & at any rate there is nothing wrong with using FC's live stacking preview, or taking short captures (keeping the exposure right down to avoid blowing out the resultant stack's image, checking it & tweaking the secondary again as per the resultant image etc...)

 

This was the way we started out being honest - I should be a bit more aware of throwing people in at the deep end I guess flowerred.gif - & of course Frank's Metaguide is designed with that as part of its raison d'etre I suspect!

 

Also, I don't want to shy you away from SCT's because of our drastic cooling regime - plus smaller scopes should reach equilibrium quite a deal sooner! wink.gif


  • LukaszLu likes this

#35 dcaponeii

dcaponeii

    Vanguard

  • -----
  • Posts: 2,411
  • Joined: 01 Sep 2019
  • Loc: Waxahachie, TX

Posted 08 November 2021 - 09:19 AM

Well it is true that when I use "video" to collimate - I am doing stacking - but it's realtime so it's all interactive.  If you combine that with a red or IR filter - it makes it very easy to see the Airy pattern.  I think there is a variety of software out there that can do realtime stacking that would basically work - but there is one software that I would recommend since it is designed specifically for stacking speckly and turbulent star images in realtime.

 

So as long as you can do realtime stacking I would keep the star as in-focus as you can for the final tweaks.

 

The reason you get a nice and steady view when you go out of focus is precisely because you have lost sensitivity to aberrations - and you start getting misleading information just like with a donut that works best when it is slightly off center.

 

But I sure encourage people not to fear collimation.  If you end up making it worse you can always use the donut to get in the ballpark - and then fine tune it.

 

A lot of the guides on the web are fairly old and based on visual collimation.  That's fine if you don't want to use a camera or don't have one - but if you do have a camera with small pixels - and if you have a red or IR filter - I would give it a try.  And be sure to play with gain, exposure and contrast to find out what works best for you to see a slight flaring or coma shape to the star, if you don't see the Airy pattern.

 

And for my EdgeHD11 I have wingnuts on the collimation screws so I can just reach up and tweak them precisely.

 

Frank

I need to show you guys some of the collimation images I took last night.  Diffraction rings clearly visible but as I approach the focus point instead of a single star image at the center my scope delivers a little cross. Even near focus my pattern that matches the one Daryl posted above is virtually identical EXCEPT the center "point" is a small cross not a point.   I can't think of anything in the scope that would yield four-fold symmetry but there it is EVERY time right at the center of the diffraction pattern.  It confuses the heck out of Metaguide as it always interprets the brightest arm a flare and puts the red dot in that direction.  Doesn't happen with my Orion DSO camera but it's got 4.5 micron pixels and usually operates with my scope at f/5 or f/8.  At f/15 with my ASI290MC camera there it is.  I did manage to cause distortion of one side of the diffraction pattern after getting too close to tight on one of the adjustment screws which at least indicated I needed to back off.  It's going to have to be only a DSO scope I guess as I spent a good 2 1/2 hours last night imaging collimation efforts.  This was after getting some excellent Lunar shots over the roof of my neighbors garage with heat plumes I could see on camera.  Any idea which I can work at f/15 for Lunar but not for planets?



#36 LukaszLu

LukaszLu

    Viking 1

  • -----
  • topic starter
  • Posts: 823
  • Joined: 28 Sep 2020
  • Loc: Poland

Posted 08 November 2021 - 11:43 AM

LukaszLu --

 

Please add this on-line resources to the web sites others have shared with you above.  

WOW - that's a lot of reading, especially since my English is not fluent - but I will try to face it :-) Thanks!

 

 

A lot of the guides on the web are fairly old and based on visual collimation. 

Perhaps this is the answer to my question whether all SCT owners face a similar scale of problems. Perhaps those who limit themselves to visual observations or use focal length reducers to photograph DSO at the smallest possible scale, do not pay attention to the precision of the detail?

 

 

...apologies Lukas, I must be getting old! (then again, I am 72..! rofl2.gif ) I do this sort of thing regularly - when I speak of "AD" or Airy Disk I of course mean Airy Pattern: the (false) disk & surrounding first diffraction ring...not sure whether I'm downright senile or if it is because I frequently use the term "AP's" to reference "Alignment Points" as in Stack processings. gramps.gif

(...)

 

Also, I don't want to shy you away from SCT's because of our drastic cooling regime - plus smaller scopes should reach equilibrium quite a deal sooner! wink.gif

 

I understood you perfectly - I am observing the disks in my vintage classic refractors with a diameter of 2.4 '', but I do not expect to see the disk at 8 '' aperture :-)

 

What do you think "sooner" means in the case of my 8'' "concrete mixer"? 2 hours should be enough to achieve equilibrium?

 

 

I've made AVI based on the video from the session presented above, in post # 23. I think it illustrates the situation better. There you can see that Poisson Point is shifted strongly to the right. When you look closely, diffraction rings appear at times.

AVI 11 mb: https://drive.google...iew?usp=sharing

 

19_39_44_pipp.jpg

 

So as soon as the weather improves, I keep turning the screws :-)

 

Meanwhile, I've processed the stacks from that night. The image of Jupiter has not changed a bit - it is still blurry and imprecise, I think at the level of a cheap refractor from a supermarket...

 

JUPITER-2021-11-06.jpg

 


Edited by LukaszLu, 08 November 2021 - 06:04 PM.


#37 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 11,220
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 08 November 2021 - 04:24 PM

I need to show you guys some of the collimation images I took last night.  Diffraction rings clearly visible but as I approach the focus point instead of a single star image at the center my scope delivers a little cross. Even near focus my pattern that matches the one Daryl posted above is virtually identical EXCEPT the center "point" is a small cross not a point.   I can't think of anything in the scope that would yield four-fold symmetry but there it is EVERY time right at the center of the diffraction pattern.  It confuses the heck out of Metaguide as it always interprets the brightest arm a flare and puts the red dot in that direction.  Doesn't happen with my Orion DSO camera but it's got 4.5 micron pixels and usually operates with my scope at f/5 or f/8.  At f/15 with my ASI290MC camera there it is.  I did manage to cause distortion of one side of the diffraction pattern after getting too close to tight on one of the adjustment screws which at least indicated I needed to back off.  It's going to have to be only a DSO scope I guess as I spent a good 2 1/2 hours last night imaging collimation efforts.  This was after getting some excellent Lunar shots over the roof of my neighbors garage with heat plumes I could see on camera.  Any idea which I can work at f/15 for Lunar but not for planets?

That just sounds like astigmatism - possibly from an element mounted too tightly.  Does the star go oblong one way and then the other as you go through focus?

 

If the star in focus appears as a cross and one side is slightly flaring - it is likely showing actual coma and you want to adjust so the flaring is uniform.

 

It may also involve a tube current - in which case you should *not* try to fix it by collimation.  You just need to let the scope cool down.

 

Frank


  • dcaponeii and LukaszLu like this

#38 LukaszLu

LukaszLu

    Viking 1

  • -----
  • topic starter
  • Posts: 823
  • Joined: 28 Sep 2020
  • Loc: Poland

Posted 08 November 2021 - 08:00 PM

(...)...I've seen a lot of.I see it as akin to  Bahtinov mask focusing, or those that claim their SCT's maintain collimation for months or years on end.....(...)

I forgot to ask one more question. I have never used a Bahtinov mask myself - never had the need to do so. Obtaining a perfect image in my APO was child's play - setting the focuser in the best possible position between the two extreme positions, where the sharpness began to deteriorate, was never a problem, and the effects in the photos confirmed it.

 

I found out however, that people experiencing issues with obtaining a sharp image on SCT often believe that buying a mask will miraculously solve their problems.

 

Of course I understand that no mask can replace precise collimation. However, I am curious about your opinion as a specialist in planetary imaging - do you think that using a mask in this field has any sense or advantages?



#39 Kokatha man

Kokatha man

    James Webb Space Telescope

  • *****
  • Posts: 16,146
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 08 November 2021 - 08:32 PM

Don knows my rather pessimistic thoughts about his scope. frown.gif

 

Lukas, the "delta" in common lingo, or the rate of change of temperature is important when considering how long it takes for the scope to reach equilibrium...here in Oz we experience very rapid temperature drops after nightfall for a lot of the year & usually a primary mirror (especially a C14) does not accommodate this very well - hence the need for knocking its temperature down quickly. But once cooled to a couple of degrees below the anticipated temp at the start of imaging it follows subsequent drops pretty closely most of the time.

 

If that rate of change is slow, such as at home in the Summer (we're near the sea) there's no problem unless the scope has been sitting in the Sun for long periods...but we also "go bush" frequently were temperature drops are quite severe.

 

Keeping the scope covered with some insulating material...with perhaps one of those inexpensive, silver tarpaulins might be worthwhile if kept outside a lot of the time...

 

If there is only a few degrees of cooling required & you aren't about to start any imaging for a couple of hours then leaving it outside to get to the temperature might be fine...or what we do at times which is have the scope facing downwards with an elasticised cloth cover over the corrector & playing a household fan straight up onto it - this can work quite well!

 

We've tried it facing upwards wrt all that "heat radiating out into black space" stuff but it works better the way I described - at least for the C14! lol.gif

 

What I'm about to say is well-nigh heretical for someone like me but I'm sure you've read all the hype about covering your scope with "Reflectix" during observing, where it is claimed that you can start doing so straight away regardless of the scope or air temperatures: I mention it as it was only after Rik ter Horst endorsed this approach by experimenting himself that I gave it real credence...although he did make the caveat that perhaps this was confined to visual observing: he is a Dutch optical engineer & someone I've corresponded with at times - whom I hold in the highest regard - so you might like to think about that as an alternative...

 

For myself, as I keep saying, I'm Old School lol.gif & one of the reasons that I don't use Metaguide...same with the cooling which we have finessed to a fine degree...too old to want to try something different than that which already works to the degree that we think it does for our own imaging. wink.gif

 

You're correct, very skewed to the right, as are the DR's - sometimes if you take these sorts of images into any software where you can raise the levels it gives a more vivid display as per your image below. (one reason why I said in my last post that using Metaguide or capturing short stacks, then processing "on the fly" before making further adjustments might prove more satisfactory & illuminating for you!)

 

LukasDefousedStar.jpg

 

 


  • dcaponeii likes this

#40 Kokatha man

Kokatha man

    James Webb Space Telescope

  • *****
  • Posts: 16,146
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 08 November 2021 - 08:47 PM

Ooops - a mask for focusing...no way imho! I think Frank & are in total agreement on the use of a star for collimating btw - people like Torsten atc have experimented with focus assist tools in FireCapture etc but Torsten himself has gone on record as advocating your eyes to gauge optimum focus.

 

There are indicators for different planets - Saturn is easy...just like the grayscale with 256 "shades" from white to black, when the Cassini Division is blackest that is optimum focus.

 

Jove, look for any fine festooning in the EZ or white ovals elsewhere to get the sharpest...if there are fine lines (either festoons or cloud streams, another approach when seeing is poorer is to settle on when these are darkest. (ie, showing greatest contrast)

 

Mars, very fine features which depend upon the resolving power of your scope. (like the Jovian markers) Sometimes you might have to rely on the polar caps, other times various finer marking such as Gomer Sinus here http://momilika.net/...eSet-LAYERS.png or else coarser delineation of some marking like S. Meridiani etc here http://momilika.net/...atedDiskMap.png  These are all to be considered within the limits of different apertures of course!

 

Uranus & Neptune present different challenges again - possibly the boreal brightening of Uranus is within reach of your C8 & although the Mars above are with much more aperture, you'll be amazed how good some Mars images with C8's are when conditions are favourable..! waytogo.gif



#41 jkmccarthy

jkmccarthy

    Mariner 2

  • *****
  • Posts: 253
  • Joined: 10 Mar 2021
  • Loc: Los Angeles, California U.S.A.

Posted 09 November 2021 - 12:25 AM

I need to show you guys some of the collimation images I took last night.  Diffraction rings clearly visible but as I approach the focus point instead of a single star image at the center my scope delivers a little cross. Even near focus my pattern that matches the one Daryl posted above is virtually identical EXCEPT the center "point" is a small cross not a point.   I can't think of anything in the scope that would yield four-fold symmetry but there it is EVERY time right at the center of the diffraction pattern.  It confuses the heck out of Metaguide as it always interprets the brightest arm a flare and puts the red dot in that direction. [...]

That just sounds like astigmatism - possibly from an element mounted too tightly.  Does the star go oblong one way and then the other as you go through focus?

 

If the star in focus appears as a cross and one side is slightly flaring - it is likely showing actual coma and you want to adjust so the flaring is uniform.

 

It may also involve a tube current - in which case you should *not* try to fix it by collimation.  You just need to let the scope cool down.

 

Frank

+1 for freestar8n's astigmatism diagnosis.

 

Here is an excerpt of selected examples from the FIGURE 96 at the link I provided above in post #32, to illustrate and compare perfect vs. astigmatism vs. pinched-optics.   See the full version on-line for other examples illustrating Coma and separately Heat-Plume effects ...

 

Star-Test_Examples.jpg

 

Regards,

 

         -- Jim

 


  • dcaponeii and Borodog like this

#42 dcaponeii

dcaponeii

    Vanguard

  • -----
  • Posts: 2,411
  • Joined: 01 Sep 2019
  • Loc: Waxahachie, TX

Posted 10 November 2021 - 02:05 PM

+1 for freestar8n's astigmatism diagnosis.

 

Here is an excerpt of selected examples from the FIGURE 96 at the link I provided above in post #32, to illustrate and compare perfect vs. astigmatism vs. pinched-optics.   See the full version on-line for other examples illustrating Coma and separately Heat-Plume effects ...

 

attachicon.gifStar-Test_Examples.jpg

 

Regards,

 

         -- Jim

I've reviewed that document myself and appreciate you posting the figure.  However, even for the examples shown for astigmatism above, ALL have a central bright region.  My scope yields a DARK central spot surrounded by a four-fold symmetric set of lobes.  The lobes are not flared out away from the center but in fact have elliptical symmetry away from the central spot (which is DARK not light).  The look like little flower petals.  Weirdest artifact I've seen in 50 years of using telescopes.  When the seeing is excellent then the four-fold symmetry is greatly reduced and I do have a bright central spot.

 

Daryl is likely correct that the scope is a dud for planetary at high f/number but I'd still like to understand why the central spot is DARK (Note: On BOTH sides for best focus.)  I was able to pinch the secondary the other night and distort the diffraction pattern exactly where you'd expect but still can't eliminate the four-fold lobes.



#43 Borodog

Borodog

    Soyuz

  • *****
  • Posts: 3,804
  • Joined: 26 Oct 2020

Posted 10 November 2021 - 07:49 PM

+1 for freestar8n's astigmatism diagnosis.

 

Here is an excerpt of selected examples from the FIGURE 96 at the link I provided above in post #32, to illustrate and compare perfect vs. astigmatism vs. pinched-optics.   See the full version on-line for other examples illustrating Coma and separately Heat-Plume effects ...

 

attachicon.gifStar-Test_Examples.jpg

 

Regards,

 

         -- Jim

Every single Airy disk image I have ever seen from a C11 shows that trefoil/trilobal first diffraction ring, including mine.



#44 Borodog

Borodog

    Soyuz

  • *****
  • Posts: 3,804
  • Joined: 26 Oct 2020

Posted 10 November 2021 - 08:28 PM

Regarding collimating an SCT, it can be frustrating, but there are things you can do to make it easier on yourself.

 

First, you absolutely MUST align your camera with your collimation screws, such that two are on the bottom of the sensor and one on the top. If you don’t do this and still continue to struggle, I wash my hands of you. ;O)

 

Second, work ONLY on the bottom two screws first. Completely ignore the top screw. Adjust the bottom two screws until the defocused star (or the in focus Airy disk, for that matter) is horizontally symmetric. At that point you are done with the bottom 2 screws. DO NOT TOUCH THEM AGAIN. Only then do you move on to the top screw to also make it vertically symmetric.

 

Start with a defocused “donut”, and once it looks good, move closer to focus. Keep going until something is obviously asymmetric, stop, and correct it using the same procedure as above. Keep doing this until you are at or near focus with no obvious asymmetries.

 

Pay attention to every adjustment! Know how much it was, of what screw, and in what direction. Work in known “chunks”; a quarter turn for very gross adjustments, then eighths, finally whatever small movement is required to finish up.

 

If you can’t see an obvious difference after a change, do MORE of it, at least until you are certain what it is doing and which way the image is changing. You can always undo it, because you are paying attention to what you are doing, right? If on the other hand the image is already WAY asymmetric and a large movement doesn’t make a noticeable difference, you might be going in the wrong direction and will need to reverse.

 

One thing that will help TREMENDOUSLY is having a computerized mount, preferably connected by ASCOM. MetaGuide, SharpCap, and FireCapture all have the ability to issue commands the keep the target centered. All 3 can also stabilize the image. If you aren’t using software to control the mount, you need to recenter the star after each adjustment. An exception is a coma corrected scope IF YOU ARE AT THE CORRECT BACK FOCUS. Then you can tolerate some movement of the star off axis as long as you keep it on screen. I recenter every movement anyway.

 

I used to be an advocate of doing your final collimation on an in-focus star only. But this takes good seeing, which is rare. Kokatha Man is absolutely correct; near focus allows you to see the Airy disk “turned inside out”, and the resulting Poisson point and rings are MUCH more stable, with the seeing largely relegated to a bright outer ring.


  • Kenny V., dcaponeii, Old Speckled Hen and 1 other like this

#45 Clouzot

Clouzot

    Viking 1

  • -----
  • Posts: 596
  • Joined: 09 Jul 2018
  • Loc: French Riviera

Posted 11 November 2021 - 07:05 AM

While we have the most prominent collimation experts on that thread, i was wondering: I once recorded a short video of an in-focus star with my small C6. Seeing wasn’t particularly good this night (FWHM around 3 arcsec) so I had this Proplanet 642BP in place (passes Ha and near IR).
https://youtu.be/Cb7iMLvxGT8
Is that really the Airy Pattern that I recorded without stacking, or something else? I suspect the small diameter of the C6 helped a lot, if so.
(Please ignore the bumps if possible: there was a huge party going on the balcony just above mine)

I can vouch for the “strobing” (helicoptering) effect when close to collimation: when seeing conditions don’t allow finer adjustments, it works all the time and you can clearly see it appear even at lower zoom levels
  • JMP likes this

#46 Borodog

Borodog

    Soyuz

  • *****
  • Posts: 3,804
  • Joined: 26 Oct 2020

Posted 11 November 2021 - 09:01 AM

While we have the most prominent collimation experts on that thread, i was wondering: I once recorded a short video of an in-focus star with my small C6. Seeing wasn’t particularly good this night (FWHM around 3 arcsec) so I had this Proplanet 642BP in place (passes Ha and near IR).
https://youtu.be/Cb7iMLvxGT8
Is that really the Airy Pattern that I recorded without stacking, or something else? I suspect the small diameter of the C6 helped a lot, if so.
(Please ignore the bumps if possible: there was a huge party going on the balcony just above mine)

I can vouch for the “strobing” (helicoptering) effect when close to collimation: when seeing conditions don’t allow finer adjustments, it works all the time and you can clearly see it appear even at lower zoom levels

Yes, that is the Airy disk of a well collimated scope.


  • Clouzot likes this

#47 nof

nof

    Mariner 2

  • *****
  • Posts: 263
  • Joined: 27 May 2018
  • Loc: Israel

Posted 11 November 2021 - 10:20 AM

Regarding the dew shield, i noticed this the other night with my scope. The airy circle looked pinched and darker to one side when I had the dew shield on and perfectly round and even with no dew shield. I assumed that because the dew shield is held on by pressure around the corrector plate that it might cause pinching, but why would it make it darker on one side? I’m looking at a star at high magnification in the very center of a large field. My scope is a C14. I wanted to confirm collimating after seeing teardrop stars fairly low on the southern horizon. I decided that the seeing was bad, especially lower on the horizon through all that atmosphere and above my roof. When the seeing is really good I’ll check it again on a star overhead.

I am not an expert, but to me this looks like your dewshield (or something else) partially obstructs the aperture.

From the more zoomed-in views (last and second-to-last row) it's evident that the scope is a bit out of collimation. 12 o'clock is compressed compared to 6 o'clock. This is actually a good example of something that is not visible in the big out-of-focus donut.



#48 LukaszLu

LukaszLu

    Viking 1

  • -----
  • topic starter
  • Posts: 823
  • Joined: 28 Sep 2020
  • Loc: Poland

Posted 11 November 2021 - 12:15 PM

Dear most prominent collimation experts. I thought that taking advantage of your presence, it would be worth resolving the doubt that I have already raised here.

 

It is about the difference between an intra- and extra-focus image, namely the situation when the image is regular and symmetrical on one side of the focus, and not on the other side.

 

Greg Nowell in his article "SCT Collimation under Adverse Circumstances" says:
"An out of focus star is rarely symmetrical on both sides of focus, because the temperature differential between the outside of the corrector plate and the inside will cause one side to look really bad, and the other, pretty good. Don't obsess about that. In the procedures that follow, use the side of focus that gives you the best donut. "

 

Do you agree with that? What about a situation when a clear difference appears already at the stage of a large donut?

 

I am asking this because I have a colleague who claims that in his case a similar symptom resulted from the misalignment of the primary and secondary mirrors. We are talking about an error that cannot be eliminated by adjusting the secondary mirror only. What if the image quality is mediocre despite meeting the manufacturer's collimation recommendations, and there is such a difference present? Isn't it a serious warining sign?

 

The aforementioned colleague overcame the problem with a file and adjusted the position of the main mirror. He says the image quality has improved dramatically after this treatment.


Edited by LukaszLu, 11 November 2021 - 12:16 PM.


#49 JMP

JMP

    Apollo

  • *****
  • Posts: 1,273
  • Joined: 31 Oct 2005

Posted 11 November 2021 - 12:40 PM

Hi Lukaszlu;

My first SCT years ago was a Celestron C5. I read on the internet that the secondary could be off center so I got out my caliper and measured and sure enough the secondary was about .040" off center. When I moved the secondary to the physical center of the tube my images got worse.

It took me a while, over time I would collimate on an artificial star, then view from the front, then center the secondary from the front view, then collimate again.

I wound up with the secondary about where it came from the factory. The optical axis defined by the primary was not the same as the physical axis defined by the tube.

Darryl says it helps to be a bit OCD (obsessive compulsive disorder). It works for me. Sounds like you have it also. It sounds like your secondary may not be on the optical axis defined by the primary. Good luck!

Jeff Phillips
Eugene, Oregon USA

#50 Old Speckled Hen

Old Speckled Hen

    Viking 1

  • -----
  • Posts: 673
  • Joined: 18 May 2020
  • Loc: 56.4451° N, 3.1670° W

Posted 11 November 2021 - 02:27 PM

Why did I sell my APO ????? !!!!!! :-)

Because once you get this "advanced" colimation "tided away"..

 

which is an English saying meaning :-  to put (something) in its proper place in order to make (something) tidy 

 

..you'll start doing it just to prove to yourself you can...




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics