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

Does this look collimated?

  • Please log in to reply
34 replies to this topic

#26 Slashzero

Slashzero

    Explorer 1

  • *****
  • Posts: 83
  • Joined: 27 May 2016
  • Loc: USS Defiant

Posted 11 August 2019 - 07:22 PM

I tried this with a flashlight at the end of the living room, but it was too close. I think you probably could if you were about 30-40 yards away, but not 30-40 feet away. I think street lights would work too, but I haven't tried with them before.
I usually set my SCT out an hour before darkness falls to try and get it thermally acclimated. I also set a fan on my back porch and have it oscillating, but that's more just to cool off the area where I'll be sitting than trying to cool the scope.


There are bright lights at the apartment complex across the way... I’m going to point there since the clouds are still out (I set it up outside to cool, and hope that by 10PM the clouds are gone, as forecast).

#27 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15489
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 11 August 2019 - 07:35 PM

 I also found that it was easier to make adjustments when the star was a little out of focus. When it was severely out of focus, it was more difficult to see the changes that were made.

 

 

RK

There are too many misleading illustrations, including the instructions that came with some of my SCTs, showing collimation of a centered obstruction shadow. That's not good enough. We're glad you got it collimated and are enjoying the views. I liked most every comment above because everyone is hitting on it and sharing their experience.

 

It's important to understand we do not collimate on shadows seen far from focus, we collimate on diffraction in the center of the FOV close to and in focus. That is, the Poisson spot (caused by the secondary) centered in the concentric rings (caused by the aperture) as close to focus as seeing will allow. Then in focus to the extent we can to get a nice symmetrical pattern.

 

Sometimes, in lesser seeing, I look for tendencies of the in focus pattern to flare more to one side than any other, then collimate that out until the flaring itself is more or less symmetrical, too. It's not easy and it takes some time to watch the image and estimate any asymmetrical flare, but it's doable. 

 

The point I wanted to drive home is, in addition to what's been said above, when collimating think diffraction. Not shadows. 


  • FlankerOneTwo and rkelley8493 like this

#28 Dale Penkala

Dale Penkala

    Now: Regular Joe

  • *****
  • Posts: 348
  • Joined: 03 Apr 2004
  • Loc: Auburn, MI

Posted 11 August 2019 - 07:46 PM

This is a very good thread! I’ve used my SCT’s for 20+ years and found that yes collimating is defiantly an issue. Just a tweak makes a big difference! Especially on planets!  Collimating when your deflation pattern closer to focus is key to getting your collimating spot on.  I seen it mentioned a couple times but I want to reiterate it more is the cooling of the optics! Now an 8” - 10” really cools fairly quick. (1-2 hours) my 14” lx200gps takes minimum of 3-1/2 hours especially if my observatory gets over 100 degrees.

Something I found that helps my scope cool is tipping the scope downward so the visual back is sticking up in the air. I remove my diagonal to allow the warm air currents to escape out thru the back.  I actually use a small fan I mount on the back so that it helps draw the warm air tube currents out of the OTA.

I have very good luck with this process for cooling.


  • elwaine and rkelley8493 like this

#29 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15489
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 11 August 2019 - 07:46 PM

There are bright lights at the apartment complex across the way... I’m going to point there since the clouds are still out (I set it up outside to cool, and hope that by 10PM the clouds are gone, as forecast).

With an artificial star, including some glint of direct or reflected light at a distance, try to find something as close to a point source as possible. A tiny pin hole in a piece of aluminum foil, for example, or a reflection off of anything round and shiny at a distance. This way, you see an actual diffraction pattern instead of magnifying a small bright spot. For example, one of my pinholes in aluminum foil was pretty big (the pin went too far through the foil enlarging the pinhole). I could see diffraction rings around it in focus, but I could also see the magnified hole. It was more of an extended object than a point source and not very helpful for collimation. Thankfully, I had smaller holes in the foil that gave a true diffraction pattern. They were a little dimmer and easier to use. When you find a suitable source, defocus it slightly. If it smears into a more uniform disc, it's not good. If it immediately breaks into a diffraction ring or two with a Poisson spot inside it, that's what you want. 


  • Slashzero and rkelley8493 like this

#30 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 15489
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 11 August 2019 - 07:52 PM

For initial cooling in modest temperatures, try using a frozen medical gel pack. Lay it on the OTA near the mirror end during dinner. In about a half hour, the entire OTA will be cool to the touch as if it had been sitting out all night. It should be ready to go right out the door after dinner. I use mine on a smaller 6" MCT, but you can size the gel pack to your OTA. It's cheap and very effective at dampening heat retained in the OTA. I always have textbook stars right out the door. Always. In modest ambient temperatures, anyway. I have not heard of anyone using it in colder temps, but my conjecture is it will still give you a boost toward thermal stability in much colder ambient temperatures. During dinner...


  • Slashzero and rkelley8493 like this

#31 rkelley8493

rkelley8493

    Viking 1

  • *****
  • topic starter
  • Posts: 855
  • Joined: 19 Jan 2019
  • Loc: Southeast USA

Posted 11 August 2019 - 09:56 PM

There are too many misleading illustrations, including the instructions that came with some of my SCTs, showing collimation of a centered obstruction shadow. That's not good enough. We're glad you got it collimated and are enjoying the views. I liked most every comment above because everyone is hitting on it and sharing their experience.

 

It's important to understand we do not collimate on shadows seen far from focus, we collimate on diffraction in the center of the FOV close to and in focus. That is, the Poisson spot (caused by the secondary) centered in the concentric rings (caused by the aperture) as close to focus as seeing will allow. Then in focus to the extent we can to get a nice symmetrical pattern.

 

Sometimes, in lesser seeing, I look for tendencies of the in focus pattern to flare more to one side than any other, then collimate that out until the flaring itself is more or less symmetrical, too. It's not easy and it takes some time to watch the image and estimate any asymmetrical flare, but it's doable. 

 

The point I wanted to drive home is, in addition to what's been said above, when collimating think diffraction. Not shadows. 

Very well said Norme! I had never heard it put that way before, but it definitely puts things into perspective on how to collimate the right way!

 

This is a very good thread! I’ve used my SCT’s for 20+ years and found that yes collimating is defiantly an issue. Just a tweak makes a big difference! Especially on planets!  Collimating when your deflation pattern closer to focus is key to getting your collimating spot on.  I seen it mentioned a couple times but I want to reiterate it more is the cooling of the optics! Now an 8” - 10” really cools fairly quick. (1-2 hours) my 14” lx200gps takes minimum of 3-1/2 hours especially if my observatory gets over 100 degrees.

Something I found that helps my scope cool is tipping the scope downward so the visual back is sticking up in the air. I remove my diagonal to allow the warm air currents to escape out thru the back.  I actually use a small fan I mount on the back so that it helps draw the warm air tube currents out of the OTA.

I have very good luck with this process for cooling.

Thanks for that Dale! I saw this very interesting video that covers the topic on how crucial cooling the optics is for getting the most out of your scope [link below]. I was mainly watching it because it's about the 10" Takahashi Mewlon [wishful thinking lol.gif ], but this guy [Dr. D] did a really good job on discussing thermal acclimation and ways you can go about getting your Cats/Casses cooled down.

 

https://www.youtube....h?v=s551j7f3W8I


  • Dale Penkala likes this

#32 Slashzero

Slashzero

    Explorer 1

  • *****
  • Posts: 83
  • Joined: 27 May 2016
  • Loc: USS Defiant

Posted 11 August 2019 - 11:32 PM

So, the skies cleared. I pointed at Altair, and put in my 9mm EP. Centered the star, and started slowly getting it out of focus. Got rings, and everything looked centered to me.
Observing the donut as I brought Altair in to focus, it seemed to collapse on itself. Maybe my last attempt at collimating wasn’t as bad as I thought.

So I pointed at Jupiter, and it looked pretty sharp. Then I pointed at Saturn. Again pretty sharp.

Maybe the viewing conditions were not good, or I had not let my SCT cool enough last time? Tonight I was pretty happy with the sharpness I was getting, even at 9mm. Of course, I didn’t have the 12 inch dob out compare it to.

I hooked up the DSLR to get some pics of the Moon. Looks sharp enough to me!

9299D91F-8473-48F0-8B0C-7D377F081370.jpeg


Edited by Slashzero, 11 August 2019 - 11:38 PM.

  • eros312 and rkelley8493 like this

#33 rkelley8493

rkelley8493

    Viking 1

  • *****
  • topic starter
  • Posts: 855
  • Joined: 19 Jan 2019
  • Loc: Southeast USA

Posted 11 August 2019 - 11:47 PM

So, the skies cleared. I pointed at Altair, and put in my 9mm EP. Centered the star, and started slowly getting it out of focus. Got rings, and everything looked centered to me.
Observing the donut as I brought Altair in to focus, it seemed to collapse on itself. Maybe my last attempt at collimating wasn’t as bad as I thought.

So I pointed at Jupiter, and it looked pretty sharp. Then I pointed at Saturn. Again pretty sharp.

Maybe the viewing conditions were not good, or I had not let my SCT cool enough last time? Tonight I was pretty happy with the sharpness I was getting, even at 9mm. Of course, I didn’t have the 12 inch dob out compare it to.

I hooked up the DSLR to get some pics of the Moon. Looks sharp enough to me!

attachicon.gif 9299D91F-8473-48F0-8B0C-7D377F081370.jpeg

Nice! 

There's one thing I've learned about judging equipment, and that's you can't accurately judge something based on one night of observing. There are too many variables that can affect the outcome. In my experience, it usually takes 3-5 nights depending on the seeing conditions.


  • Slashzero likes this

#34 Jeffmar

Jeffmar

    Viking 1

  • -----
  • Posts: 591
  • Joined: 18 Mar 2012
  • Loc: salt lake city, utah

Posted 14 August 2019 - 05:55 PM

Very good point.  I wonder how many tales of how bad my scope is was just the scope not being aligned properly.   

Many years ago I bought a C11 very cheaply because the owner thought the optics weren't good. Once I learned how to collimate it everything was crisp and sharp. With my more recent C11 the difference between collimated and not collimated is even more obvious. 



#35 Astrojedi

Astrojedi

    Soyuz

  • *****
  • Posts: 3958
  • Joined: 27 May 2015
  • Loc: SoCal

Posted 15 August 2019 - 09:10 AM

Nice! 

There's one thing I've learned about judging equipment, and that's you can't accurately judge something based on one night of observing. There are too many variables that can affect the outcome. In my experience, it usually takes 3-5 nights depending on the seeing conditions.

I would say at least a couple of months especially if you are moving up in aperture. Too many variables at play.


  • rkelley8493 likes this


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