Jump to content


Photo

Not sure if I goofed or what!

  • Please log in to reply
17 replies to this topic

#1 PGT

PGT

    Lift Off

  • -----
  • Posts: 13
  • Joined: 29 Oct 2012
  • Loc: Arizona

Posted 06 August 2014 - 05:02 PM

I was doing some observing last night.  Everything looked fine as I viewed the moon first under 94x and then with my 9.7mm at 242x magnification.  The craters along the terminus were really clear.  I then decided to view my first deep-sky object and things got strange.  I was trying to view M51 and so decided to use Alkaid as a jumping off point.  I centered it in my telrad and started to focus.  Suddenly I was looking at something that I can only describe as an image of a dull CD.  There was a circle that took up most of the objective with a smaller dark circle in the middle.  Between the two circles there appeared "smudges".

 

At first I thought there was something wrong with my eyepiece so I switched to a second 25mm that I have.  The same CD showed up.  Then I thought it was perhaps some sort of reflection of the primary and secondary mirrors.  That thought didn't last long because as I adjusted the telescope up and down and side to side a little bit the image moved off center.  When I moved away from Alkaid to where I thought I might find M51 there was no image - but I couldn't find M51 either.

 

Any ides on what might have happened.



#2 Ed Whitney

Ed Whitney

    Messenger

  • -----
  • Posts: 481
  • Joined: 08 Jul 2010
  • Loc: Palm Coast, Florida

Posted 06 August 2014 - 05:20 PM

Hi Pete,

 

Going from the moon to a DSO you must refocus. The moon is far away, but DSO's are further and it always takes "some" refocusing. I'm asumming you have done this already, but from your post it almost seems like you didn't try refocusing. It does sound like you are seeing the dark shadow of the secondary in the center of the "CD" you spoke of. Also, a bit more info may help others here to help you figure this out.



#3 PGT

PGT

    Lift Off

  • -----
  • Posts: 13
  • Joined: 29 Oct 2012
  • Loc: Arizona

Posted 06 August 2014 - 08:34 PM

Thanks Ed.  I did refocus as best as possible for the DSO.  That's when I got the "CD" image.  I was using my 25mm eyepiece, which I switched to after viewing the moon with my 9.7mm.



#4 cadfour

cadfour

    Vostok 1

  • *****
  • Posts: 188
  • Joined: 26 Jul 2012
  • Loc: Melbourne, Florida

Posted 06 August 2014 - 08:49 PM

the description you gave of a cd with a black hole in the middle sounds like a defocused star. I would suggest turning the focus knobs in the direction that shrinks the cd and black hole until the cd becomes a single pinpoint dot. If you get to that point you are looking at a focused star.



#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43862
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 06 August 2014 - 08:57 PM

Hi Pete,

 

Going from the moon to a DSO you must refocus. The moon is far away, but DSO's are further and it always takes "some" refocusing. I'm asumming you have done this already, but from your post it almost seems like you didn't try refocusing. It does sound like you are seeing the dark shadow of the secondary in the center of the "CD" you spoke of. Also, a bit more info may help others here to help you figure this out.

Both the moon and deep space are at optical infinity.  For an F/10 scope with a 2350mm focal length, an object would have to closer than about 25km to exceed the depth of focus.  The difference between infinity and the moon, about 13nm in focuser travel. The depth of focus is about 0.22mm or 220,000 nm.. 

 

Something else much have happened, something slipped, the eyepiece in the Barlow maybe.

 

Jon



#6 cadfour

cadfour

    Vostok 1

  • *****
  • Posts: 188
  • Joined: 26 Jul 2012
  • Loc: Melbourne, Florida

Posted 06 August 2014 - 09:07 PM

Jon, It does seem odd....but in his post he states that he turned the scope to the star and started to focus. I am not sure that was required if both the moon and a star focus at infinity, or very close to it.  I figure maybe he accidentally unfocused it while trying to refocus.



#7 Phil Sherman

Phil Sherman

    Surveyor 1

  • -----
  • Posts: 1516
  • Joined: 07 Dec 2010
  • Loc: Cleveland, Ohio

Posted 07 August 2014 - 02:22 AM

Thanks Ed.  I did refocus as best as possible for the DSO.  That's when I got the "CD" image.  I was using my 25mm eyepiece, which I switched to after viewing the moon with my 9.7mm.

 

Most eyeieces will require refocusing when switching between them. The exception is matched sets of eyepieces that will be very close to focused when swapping them in and out. Next time you're planning on switching from the moon to a DSO with an eyepiece switch, change the eyepieces before moving the scope and get the moon in focus first. It's always a good idea to recheck focus after moving from a bright object to a dim one.

 

You can buy parafocal rings for your eyepieces. These slip over that 1.25" tube portion of the eyepiece and can be locked in place with setscrews. Once set properly, your eyepieces will be parafocal (all in focus without moving the focuser) and can be swapped without more than a little touchup of the focuser.

 

Phil



#8 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11179
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 07 August 2014 - 04:28 AM

I was doing some observing last night.  Everything looked fine as I viewed the moon first under 94x and then with my 9.7mm at 242x magnification.  The craters along the terminus were really clear.  I then decided to view my first deep-sky object and things got strange.  I was trying to view M51 and so decided to use Alkaid as a jumping off point.  I centered it in my telrad and started to focus.  Suddenly I was looking at something that I can only describe as an image of a dull CD.  There was a circle that took up most of the objective with a smaller dark circle in the middle.  Between the two circles there appeared "smudges".

People normally don't try to view galaxies when the Moon is up and more than 50% lit. But if you want to view deep-sky objects and the Moon in the same session, view the DSOs first and the Moon last. Viewing the Moon will utterly destroy any dark adaptation you might have. The chances of seeing a galaxy any time in the next 20 minutes are very slim.

 

Is it possible that you were seeing an afterimage of the Moon?



#9 twoballscrewball

twoballscrewball

    Mariner 2

  • *****
  • Posts: 225
  • Joined: 25 Apr 2014
  • Loc: Southern NJ

Posted 07 August 2014 - 11:55 AM

That is textbook unfocused star.   Switching EP's, unless they are a parfocal set (all have same focus position) will always require a refocus.   As said, turn the focuser in the direction that makes this large light disc smaller, until you have a pinpoint star.  



#10 PGT

PGT

    Lift Off

  • -----
  • Posts: 13
  • Joined: 29 Oct 2012
  • Loc: Arizona

Posted 07 August 2014 - 12:37 PM

Thanks for all your help.  To be honest, I can't remember if I tried refocusing or not.  As for the moon and the star, at least on my Celestron 9.25, the moon is nowhere near infinity.  The comments about not viewing DSOs when the moon is up are very pertinent.  I simply wanted to view something as the monsoon clouds here in Arizona have prevented viewing for a month or so. I jumped on a chance to view the moon, and it was great.  I figured the nearby star would be a compromise between the moon and a galaxy - especially since I could put the finder on the star, in order to help focus.

 

If the image comes up again I'll be sure to focus to shrink the image.  I'll post again if that does the trick.  Also, thanks for the tip about the parafocal rings.  Anything to make EP switches easier will be a great help.  I also just bought an 8-24 zoom eyepiece.  It hasn't arrived yet but I hope it will help determine which magnification is best for a given object (and then insert the fixed EP to enhance viewing).

 

Again thank you everyone for all your help.



#11 SteveNH

SteveNH

    Viking 1

  • *****
  • Posts: 711
  • Joined: 09 Jul 2011
  • Loc: Millbrae, CA

Posted 07 August 2014 - 02:29 PM

Hi Pete, I just wanted to add that you should try practicing relaxing your eyes as you look through the eyepiece. Most people, when looking through a camera viewfinder or telescope/ binocular eyepiece, tend to have a natural tendency to focus their own eyes to a very close distance. This requires tensioning of the focusing muscles, where a relaxed eye is one that is focused at distant objects (without optical aid). As you gain experience in observing through instruments, you will find that relaxing your eyes to infinity focus will increase comfort and endurance, minimize eye strain, and reduce the possibility of getting headaches from eye fatigue. It also provides a method to be more consistent with focus as you change eyepieces or swap instruments. You can practice in the daytime by keeping both eyes open as you look through the telescope and concentrating only on the eyepiece eye.

 

I believe what happened in your observing session is that the moon at the terminator only appeared to be in focus because of its relative brightness, allowing your iris to stop down and increase your depth of focus. The star field is comparatively very dark, and will cause your iris to dilate, making your eye's focus error much more apparent. As Jon mentioned, difference in focus of things in space are all essentially negligible, and are far overpowered by shifting optics and changes in elevation above the horizon. The moon does not focus at a noticeably different point than a star, as long as both are generally located in the same area of the sky; so once you focus on a star, you should be good to go for all objects nearby, including the moon and distant galaxies.



#12 Cames

Cames

    Ranger 4

  • -----
  • Posts: 338
  • Joined: 04 Aug 2008

Posted 07 August 2014 - 03:17 PM

SteveNH said:

 

"I believe what happened in your observing session is that the moon at the terminator only appeared to be in focus because of its relative brightness, allowing your iris to stop down and increase your depth of focus. The star field is comparatively very dark, and will cause your iris to dilate, making your eye's focus error much more apparent. As Jon mentioned, difference in focus of things in space are all essentially negligible, and are far overpowered by shifting optics and changes in elevation above the horizon. The moon does not focus at a noticeably different point than a star, as long as both are generally located in the same area of the sky; so once you focus on a star, you should be good to go for all objects nearby, including the moon and distant galaxies."

 

The part of his expanation that I would like to emphasize is his mention of the pupillary constriction that accompanies looking at a bright light like the Moon in your case.

If you ever have your telescope out in the day time, insert your 25 mm eyepiece and just look up at the bright sky. It helps if the sky would be clear blue or uniformly overcast.  With the 25 mm eyepiece in the telescope in broad daylight, the size of the exit pupil that eyepiece produces is many times the size of your eye's pupil. under those conditions, it's very common to see the central darkening of the field of view when looking through a long-focal-length eyepiece in daylight and that image reminds me of the 'dull CD effect' that you described.

 

At night when switching from the Moon to a more subdued illumination, it may take a few seconds for your iris to open up enough so that your eyepupil is equal to or greater than that produced by your eyepiece. That brief interlude is why you may have been able to see the effect at night.

 

Try it for yourself one of these days when you have a uniformly bright sky. Insert your 25 mm or greater eyepiece and see if you can catch the darkening in the FOV usually thought to be caused by the obstruction of your secondary mirror.  The blotches may just have been after images something like when a flashbulb image lingers for a short while. I can't say for sure that's what you saw but it's my guess.

--------

C



#13 SteveNH

SteveNH

    Viking 1

  • *****
  • Posts: 711
  • Joined: 09 Jul 2011
  • Loc: Millbrae, CA

Posted 07 August 2014 - 04:23 PM

Hi Cames, thanks for emphasizing the constriction part. I'm fairly certain that what Pete saw in the star field was the out of focus image you get when you look at a star with an SCT which, as Steve (twoballscrewball) pointed out, has the unmistakable classic appearance of dim, tiny white CD's. I think Pete's telescope was re-aimed at the stars after focus was first set improperly on the moon. His eyes were probably at his near-focus limit at this point, but because his eyes were "stopped down", the moon still appeared to be in acceptable focus to him. (In addition, obtaining good focus tends to be more challenging and ambiguous on extended objects as opposed to a star, especially for beginners.) In the darker star field, his eyes were no longer stopped down (or as you say, eventually would dilate), and so could no longer compensate for the focus error of his telescope. The result - CD's!

 

Edit: (Sorry about the bold type near the end - I didn't do it!)


Edited by SteveNH, 07 August 2014 - 04:25 PM.


#14 areyoukiddingme

areyoukiddingme

    Viking 1

  • *****
  • Posts: 654
  • Joined: 18 Nov 2012

Posted 08 August 2014 - 12:46 AM

Hmm, rather odd. Sounds a bit like an after image of the moon. In any case, after viewing the very bright moon, how much time elapsed before re-focus on Alkaid etc.? Could be a good few minutes worth of dark adapting would help. M51 is rather low in brightness, so if only a minute or so elapsed after viewing the moon, I'd be surprised if you DID see it. 



#15 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43862
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 08 August 2014 - 08:12 AM

 

 

As for the moon and the star, at least on my Celestron 9.25, the moon is nowhere near infinity. 

 

The optical reality is that the moon is at infinity. I went though the analysis based on the 2350 mm focal length of your scope and an object that is 250,000 miles is indeed at infinity... 

 

Jon



#16 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11179
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 08 August 2014 - 09:28 AM

The optical reality is that the moon is at infinity. I went though the analysis based on the 2350 mm focal length of your C9.25, and an object that is 250,000 miles is indeed at infinity...


Not quite! You need to move the mirror 14 nanometers to refocus from infinity to the Moon. To put that in context, it's about 1/30th the wavelength of light. Smaller than a transistor on a microchip. A couple hundred atoms wide.

Better have a mighty accurate focuser!

#17 PGT

PGT

    Lift Off

  • -----
  • Posts: 13
  • Joined: 29 Oct 2012
  • Loc: Arizona

Posted 08 August 2014 - 02:46 PM

There was a few minutes between the moon and trying to see Alcaid as I manually repositioned the scope. (I'm trying to do some things without the GOTO.  One thing to comment on was that the CD image was approximately 1/2 the size of the eyepiece's field of view.  So if my mirror is 9.25" this was about half of that in diameter.



#18 Thomas Karpf

Thomas Karpf

    Surveyor 1

  • -----
  • Posts: 1754
  • Joined: 09 Feb 2009
  • Loc: Newington, CT

Posted 08 August 2014 - 04:22 PM

Hi Pete,

 

Going from the moon to a DSO you must refocus. The moon is far away, but DSO's are further and it always takes "some" refocusing. I'm assuming you have done this already, but from your post it almost seems like you didn't try refocusing. It does sound like you are seeing the dark shadow of the secondary in the center of the "CD" you spoke of. Also, a bit more info may help others here to help you figure this out.

 

No you do NOT need to refocus going from the moon to anything else in the sky.  As far as your telescope is concerned, EVERYTHING beyond the Earth's atmosphere is at infinity.  As proof of that, there are lots of occultation images on the web where the moon is passing in front of a planet.  In ALL of those images the image of the planet is sharp at the same time the image of the moon is sharp.  .

 

Yes, that sounds like a badly out-of-focus star.  Perhaps something (the mirror, the focuser, etc.) slipped when you switched eyepieces.








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics