Jump to content


Photo

Seeing fainter stars

  • Please log in to reply
13 replies to this topic

#1 helpwanted

helpwanted

    Skylab

  • *****
  • Posts: 4493
  • Joined: 04 Jul 2007
  • Loc: Phoenix, AZ

Posted 13 January 2013 - 12:00 PM

I was reading the Feb issue of Astronomy Magz, pg 68, an article from Erika Rix, Astro Sketching, and she mentions something I have read many times before, and always wanted to ask other people's thoughts about.

In the article, she says: "Take advantage of averted vision and slight telescope movement (as you lightly tap the tube) to detect faint stars and changes in surface contrast."

I understand the reason behind this, your eye is more sensitive to motion.

I have heard that comment many times and have always wondered: Does this mean that using a non-tracking telescope, where the stars drift through the fov, by default, allow you to see fainter than a tracking telescope?

Same observer, same size telescope, just tracking vs non-tracking.
If anyone out there with a tracking scope wants test this and turn off the motor, please let me know your results!

David

#2 Astrojensen

Astrojensen

    Fly Me to the Moon

  • *****
  • Posts: 5070
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 13 January 2013 - 03:13 PM

In my experience, I can see much fainter stars, when the scope tracks and I can relax and look exactly in the right place with averted vision. A non-tracking scope can also see very faint, but if the magnification gets high, the objects begin to move too fast and I can't see as faint as I can with a tracking scope. Since high magnification is an essential trick to push any scope to its limits, I'd say that a large, non-tracking scope can't go as faint as one with tracking can. At magnifications below 200x I don't need tracking, but above 300x, I do. This makes tracking nearly essential on any scope larger than 12", if you want to push its limits.

That is at least my experience.


Clear skies!
Thomas, Denmark

#3 IVM

IVM

    Apollo

  • -----
  • Posts: 1056
  • Joined: 07 Jan 2008
  • Loc: USA

Posted 13 January 2013 - 03:50 PM

Sidereal movement is too slow in my experience to affect the view either way. Nudging a non-driven scope so as to track does affect the view - often it helps with nebulous objects and detail, but I'd say never with faint stars.

Nudging a non-driven scope is when I often see the faintest nebulous objects or details ~1 arcmin in size. During critical observation I move the scope much more than necessary for tracking - I can move it around almost continuously just for the sake of detection of faint nebulous objects and detail.

In a driven scope I often emulate the same kind of movement with the motors. Frankly I find a driven scope a disadvantage compared with a Dobsonian or a manual mount with a paddle (not manual slow-motion gears). It is more difficult to control the motors than your hand to produce the right kind of movement of the field. It is, of course, a disadvantage only when trying to use the motion trick. In the final analysis a driven scope probably shows me more than even a larger non-driven aperture, because I don't have to track, lose, and find again an object or field that simply requires more time to be "cracked". A manual scope pushes me to give up; a driven scope pushes me to persevere. All this, again, is as far as detecting sizable (~1') nebulous objects and detail - not stars.

Movement in my experience does not help with stars or starlike objects - on the contrary, movement faster than sidereal makes them disappear. For stars, magnification is the surest way. And as Thomas pointed out, increasing the magnification quickly becomes impractical with nondriven scopes.

#4 Sasa

Sasa

    Viking 1

  • *****
  • Posts: 780
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 13 January 2013 - 04:10 PM

I second Thomas arguments. My experience is similar. I'm able to push my 100mm refractor below magnitude 14 (for that I was pushing magnification up to 350x). But with my former 250mm undriven dobsonian Newton, I had hard time to push it close to magnitude 15 although in theory I should be able to get it down to magnitude 16 based just on the photon collecting area.

My understanding was that this was mostly caused by rather ineffective observing with the dobson at higher magnifications close to 200x and above. Here I mostly mean standing-vs-sitting, pushing-vs-slow motion (or EQ mount with tracking even better) and ability to shield my eyes while operating the telescope.

So I think that if you take same large dobsons, one driven one undriven, it would be much easier to push the driven one towards it limits.

#5 blb

blb

    Skylab

  • -----
  • Posts: 4440
  • Joined: 25 Nov 2005
  • Loc: Piedmont NC

Posted 13 January 2013 - 06:32 PM

Does this mean that using a non-tracking telescope, where the stars drift through the fov, by default, allow you to see fainter than a tracking telescope?


I think the problem lies in the fact that more magnification means that you can see fainter stars but the more magnification I use the smaller my field-of-view is and the faster an object drifts out of myview. With a scope on a mount that tracts the object stays in my field-of-view and I can tap the tube and use averted vision to see much fainter stars than can ever be seen as the field passes quickly through the field-of-view. That does not mean that you can not see faint stars with a Dob, it just means it is harder for most of us.

#6 helpwanted

helpwanted

    Skylab

  • *****
  • Posts: 4493
  • Joined: 04 Jul 2007
  • Loc: Phoenix, AZ

Posted 13 January 2013 - 08:40 PM

Interesting answers, thank you all, I have been meaning to post this Q for some time, I am glad to read a discussion on it!

#7 aa6ww

aa6ww

    Viking 1

  • *****
  • Posts: 922
  • Joined: 23 Oct 2011
  • Loc: Sacramento, Calif.

Posted 16 January 2013 - 03:51 PM

I was reading the Feb issue of Astronomy Magz, pg 68, an article from Erika Rix, Astro Sketching, and she mentions something I have read many times before, and always wanted to ask other people's thoughts about.

In the article, she says: "Take advantage of averted vision and slight telescope movement (as you lightly tap the tube) to detect faint stars and changes in surface contrast."

I understand the reason behind this, your eye is more sensitive to motion.

I have heard that comment many times and have always wondered: Does this mean that using a non-tracking telescope, where the stars drift through the fov, by default, allow you to see fainter than a tracking telescope?

Same observer, same size telescope, just tracking vs non-tracking.
If anyone out there with a tracking scope wants test this and turn off the motor, please let me know your results!

David


Like others out here have said, it's nearly impossible to enjoy a non tracking scope at higher powers with out tracking. You spend your time chasing the object instead of either searching for it or observing it.
The difference between a big cassagrain and a big dob is, you usually have about twice the field of view in a dob which making following an object by hand easier.
In my case, with my C14 and G11, I keep my clutches free and loose, which makes following an object easy since the mount is so smooth, but the scope does have to have perfect balance, then it just moves like the proverbial hot knife in butter.
At powers above 200x, its a pain and a chore to chase an object with out tracking. You just cant enjoy what your looking at, and you cant study in details what you want to because your too busy trying to keep that object in the eyepiece. Also, as one of my friends found out who has a dob, you cant show what your looking to anyone when your using high power, because once a person gets over to the eyepiece, the object is gone or close to leaving the field of view.

As far as tapping the eyepiece, I do that when I'm fairly sure what I'm looking at is in the field of view, but I'm right at the threshold of seeing it. I don't like using advert vision, if I cant see something head on, whats the point, unless its just to make a confirmation. It does work however. Trying to catch that 15.3 Mag central star in the ring nebula, helps using advert vision if your right at the threshold, for example.

The last object I remember taping my eyepiece on, was when looking for the cocoon nebula. All the stars I needed to see were in the field of view, based on some photos I had up on my laptop of the object, but I couldn't make out the cocoon. After sitting motionless for a few minutes with my eye at the eyepiece, gathering up as many photons as I could, I started taping the eyepiece and the outline of the cocoon began to appear.

I did this also in trying to observe Einsteins Cross. I was so far off in my observable magnitude, that I was just barely able to make out the lensing galaxy, so I could have danced on the eyepiece and I wasn't going to make that dim light from the quasar to appear.

Sometimes, on large objects, like the Rosetta Nebula, I have to grab the scope and move it slightly in each direction, to see where the nebulosity starts, so that helps also.

Keeping your eyes dark adapted also definitely helps, wearing red glasses when you can, to avoid stray light from annoying nearby friends.

.. its all pretty fun, and beats watching TV 100 percent of the time!!

...Ralph

#8 BillFerris

BillFerris

    Soyuz

  • *****
  • Posts: 3687
  • Joined: 17 Jul 2004
  • Loc: Flagstaff, Arizona, USA

Posted 16 January 2013 - 06:34 PM

In my experience, I can see much fainter stars, when the scope tracks and I can relax and look exactly in the right place with averted vision. ...


This ability to spend several minutes studying an eyepiece view in search of a faint star or extended object also allows dark adaptation to progress. Even a pristine dark sky is bright enough that dark adaptation is only partially achieved. This is why we're able to see color in stars with the naked eye. Dark adaptation is not complete under these conditions. Observing a high magnification view of the night sky for some period of time allows dark adaptation to progress to completion, which makes us more sensitive to faint point sources and extended objects. A skilled observer with a well-built scope can certainly manually track objects at high magnification. Personally, I've always appreciated being relaxed at the eyepiece and letting the mount do the tracking.

Bill in Flag

#9 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10229
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 16 January 2013 - 11:04 PM

I was reading the Feb issue of Astronomy Magz, pg 68, an article from Erika Rix, Astro Sketching, and she mentions something I have read many times before, and always wanted to ask other people's thoughts about.

In the article, she says: "Take advantage of averted vision and slight telescope movement (as you lightly tap the tube) to detect faint stars and changes in surface contrast."

I understand the reason behind this, your eye is more sensitive to motion.

I have heard that comment many times and have always wondered: Does this mean that using a non-tracking telescope, where the stars drift through the fov, by default, allow you to see fainter than a tracking telescope?

Same observer, same size telescope, just tracking vs non-tracking.
If anyone out there with a tracking scope wants test this and turn off the motor, please let me know your results!

David


I've wondered this myself and at lower magnification which isn't great for faintest stars the slow movement is probably a good thing. At 500x though its a pure pain in the a$$. It detracts not enhances.

Pete

#10 starrancher

starrancher

    Mercury-Atlas

  • *****
  • Posts: 2960
  • Joined: 09 Jun 2009
  • Loc: Northern Arizona

Posted 18 January 2013 - 10:00 PM

The technique is tapping the tube . Tracking or not tracking has nothing to do with this technique . It can be used with both types of scopes . Just turning the motors of isn't going to produce the same result .

#11 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10229
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 18 January 2013 - 10:08 PM

Lol it was a nice try!

Pete

#12 helpwanted

helpwanted

    Skylab

  • *****
  • Posts: 4493
  • Joined: 04 Jul 2007
  • Loc: Phoenix, AZ

Posted 20 January 2013 - 01:08 AM

What was a nice try? I wasn't trying anything, just opening up a topic for conversation?

#13 Dennis_S253

Dennis_S253

    Surveyor 1

  • -----
  • Posts: 1660
  • Joined: 22 Nov 2011
  • Loc: West Central Florida

Posted 20 January 2013 - 07:49 PM

A tracking motor doesn't speed up just because your using a higher magnification. Just as a slow motion knob does not have to be turned faster either. The techique to touch the scope can always be used with good results.

#14 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8051
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 21 January 2013 - 09:40 AM

From what little I can gather reading about the complex eye function, it does seem magnification allows fainter detail to be seen. This is because magnification draws the image into the eye's dim low contrast response range. Rods are, IIRC, not wired like cones. (Rods are wired in groups, while single cones are hard wired directly.) More rods have to fire for the brain to register a "hit." So, expanding the objects size helps more rods fire.

Now, I understand the eye is also very sensitive to movement when in mesopic and scotopic mode. So, tapping produces a response, presumably allowing dim features to put photons on more rods allowing the rods to fire a signal to the brain. And it might make some sense human sight evolved to detect movement in the dark.

This seems to work best with extended objects, and maybe not so much for point sources. Here, the airy disc of a faint star covers very few widely scattered rods. The rods register a 'hit' because the star is just bright enough to do so and might not be large enough to register on one or more, depending on the image size on the fovea. However, if you tap the scope with your eye completely stationary, the faint star image might hit a few adjacent rods and trigger a response.

I dunno, though, I have tried tapping the scope, but found it not as useful as others might. I'm just tossing that out there to spur some discussion. The eye is a complex thing, and I am far from understanding it.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics