Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
09-02-2012, 11:25 AM   #121
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
QuoteOriginally posted by noste99 Quote
Dear Forum members,

I just bought a K-5 and tried it out on the Belgian (not so dark) summer night sky.
After zooming the raw picture on the camera up to 32x I was surprised to see that the stars did not show up as dots or segments of a circle but
as strange forms (as the picture was not really sharp but the lens was put on infinity).

The tripod I used was not the most robust one, and the lens is a standard Pentax 50-200mm that came with the camera.

What is going wrong here? The tripod? The lens? The mirror flapping up and down and creating vibrations?

Thanks a lot in advance for the help and suggestions.

Best regards,

Stefan
You really should post one of those pictures (preferrably a couple of 100% crops some 800 pixels wide), because there are many things that can go wrong. First of all, zooming to 32X is an awful lot, and stars will rarely be perfect round dots at that magnification for a number of reasons of which the most prominent are:
  • no lens is truly ideal/perfect. There will almost aleways be some flaws such as coma, astigmatism, distortion - usually more obvious the further you are away from the image center.
  • poor atmospheric conditions (turbulence in the air = poor seeing), which may result in blurred stars throughout the image.
Besides there are some operator related pitfalls, as you mention yourself, that are easier to deal with and that all have to do with vibration:
  • you should always use a remote release (cable, radio or IR) and
  • you should always use mirror lock-up
And finally, unless you use a tracking mechanism: At 32X magnification you will always see some sort of trailing of your star images as soon as you expose for more than a couple of seconds with a 50mm lens.

So, if you upload a couple of pictures, it would be much easier to give you a proper diagnosis. Don't be afraid if those images aren't stellar masterpieces. We all had to experiment and learn at some point in time!

09-02-2012, 01:32 PM   #122
New Member
noste99's Avatar

Join Date: Sep 2012
Posts: 15
QuoteOriginally posted by Stone G. Quote
You really should post one of those pictures (preferrably a couple of 100% crops some 800 pixels wide), because there are many things that can go wrong. First of all, zooming to 32X is an awful lot, and stars will rarely be perfect round dots at that magnification for a number of reasons of which the most prominent are:
  • no lens is truly ideal/perfect. There will almost aleways be some flaws such as coma, astigmatism, distortion - usually more obvious the further you are away from the image center.
  • poor atmospheric conditions (turbulence in the air = poor seeing), which may result in blurred stars throughout the image.
Besides there are some operator related pitfalls, as you mention yourself, that are easier to deal with and that all have to do with vibration:
  • you should always use a remote release (cable, radio or IR) and
  • you should always use mirror lock-up
And finally, unless you use a tracking mechanism: At 32X magnification you will always see some sort of trailing of your star images as soon as you expose for more than a couple of seconds with a 50mm lens.

So, if you upload a couple of pictures, it would be much easier to give you a proper diagnosis. Don't be afraid if those images aren't stellar masterpieces. We all had to experiment and learn at some point in time!
Here is one of them.
After checking the property fields I see now it is using an 18mm F/3.5 20sec. The bright stars out of the centre have strange forms when zoomed.
A second picture is made with an old 50mm F/2 10sec. Here the strange forms can be seen even better and especially outside the centre of the picture. Is this astigmatism?
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
09-02-2012, 03:38 PM   #123
Veteran Member
pixelsaurus's Avatar

Join Date: Jul 2007
Location: Levin, NZ
Photos: Gallery | Albums
Posts: 794
QuoteOriginally posted by noste99 Quote
Is this astigmatism?
Perhaps. The star images definately look comatic. Stopping down will minimize it but that will mean you will have to bump your exposure time/ISO to compensate. Coma and astigmatism are the products of lens design and construction and is usually worse in zooms. You should just run some tests at various f stops until you find the sweet spot, or at least an f stop you can live with
Primes usually are better than zooms in astro.
09-02-2012, 04:05 PM - 1 Like   #124
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
Very good, Stefan: There is nothing wrong with neither your camera nor your skills as an astrophotographer. It is just your lenses that are to blame.

I can show you similar results with some of my lenses. Here are just a couple of examples:


On top, you have an off-centre crop (200%) of an image talken with a Tamron Adaptall-2 35-80mm lens and below you
have a similar 200% crop from an image in the same region (not the exact same spot) taken with a Super Takumar 55mm
lens. The difference is quite obvious, isn't it?

And here's another example:


An image taken with an SMC Pentax-M f/2 50mm lens. Looks fairly decent perhaps,
but click on image to get a closer look........

Mind you, there isn't really anything "wrong" with these lenses. They were just designed with terrestrial use in mind and not those point-like sources of light that we try to capture. Therefore, coma will often be a problem with even good wide field primes from about 50mm focal lenght and below.

Anyway, as long as we do not blow up our pictures enormously, they are still quite decent, I think.


Last edited by Stone G.; 09-02-2012 at 04:11 PM.
09-02-2012, 07:06 PM   #125
Inactive Account




Join Date: Jun 2012
Location: shanghai
Photos: Albums
Posts: 30
I suggest that if you want to buy an lens for astrophotography you should view this site - lenses reviews - Lenstip.com
In that lens review you can see the star coma like this.
Pentax smc DA 15 mm f/4 ED AL Limited

09-02-2012, 11:05 PM - 2 Likes   #126
Senior Member




Join Date: Oct 2011
Location: Rochester, MI
Posts: 141
Here's my most recent shot, M31. Taken a few weeks ago with my K-5.

09-02-2012, 11:26 PM   #127
Inactive Account




Join Date: Jun 2012
Location: shanghai
Photos: Albums
Posts: 30
QuoteOriginally posted by TopherTheME Quote
Here's my most recent shot, M31. Taken a few weeks ago with my K-5.
nice shoot!but the corner stars looks not very well.
09-03-2012, 08:44 AM   #128
Senior Member




Join Date: Oct 2011
Location: Rochester, MI
Posts: 141
Yeah, I didn't really have the field flattener positioned correctly. Normally I'd crop the image a little more but in this case the object took up nearly the entire field.

09-03-2012, 10:05 AM   #129
New Member
walky's Avatar

Join Date: Jun 2012
Posts: 6
I think some of it could be the effect of trailing. Without the proper device what you get is a view of each star in time, let's say as it move sin 10 secsof exposure .Plus of your tripod is not that steady you may get movement and elongation of the stars/
09-03-2012, 03:51 PM   #130
Senior Member




Join Date: Oct 2011
Location: Rochester, MI
Posts: 141
QuoteOriginally posted by walky Quote
I think some of it could be the effect of trailing. Without the proper device what you get is a view of each star in time, let's say as it move sin 10 secsof exposure .Plus of your tripod is not that steady you may get movement and elongation of the stars/

Its entirely from the field curvature of the telescope. The image is from a stack of 112x90sec exposures taken on an equatorial mount. There's very little star trails in that image, you have to look really really close at the original RAWs to see it.
09-03-2012, 09:10 PM   #131
Site Supporter




Join Date: Apr 2008
Location: Anacortes, WA
Photos: Albums
Posts: 88
Tamron SP400 f4 for astrophotography

You are all encouraging me to get into astrophotography with my K-5 by starting with the 0-GPS1.
I happen to have the Tamron SP400 f4 (65B) which is killer sharp across the frame at f4.5. Would this lens be good for astro work? Does anyone know how the coma and astigmatism is on this lens?
Anything else I should know about using this lens with the 0-GPS1?
Keep posting your photos!
thanks
09-03-2012, 10:07 PM   #132
Senior Member




Join Date: Oct 2011
Location: Rochester, MI
Posts: 141
It all depends on the kind of photos you want to take. I've found that lenses can be very hit and miss when it comes to astro photos. Many long FL lenses that are very sharp and work well for terrestrial photography can be terrible for astrophotography and vice versa. The most important things to consider are sharpness at infinity and CA. The easiest way to tell is just to try it.

As for the GPS1, its a great and economical way to get into AP if you're on a budget. EQ mounts that are astrophotography worth start at around $400 for the whole setup. The GPS1 isn't perfect, don't except pinpoint stars every time, especially at 400mm. At 300mm I usually had a keep rate of around 40% with 1minute exposures.
09-03-2012, 11:40 PM   #133
Inactive Account




Join Date: Jun 2012
Location: shanghai
Photos: Albums
Posts: 30
I had used the ogps1 with 220mm telescope to take astrophotography.It could not hold 20 seconds.

But you can shooting a number with high iso.Finaly stack in computer.Usually you can get a nice effect.Like I before shooting M8 & M20.
09-04-2012, 12:48 AM   #134
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
QuoteOriginally posted by seadog Quote
You are all encouraging me to get into astrophotography with my K-5 by starting with the 0-GPS1.
I happen to have the Tamron SP400 f4 (65B) which is killer sharp across the frame at f4.5. Would this lens be good for astro work? Does anyone know how the coma and astigmatism is on this lens?
Anything else I should know about using this lens with the 0-GPS1?
Keep posting your photos!
thanks
I use the Adaptall-2 SP 350mm/f5.6 (06B), SP300mm/f5.6 (54B), 200mm/f3.5 (04B) and SP 35-80mm/f2.8-3.8 (01A) together with my K-5 and K200D to my absolute satisfaction - so, you should expect excellent results with yout 65B, since this lens should be one of the very best.

QuoteOriginally posted by TopherTheME Quote
--------------

As for the GPS1, its a great and economical way to get into AP if you're on a budget. EQ mounts that are astrophotography worth start at around $400 for the whole setup. The GPS1 isn't perfect, don't except pinpoint stars every time, especially at 400mm. At 300mm I usually had a keep rate of around 40% with 1minute exposures.
I can confirm that as my own experiences too with my 06B and 54B lenses. One minute is about maximum and yes, the keep rate would be around the same as TopherME finds.


Globular cluster M13 in Hercules captured with Tamron Adaptall-2 SP 350mm f/5.6 lens and Pentax K-5 using
the PENTAX O-GPS1 Astrotracer function.

Combined stacks in DSS of 5 45 second exposures, 10 25 second exposures and 10 15 seconds exposures.
All taken at ISO 1600. Resulting image post-processed in PhotoImpact.

Click on image to see a non-resized 100% crop (1280 pixels wide).

Last edited by Stone G.; 09-04-2012 at 01:52 AM. Reason: missing image info
09-08-2012, 02:08 AM   #135
New Member
noste99's Avatar

Join Date: Sep 2012
Posts: 15
QuoteQuote:
I have found the Remote Continuous Shooting drive mode better than using the interval timer for the K-5. One press of the remote locks the mirror up and begins the first exposure. The next frame will follow immediately after the first and while the 2nd frame is exposing, the first frame will be written to the card.
Hello,

Does this mean that it is impossible to combine the use of the interval timer of the k-5 together with the Mirror up function?
I could not find out whether really the mirror was locked up before starting using the interval timer.

For astrohpoto's the interval timer (function allowing to select how many pictures are taken and with which time distance) is interesting so that the pictures can be stacked. In order to reduce vibrations to the minimum, I would first make sure that the mirror is locked up. For that I use the MUP function. When activating the multi picture mode (interval timer) in MUP mode and starting the process I hear the mirror (at least this is what I think it is) swapping up (or is this only the shutter?). I expect that with MUP my K-5 start behaving as a mirror less camera. Is this really the way it works?

Regards,

Stefan
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
activations, angle, camera, dslr, flickr, gps, images, interval, k-5, k-5 ii, k-5 iis, k5, lake, leds, lens, pentax, pentax k-5, picture, shutter, sky, star, thanks, time
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Astrophotography iClick Pentax DSLR Discussion 1130 09-13-2017 11:05 AM
Astrophotography with Kr??? kooks Pentax K-r 5 04-23-2011 07:12 AM
Astrophotography Naturenut Troubleshooting and Beginner Help 4 09-09-2010 07:27 AM
Astrophotography with K10 J-Rod Photographic Technique 5 12-12-2009 06:08 PM
K7 And Astrophotography Christopher M.W.T Pentax DSLR Discussion 49 11-25-2009 08:51 AM



All times are GMT -7. The time now is 10:55 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top