Jump to content


Photo

Mallincam star shapes

  • Please log in to reply
20 replies to this topic

#1 James Cunningham

James Cunningham

    Gemini

  • *****
  • Posts: 3224
  • Joined: 07 Aug 2010
  • Loc: Maryland

Posted 15 September 2012 - 05:42 AM

When I look at most of the images of DSOs, I can see that the stars are small round dots. On my mallincam, the stars are much larger and have funny shapes. How can I reduce the size of my stars and make them rounder. Thanks.
Jim

#2 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 15 September 2012 - 03:52 PM

Lower resolution video chips necessarily produce more pixelization. you should use only images taken by similar type cameras as a baseline for comparison. Could you post an example image showing your 'poor' stars?

#3 James Cunningham

James Cunningham

    Gemini

  • *****
  • Posts: 3224
  • Joined: 07 Aug 2010
  • Loc: Maryland

Posted 15 September 2012 - 05:09 PM

Here is an image of the Trifid Nebulae taken a few nights ago. No processing but I did have to reduce the size in order to post here.
Jim

Attached Files



#4 nomosnow

nomosnow

    Ranger 4

  • -----
  • Posts: 360
  • Joined: 21 Mar 2011
  • Loc: Fort Saskatchewan,Ab ,Canada

Posted 15 September 2012 - 06:23 PM

Jim
I think that is a very nice image. Also this is an image that is typical of a Mallincam . Nothing wrong with it. :)
John

#5 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 15 September 2012 - 07:05 PM

Other than having a tad better focus or better seeing, that image is basically fine. Nothing otherwise out of the ordinary.

#6 James Cunningham

James Cunningham

    Gemini

  • *****
  • Posts: 3224
  • Joined: 07 Aug 2010
  • Loc: Maryland

Posted 15 September 2012 - 08:05 PM

Thanks for the comments. I have to work on my focusing. Even with a Bachtnov mask, its hard to get it exactly right. Perhaps my collimation is off just a bit. To me, the stars are larger than they should be but overall, I am pleased with the results that I get on good seeing nights.

#7 Sky Captain

Sky Captain

    Metal Whisperer

  • *****
  • Posts: 11225
  • Joined: 07 Nov 2004
  • Loc: Loc: Loc:

Posted 16 September 2012 - 01:44 AM

To me, the stars are larger than they should be but overall, I am pleased with the results that I get on good seeing nights.


Typically, the stars will appear larger because they are brighter objects than the relativley dimmer DSO's your live videoing.
Try using a filter such as a UHC/OIII that will cut down on the brighter objects light and will enhance the dimmer DSO's. Of course which filter you use depends on the object your trying for in the first place.

The stars will 9 times out of 10 will be the brightest objects in the FOV. Try videoing just stars at 30 sec. and at 2 seconds then compare the star sizes...star size will be smaller and "tighter" at the 2 second setting.

#8 James Cunningham

James Cunningham

    Gemini

  • *****
  • Posts: 3224
  • Joined: 07 Aug 2010
  • Loc: Maryland

Posted 16 September 2012 - 06:12 AM

Thanks. I use the Astronomik UHC filter already. I have an OIII filter and have never used it. I will try that combination.

#9 Dom543

Dom543

    Mariner 2

  • *****
  • Posts: 209
  • Joined: 24 Oct 2011

Posted 30 September 2012 - 10:06 PM

Jim,

Make sure you are also using an IR cut filter. CCD's have an almost unlimited IR range sensitivity. That can bloat anything that emits a lot of IR. Stars yes, nebulas no. The Astronomik UHC filter, if not of the CCD type, has no IR cut.

Cheers!
--Dom

#10 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 30 September 2012 - 11:51 PM

CCDs have an almost unlimited IR range sensitivity?

Hardly. I use an 80mm f/5 achromat and C8, both with focal reducers. The refractive optics in both cases do not result in any undue star bloat with my Mallincam, and I don't use any kind of IR block filter. From what I can see, folks are unduly afraid of this potential phenomenon, and in their minds blow it out of proportion.

#11 Dom543

Dom543

    Mariner 2

  • *****
  • Posts: 209
  • Joined: 24 Oct 2011

Posted 01 October 2012 - 01:20 AM

Sorry for the sloppy grammar.

I meant to say "CCD's have sensitivity in an almost unlimited range".

Meaning that, unlike the human eye, CCD's sensitvity curve doesn't go down to the zero level for a very broad range of the spectrum past 680nm.

Thank you Glenn for pointing out the need for this clarification.
--Dom

#12 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 01 October 2012 - 07:41 AM

It would be instructive to see comparison images taken with an instrument having poor near-IR focusing, with and without an IR blocking filter. So far I haven't found the need for one, and so don't have one on hand to test. It would be best, if possible, to differentiate between IR and deep blue bloating of stars, as it's the beyond-visual, extended red sensitivity which is of interest...

#13 jgraham

jgraham

    Voyager 1

  • *****
  • Posts: 13636
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 01 October 2012 - 12:01 PM

It's possible that the IR filter is built-in. Where you'll really notice it is with red stars like Mira-type variables. Without an IR filter they look like little planetary nebula in system that include refractive elements. Even in reflective systems it can whack the color ballance in CYMK color systems as all four color channels tend to pick up the IR making the colors pastel and less sarurated. The sensitivity of silicon semi-conductors peaks in the visible, then steadily declines to zero at about 1100nm in the near-IR. Above 1100nm silicon becomes transparent. I used to have some really nice silicon wafers in my lab that we used as entrance windows for near-IR spectrometers working in the 1.1 to 3.5 micron range.

In my case I always use at least an imaging skyglow filter which also happens to serve as an IR cut filter.

#14 Chris A

Chris A

    Apollo

  • -----
  • Posts: 1099
  • Joined: 03 Feb 2007
  • Loc: Toronto, Canada

Posted 01 October 2012 - 02:44 PM

All refractive type scopes whether it's an APO/Achro refractor, SCT cannot properly focus on infrared and this will show the tendency of star bloat. Only scopes that use a mirror type system WITHOUT refractive accessories (Barlow, reducer, coma corrector etc.) will focus accurately on the infrared wavelength. If you are going to use a refractive system or add refractive accessories to the imaging train, then I suggest you use an IR filter unless the IR is built into the ccd window when viewing all objects with the exception of galaxies. Check out some great articles by Jim Thompson who really knows his stuff regarding all types of filtration.

http://tech.groups.y...omical Filters/

Chris A

#15 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11401
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 02 October 2012 - 09:30 AM

Here is an image of the Trifid Nebulae taken a few nights ago. No processing but I did have to reduce the size in order to post here. Jim

Jim - you shouldn't need to over-reduce the original image pixel size - IF the jpg compression is raised to say 20% which is generally ok for web use and you'll still comply with CN TOS* - try it and we get to see the bigger picture :grin:
* <100K max file size and <=800px800p

#16 ZRX-Steve

ZRX-Steve

    Ranger 4

  • -----
  • Posts: 307
  • Joined: 31 Mar 2008
  • Loc: Phoenix, Arizona

Posted 02 October 2012 - 01:04 PM

CCDs have an almost unlimited IR range sensitivity?

Hardly. I use an 80mm f/5 achromat and C8, both with focal reducers. The refractive optics in both cases do not result in any undue star bloat with my Mallincam, and I don't use any kind of IR block filter. From what I can see, folks are unduly afraid of this potential phenomenon, and in their minds blow it out of proportion.


I did some testing last night. The tests were objects with a Baader UHC-S by itself, and a Baader UHC-S + a UV-IR cut filter. Mallincam VSS, Astrotech 12" F/4 Newtonian on a G11, Baader UHC 2" screwed into the focuser. I tried it with and without a 1 1/4" Baader UV/IR cut filter on the following objects (Ring, Dumbell, M2, and M31). M31 is the only one where I *might* have noticed a difference of less density in the body of M31 with the UV/IR cut + UHC. That would make sense since Galaxies emit light in all spectrum. The effect, if any was very subtle though.

I typically don't use a UHC on a Galaxy, but I had the UHC installed, M31 was up, I was testing, so I did.

#17 Chris A

Chris A

    Apollo

  • -----
  • Posts: 1099
  • Joined: 03 Feb 2007
  • Loc: Toronto, Canada

Posted 02 October 2012 - 02:24 PM

Hi Steve

Your Astrotech 12" F/4 Newtonian is a mirror system and would not benefit from an IR cut filter unless you added some type of refracting accessory like a Barlow, reducer or coma corrector. I used to use an IR cut filter on my SCT & TMB 80 mm triplet apo refractor and now leave it off since I often view galaxies with other objects and found the IR cut filter degrades the galaxies plus there was not a big difference in clusters or nebula with the IR cut to justify using it.

Clear skies,

Chris A

#18 jujumaster

jujumaster

    Sputnik

  • -----
  • Posts: 39
  • Joined: 26 Nov 2011

Posted 24 October 2012 - 04:27 PM

I think people have covered the bloating question, but as to the star shape, I would recommend playing with the APC function on the Mallincam. Generally in postings I've seen, most people keep the vertical APC one 'notch' higher than the horizontal APC to help maintain rounder images.

#19 nytecam

nytecam

    Voyager 1

  • *****
  • Posts: 11401
  • Joined: 20 Aug 2005
  • Loc: London UK

Posted 25 October 2012 - 03:24 AM

It would be instructive to see comparison images taken with an instrument having poor near-IR focusing, with and without an IR blocking filter. So far I haven't found the need for one, and so don't have one on hand to test. It would be best, if possible, to differentiate between IR and deep blue bloating of stars, as it's the beyond-visual, extended red sensitivity which is of interest...

Simple way is to take a spectrogram and you'll find which part of the spectrum eg blue through to red/NIR is the sweet spot for the refractor :grin: Unfortunately spectroscopy put me off refractors [and their inherent chromatic aberration] decades ago as none that I've tested brought all radiation to a common focus - hence my preference to reflectors and then I go and spoil it by adding Barlows or FR :o

#20 jujumaster

jujumaster

    Sputnik

  • -----
  • Posts: 39
  • Joined: 26 Nov 2011

Posted 04 November 2012 - 08:39 PM

Glenn,
How do Barlows spoil the common focus?

Thanks,
Ian

#21 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 05 November 2012 - 05:52 PM

Although it was Maurice who raised the point, I'll answer...

A Barlow or focal reducer has refractive optics, which will introduce chromatIc aberration. And so they wreck the pure color fidelity of a reflecting telescope, where all wavelengths would otherwise come to the same focus until lenses are thrown into the optical train.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics