Review Nikon Coolpix 995 for Deepsky Imaging



Update: Jun 01, 2002

The model 995 entered the market in late spring 2001. The following features influenced my decision to choose this model also for astroimaging:
* Bulb exposure up to 60 sec
* automatic dark frame subtraction possible (NR)
* lens thread 28mm for possible attaching to the telescope
* rotation of screen possible
* automatic sharpest image selection (seldom used)
* many different modes for recording possible
* very close macro capabilities, see cropped sample image "spider"


A detailed general review for the camera can be found here.

At a quite dark site (mag 5.5 sky) in 1665 m elevation (Weinebene) and at +1C I shot this piggiback image of Taurus constellation including the Pleiades and some light pollution near the horizon.

It is a single 60 sec raw exposure without NR with internal lens at f/2.3 in wide mode at ISO 400. Only levels have been adjusted.

click for full resolution cropped image (84 kB)

Using my Pentax 40mm adapter for imaging M45 through the 4 " refractor at wide position and 4 x 60 sec at ISO400 exposures + NR: - mag 15 + nebulosity can be reached. Outside temperature +5C

Bulb exposures through the telescope in any case require the remote shutter release.

click for full resolution image (31 kB )

Now to the dark current situation
The right series shows the different results of nightshot imaging at the same condition as following:


60sec exposure, iso 400, at 9C outside temperature, lens in wide position, resolution 2048x1536, center part window shown, it is necessary to compare the image in full resolution!

With internal noise reduction active, after the 60sec exposure the camera needs 60 sec more to finish and delivers the following result on top

Next under the same conditions with noise reduction (NR) disabled, the result is showing more noise, but also much more details in the dark areas.

The bottom part of the image shows the result with manual subtraction of a 60 sec dark frame in Photoshop the image shows still the same details at reduced noise. Note, at later imaging the NR function worked quite well, sometimes NR gives even better results than with manual subtraction.

Conclusion: Get your camera to constant temperature, then NR will work best.

click for full resolution image (68 kB )

Now a comparison showing the benefits of low ambient temperature:

Top: M31 combo 3x60 sec with 4" refractor and iso 400 at 10C

Bottom: Same setup with ambient temperature
of -5C, a significant increase of deepness is obvious

The following values are based on use of the shown self made adapter for the 40mm Pentax XL eyepiece.
A high quality adapter can be purchased at:
William Optics

Magnification considerations for this eyepiece with maximum camera resolution:

Refractor 4" f/9 with 40mm Pentax afocal
wide: 1,92 horizontal, 1,44 vertical, resolution 3,4" per pixel
tele: 0,48 horizontal, 0,36 vertical, resolution 0,84" per pixel

C11 with 40mm Pentax afocal
wide: 0,63 horizontal, 0,47 vertical, resolution 1,1" per pixel
tele: 0,16 horizontal, 0,12 vertical, resolution 0,28" per pixel
As eyepiece a symmetrical Ploessl with FL between 20 and 30 mm close to the camera lens is best suited to minimize vignetting.

On Sep. 19 I got a first impression of the imaging capabilities using the telescope:

At a hazy night with mag 4,5 sky at 13C I focused through my C11 with afocal Pentax 40mm to M57.
Camera settings: iso400, NR disabled, zoom in wide, manual release, resolution 2048x1536, quality fine.

I imaged 5 exposures each 60 sec, after each of the exposures a dark frame was done.
Guiding was poor, will be better next time.
The center portion of a raw frame is shown on the top.

Each exposure got the dark subtracted and then was aligned manually in Photoshop.
Color balance and levels were adjusted, after conversion to LAB one Lucy-Richardson deconvolution was applied to the luminance channel.

The result is shown at bottom.

Next evening I pointed my C11 in a mag 5 sky at 14C to the globular M13.
Camera settings: iso200, NR disabled, zoom in wide, manual release, resolution 2048x1536, quality fine,

2 exposures each 60 sec, after each of the exposures a dark frame was done.
The center portion of a raw frame is shown on the top at 50% size.

The 2 exposures got the dark subtracted and then were aligned manually in Photoshop.
Levels were adjusted, and a unsharp mask applied.

The result is shown at bottom (50% size).

At quite steady seeing I tried at 10C the nebula M42 in Orion.
Camera settings: iso400, NR disabled, zoom in 50% tele, f/3.8, resolution 2048x1536, quality fine.

The center portion of a 8 sec raw frame is shown on the top (50% size).

The advantage of longer exposure is amazing, compare one 60 sec exposure at iso400 +NR, zoom wide and 5C at bottom (unprocessed reduced size raw frame!)
final image see here (40 kB)

M42 at 50% size

High resolution imaging:

During daytime the turbulences caused by the sun are blurring the images a lot. A better result can be reached by selecting the best frames out a big number.
The camera was attached to the 4" refractor, Astrosolar ND5 filter, with Pentax 40mm at f/20, settings: iso100, zoom in 4x digital position, Quick shot setting 640x480, manual release, quality normal,
15 images were taken and the best one was selected - see image on top.

For comparison the Philips webcam was used at the same pixel resolution, 250 video frames were taken and the best one selected, see image on bottom.
Unsharp mask and level adjustment for both in Photoshop.

The comparison clearly shows much higher resolution for the webcam image, there is a better chance to catch the right moment of steady air, secondly with the webcam configuration the light has to pass much less lens surfaces (approx 30 !) and gets less scattered.
The same effects are evident for high resolution planetary imaging.

NOAA9632 Sep 27, 2001

A tip for focusing:

As the deep sky objects cannot be seen on the display, I point the telescope to a brighter star and focus using a Hartmann mask at maximum zoomed in position, then I slew back to the object's coordinates;
A tip: The internal preview display increases its sensitivity drastically at ISO 800!

Conclusions:

Generally I am satisfied with the light gathering capabilities and the relative low noise of this digicam, see to the right a cropped 60 sec image (a cold night!) of the Perseus cluster NGC869 with stars down to Mag 15 visible.

Please have in mind:

* Lower outside temperatures increase the image quality significantly, during summertime deepsky imaging is limited to very few objects.

* Use ISO 400 for deep sky shots, at outside temperatures below 0C you can even use ISO 800 with good results.

* Fast telescopes make imaging much easier with this limited exposure time.

* NR should be enabled normally, try to disable only for getting maximum light capture at low temperatures.

* Alternatively I prefer to shoot several images in sequence without NR, I take 2-3 seperate dark frames, combine them, darken them (levels) to avoid black holes in the final images and use the program BLACKFRAME to achieve a good and quick dark subtraction.

* Remote control device (MC-EU1) for shutter is essential for using bulb mode without vibration.

* You need experience in image processing techniques and programs like Photoshop to get reasonable quality out of the raw images.

* Nikonview software did not work, USB driver (from CD) installs the camera as an external drive, simple use with explorer

* The remote shutter control somtimes hangs up, usually it helps pressing the remote zoom button to reactivate the remote control, otherwise unplug and replug it and press a zoom button at the MC-EU1.

* Buy a second Lithium ion accu, the operation time at lower ambient is approx 45 min, the charging time is 7-8h! Regular discharging extends the battery life time.

* Digicams are easier to use for Moon, Sun and planetary imaging, as well as for piggiback imaging.

* Such a camera never can reach the performance of cooled cameras. If you already own a digicam you should not get another one, but save for a cooled astrocamera.

A serious alternative is the Canon D60, a new SLR camera with a very low noise CMOS chip;
see review of the D60: here

Perseus cluster - click for full size b&w image (54 kB)
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