Lodestar 1 vs.
Lodestar X2:
the new and old autoguiders from Starlight Xpress Text and
processing by Lorenzo Comolli - Test images by L.Comolli, A.Sala,
E.Sordini Written in September 2014
The two tested cameras: from left the "old"
Lodestar and the new Lodestar X2 (adapted from image courtesy of
Starlight Xpress)
Introduction
Every deep sky astrophotographer perfectly knows that
autoguiding is a fundamental ingredient in the recipe for a good long
exposure astrophoto. The old times of ST4 or webcam guiding have long
gone, and many great autoguiding cameras have been available on the
market for quite a while now. As shown in my Guiding cameras survey 2010,
an autoguider must produce good quality images in order to be able to
accurately track the stars: that's why I strongly advise against using 8
bit cameras, or - even worse - color sensors. Both cameras tested here
have great black and white sensors and 16 bit quantization.
The main reason for this test is to evaluate the performance of the new Lodestar X2 vs. the older Lodestar (called “Lodestar 1” in this article) I personally own since 2010, and which I'm very happy with. The idea for the test came from Emmanuele Sordini (owner of a Lodestar 1), while talking with Alessandro Sala (Lodestar X2) and myself (Lodestar
1). So we gathered together on 21 Sept 2014 in my observatory to
perform the test. By the way, you may also be interested in an older
comparative test I did between the Lodestar 1 and SXV-H5.
You may also be interested in an older test I made between the Lodestar 1 and SXV-H5.
Differences
The new Lodestar X2 has very few differences from the older Lodestar 1. Here are the main ones:
Sensor: updated from Sony ICX429 to ICX829. According to the Starlight Xpress website, the new sensor has "considerably
improved QE and read noise. This new 'Exview 2' chip approximately
doubles the Lodestar sensitivity and adds nearly a full star magnitude
to the minimum guide star brightness". Also according to Sony (see the sensor datasheet) "the new products have approximately 6 dB greater sensitivity, a vast improvement over the existing ICX428AK/AL and ICX429AK/AL"
Autoguider socket: the
older Lodestar has a very small and unreliable connector (JST, 5-way, ZH
style, 1.5 mm pitch) that can easily slip out, thus ruining the
exposures. My personal solution was easy: joining the USB connector
together with the JST cable with a plastic cable tie. Other solutions
are available on the market, such as the Telescope Service "Strain
relief for Lodestar cable" or the TecnoSky "Blocca cavi per Lodestar". The new Lodestar X2 finally uses the RJ12 standard autoguider connector which will hold the cable firmly in place.
Comparison of the autoguide connectors (adapted from image courtesy of Starlight
Xpress).
From a dimensional and functional point of view (e.g.
backfocus, compatible software, etc.) the two cameras are nearly
identical. Likewise, they both share the "problem" of interlaced download,
whereby the even-numbered and odd-numbered rows of the detector are
each exposed for half the desired time in sequence. Exposing both even
and odd lines at the same time and for the full exposure duration would
easily double the overall sensitivity (!). This behavior seems to happen
only for short exposures (i.e. < 1 s).
The Lodestar X2 shares many advantages of its predecessor: very easy to
use, compact and lightweight (only 70 g and 31.8 mm in diameter). It is
also supported by the most popular software packages, plus it doesn’t
need an external power supply since the same USB2 cable is used both for
communication and as a power source.
Comparison
of the two cameras relative quantum efficiency.
Please note that this is *relative* quantum efficiency, so no
information about the absolute sensitivity can be inferred.
From this graph the only info is that the sensitivity to colors of the new sensor is nearly the same of the older one.
Test set-up
A simple test setup was used, composed of a small refractor, with a
typical diameter used for guiding, mounted on a bigger OTA. Here are the
details:
Guiding scope (used with the cameras at the prime focus): Pentax 75, apochromatic refractor, 75
mm diameter, f/6.7, 500 mm focal length.
Main scope (not used):
310 mm Newtonian reflector.
Mount: Astrosystems fork mount (very bulky mount that sits permanently in my observatory).
Location: Tradate (VA),
a light polluted city in the Po Valley, Italy.
Temperature: about +19
°C
Sky brightness: 19.2
mag/arcsec^2 (measured with the SQM-LE)
Date: 21 September 2014
Test
protocol
Two series of images of the galaxy NGC7331 were gathered with both
cameras, over a timespan of about 30 minutes, when the object was very
high in the sky (about 65°). The conditions were stable during the
test with clear sky.
The dark images were median combined, while the light images were SD-mask combined.
Here are the registered images:
Exposure
Lodestar
1
Lodestar
X2
1 s
5 exp
5 exp
10 s
5 exp
5 exp
30 s
5 exp
5 exp
dark
1 s
5 exp
5 exp
dark
10 s
5 exp
5 exp
dark
30 s
5 exp
5 exp
bias
9 exp
9 exp
Lodestar
1
Lodestar
X2
Dark, 1 s exp, median 5 frames
Dark, 1 s exp, median 5 frames
Dark, 10 s exp, median 5 frames
Dark, 10 s exp, median 5 frames
Dark, 30 s exp, median 5 frames
Dark, 30 s exp, median 5 frames
Bias, median 9 frames
Bias, median 9 frames
Here are images at 50% resolution. Click
on the images for 100%.
Visualization threshold computed as [ average-5*std ; average+10*std ]
Image
analysis
The analysis of the recorded images was especially focused on the following aspects.
SNR of guide star: the
better the SNR (signal to noise ratio), the better the guiding accuracy.
This is very frequently a highly underestimated factor in guiding, but this is the key to good subpixel accuracy.
Cameras with high sensitivity and quality (usually associated with a
higher number of bits), will produce better results. Both cameras have
16 bit digitization and a very similar SNR, so that there is virtually
no difference in guiding accuracy. See "Reference star SNR" in the table below.
Background noise: to measure this quantity, the standard deviation (std)
of the background must be compared to the sensitivity, measured by
means of the flux intensity of a reference star. The result is similar
in the two cameras. Oddly enough, the relative std of the Lodestar X2
improves with longer exposures. See "Std relative to flux" in the table below.
FWHM of guide star: the
pixel size is the same, so no difference is expected. The measurements
show nearly identical results, with stars from 6" to 8" in
diameter. See "Fwhm ["]" in the table below.
Hot pixels: the
approximate number of hot pixel was measured in the 1 s dark frame. As
guiding cameras are frequently used without dark calibration, 1 s of
exposure yields good sensitivity with a reasonably low hot pixel count.
Hot pixels are particularly nasty because they produce spurious signals
that can ruin the guiding. Both cameras were found to produce very few
hot pixels (Lodestar 1 ~30, Lodestar X2 ~10). The brightness of the
hottest pixel was found to be 564 times the background standard
deviation in the Lodestar 1 and 143 times in the Lodestar X2, so the new
camera has fewer and dimmer hot pixels. Moreover my old Lodestar 1
shows a vertical hot column, while the Lodestar X2 is clean. See "Number of hot pix (approx)" in the dark data table below.
Spurious glow: both cameras have a noticeable glow on the left-hand side of the frame, which is by far dimmer in the Lodestar X2.
Comparison of images of the galaxy NGC 7331. Pentax 75
refractor, 500 mm focal length.
Each panel is an sd-mask combine of 5 exposures of 1-10-30 s each, dark calibrated. Click on the image for 100%
The above analysis was derived from the measurements and computations
shown in the below tables.
NGC7331 combine
5 x 1 s
5 x 10 s
5 x 30 s
Camera
Lodestar 1
Lodestar X2
Lodestar 1
Lodestar X2
Lodestar 1
Lodestar X2
Reference star flux
13256
10071
134520
103755
354683
292224
Flux relative to Lode X2
132%
100%
130%
100%
121%
100%
Reference star SNR
179
134
1262
1125
2077
2520
Std of background
5.27
5.35
7.59
6.57
12.17
8.26
Std relative to flux
75%
100%
89%
100%
121%
100%
Fwhm [pix]
1.85
2.04
1.85
1.95
2.28
2.13
Fwhm ["]
6.44
7.10
6.43
6.78
7.94
7.41
And from these dark data.
Dark median 5 x 1 s
Lodestar
1
Lodestar
X2
Std of background
8.4
8.7
Ave
2180
950
Max
6916
2198
Max-Ave
4736
1248
Max-Ave normalized
564
143
Ave+20std
2348.0
1124.0
Ave+10std
2264.0
1037.0
Ave-5std
2163.2
932.6
number of hot pix (approx)
30
10
Limiting
magnitude measurements
I measured the limiting magnitude from the 5x30s combined exposure. The
Lodestar 1 can detect stars a little fainter than those detected by the
Lodestar X2. So, the manufacturer’s claim that the Lodestar X2 is more
sensitive by "nearly a full star magnitude" could not be confirmed: in fact, the Lodestar X2 was found to be somewhat less sensitive (about 0.3 mag). This really small difference goes nearly unnoticed while guiding, but is surprising nonetheless.
Limiting magnitude measurements on the two
Lodestar cameras and on other guiders. For the measurement
method see here or read the article on Nuovo Orione Feb 2010.
Limiting magnitude results on 5x30s combine (at snr=6)
Lodestar
1
16.4
Lodestar
X2
16.1
I-nova
Pla-mx
16.1
Philips
Vesta Pro BN
14.7
Magzero
Mz5m
12.8
Conclusions
Both cameras were found to have similar (great) sensitivity and
features. The two improvements claimed by the manufacturer were tested.
Undoubtedly, the new autoguider socket is by far better, but the
Lodestar X2 is not more sensitive than the older model.
So, given the above results, two questions come to mind: 1. Would I recommend the new camera to new customers?
Sure! All in all it's a great autoguider, just like its predecessor. 2. Is the Lodestar X2 worth the upgrade from Lodestar 1?
Not really, because the image quality and sensor sensitivity are pretty
much the same. The only real improvement over the older model is the
better cable socket.
Test results
Image quality
tie
Sensitivity
Little better Lodestar 1
SNR of guide star
Little better Lodestar 1
Number of pixels
tie
Hot pixels
Little better Lodestar X2
Pixel dimension (smaller is
better)
tie
Dimensions
tie
Cable complexity
tie
Autoguider cable reliability
Much better Lodestar X2
Power consumption
tie
Acknowledgments I'd like to acknowledge Emmanuele
Sordini for the (many!) English text revisions, and again Emmanuele
Sordini and Alessandro Sala for gathering with me and taking the test
images. Thanks also to Angelo Besani for pointing out the Sony sensor datasheet.
HTML Editing and Publishing by Lorenzo
Comolli. Email me at comolli@libero.it.