Off-axis-guider (OAG) vs. Guide Scope
Back in the days of film imaging and manual guiding this was a great
controversy. A guide-scope system was much easier to use, but would
flexure problems that were difficult or even impossible to solve. OAGs
have no flexure, but finding a guide-star was sometimes impossible, and
sometimes too difficult to yield good results. I used an OAG system
when I used to image with film.
But now we have digital cameras, computers, and specialized software.
The result is that guide-stars are selected by point-and-click, mounts
are guided automatically, and sub-frame exposure times are typically 5
minutes, which in most setups would not be long enough to present
flexure problems. Modern guide cameras such as the
Starlight Xpress LodeStar or the QHYCCD QHY5L-IIM are so sensitive they
can be used on an OAG with little
difficulty in finding guide-stars. The decision between OAG or
guide-scope is less of a
controversy and more a matter of choice.
But oblong stars are still reported by guide-scope users, even
when they image with a refractor (fixed optics) and have a very solid
setup. I suspect that often overlooked is
that the guide-scope
focal length is too short. There is a tendency these days to use short
focal length guide-scopes, even finder-scopes, which degrades the
ability of the software to calculate star position.
Through testing I have demonstrated the following equation in regards
to the guide-scope and guide-camera:
Guiding Accuracy =
Guide-scope-focal-length * Guide-star-image-quality
Yes, guiding software is capable of sub-pixel guiding down to 1/10
pixel, but that is
only true if the star-image quality is large enough and good enough for
the software to
calculate the guide-star position accurately. Poor image = poor
guiding, and you may not get a star image that is good enough when
using a very short focal-length guide-scope.
If using a guide-scope:
focal-lengths that are too short - no less than 1/4th of the imaging
The longer the guide-scope
focal length - the better
The more sensitive the guide
camera - the better
Try to keep sub-frame exposure
times to 5 minutes or less
Understand that guiding accuracy may suffer when a dim guide-star is
If using an OAG, or the new On-Axis-Guider from Innovations Foresight
Use a very sensitive guide
camera, such as the LodeStar or the QHY5L-IIM (see Links)
Sub-frame exposure times may
be as long as you want
I am about to enter a perilous area of controversy by stating the
I think in many cases the importance of precise polar-alignment is over
Back in the days of imaging with film, polar alignment was very
important. Exposure times were an hour or longer, and guide-scopes
might have to be pointed a few degrees away from the center of the
image or order to point to a bright enough guide-star. The combination
of the two could result in stars that were shaped like an arc, unless
the mount was perfectly polar-aligned.
But with digital imaging and guide cameras, typical exposure times are
5 minutes, and guide-scopes don't have to be slewed away from the
center of the image. The importance of polar-alignment has been greatly
reduced. I've even heard arguments that a small amount of
polar-misalignment is better since it causes the mount to always
perform DEC corrections in one direction, and avoid backlash problems.
In many cases, the polar-alignment scope of a mount allows for
polar-alignment within 0.1 degree, which is good enough for most
But there are 2 important exceptions to this rule:
1. When imaging without any
guiding at all, fairly precise polar-alignment is needed to keep the
frames reasonably similar.
2. When imaging an object very
close to the poles (10 deg or so), the mount may not be able to keep up
with the needed R/A corrections. This occurs because DEC corrections
cause increasingly larger R/A movements as the you get closer to the
poles. Precise polar alignment keeps this problem to a minimum.
Because of the nature of digital sensors, digital images need a certain
amount of processing. The processing required by modern digital cameras can be divided into these areas:
This noise is caused by the sensor and the electronics, and is
visable in a DSLR as a red glow toward the edges of the image, or as
dim multi-colored pixels everywhere. The amount of noise increases with
the exposure time as well as the temperature. Much of the noise is
easily removed in software by subtracting a dark-frame. To take a
dark-frame, simply cover the front of the camera and take an exposure
of the same time and temperature as the real image. Modern CCDs
cameras designed for astro-imaging also have electronic cooling which reduces noise even further.
These are pixels that are hot, dead, or stuck (too bright,
black, or colored) and appear as if colored sand was sprinkled on the
image. Sometimes they can be removed in software automatically by
functions called something like "Bad Pixel Mapping", but many times
they must be removed by manual editing.
Digital sensors operate in a linear fashion, while film is
designed to act in a logarithmic fashion. For daylight images, all
digital cameras apply some kind of logarithmic function to the image to
make it presentable, otherwise it would appear to dim. For
astro-images, a stronger logarithmic function is needed to get the image to look like it would on film.
No Reciprocal Failure:
Film has reciprocal failure, which means the film looses
sensitivity as the light level gets lower. This has the negative effect
of losing dim detail, but the positive effect of losing light
pollution. Digital sensors don't have reciprocal failure, which means
they catch more faint detail, but more light pollution as well. This
increased light pollution can be partially removed by software.
This technique removes the viginetting effect that makes
the center of an image brighter than the edges. The easiest way to take
a flat-frame is in the daytime: Attach the camera to the scope, use
the same focus setting as astro images, aim the scope to an even area
of the sky, cover the front with white cloth, and take an image with an
exposure time that results in light gray.
It is amazing what modern day software can do. Indeed, it
seems like cheating.
DSLR Camera Settings
In the past when I used the Nikon D50 for imaging, I used the following settings. Other digital SLR
cameras may use similar settings.
(more sensitive, but more noisy too)
The longer the total exposure time, the better, but for acceptable images here's a general
rule-of-thumb for DSLRs at ISO 1600:
Bright open clusters...15 min
objects..................2 hours or longer
- It works!
- Supports a wide range of cameras: DSLR, color CCD, mono CCD.
- Full-featured image-capture and pre-processing, including support for LRGB and narrowband.
- Advanced debayering and noise reduction technighes.
- Easy to use, but you need to read the manual (darn).
This popular freeware performs all the pre-processing steps (convert DSLR raw files, calibrate, register, stack). It
works well, but you may have to experiment a bit, and the hot/cold
pixel removal leaves undesirable artifacts. I used to use this when I
imaged with the Nikon D50.
PhotoShop CS2 (freee school version, because we're all students, right?):
Used for final processing.
PhotoShop Hints - Here are some hints in getting started using PhotoShop for astro-images:
Use the Image / Adjustments /
Curves MainMenu-item to remove background noise and bring out detail.
Curve to reduce background
Curve to bring back faint
To apply a curve to only a section, use these steps:
1. Polygonal Lasso
2. Select / Feather
3. Image / Curves
Light pollution and noise may be further reduced with:
Image / De-speckling
Image / Noise Medium (careful,
this also reduces detail)
To reduce noise only in certain spots, use the blurring or patching
Evolution of an Image
Single Frame (Nikon curves
applied for visability)
Dark-frame (Nikon curves
applied for visability)
Calibrated frame - Single
Frame minus Dark-frame (Nikon curves applied
8 stacked frames - note
increase of signal/noise ratio (Nikon curves
applied for visability)
IRIS Modified Equalization -
more detail, but more noise
Photo Shop Curves to reduce
curves for more red detail ..... Done!