Typically the beamsplitter is placed about 85mm in front of the entrance slit. In an f7 beam ( the fastest for the spectroscope) this means a minimum size of 12.2mm; I use about 20mm x 35mm plates were possible.
Generally, yes.
The target star is on the optical axis.
(However, if there was a nearby field star which was brighter than the target star - I'd probably go for that! Why pass up on the opportunity!!)
Most stars are from class M (<3700K) , which in NIR have about 70% as much as energy than in visible. Since the ONAG allows the guider to access the all scope FVO, and associated F number, most of the time there is plenty of signal level for guiding in NIR.
I used the SBIG remote guiding head with an AO8 unit, which is based on the TC237 CCD ship as well.
The performance is a matter of scope FOV and F number. With my C11 @F/10 I can track with stars form 9 to 10mag. Since the ONAG addresses a very large FOV for guide star searching there is a very high probability to find a suitable one for the job.
I use the AO8 at 3 to 5 HZ because faster does not make much difference. Unless you are looking at very close double stars, fast AO correction rate is not a good idea.
The isoplanitc angle for which the guide star has still enough correlation with the target is quite small, few arc-second. Above this atmospheric turbulences can not be corrected with an AO. As a matter of fact doing so will decrease the over all performance by chasing the seeing from the guide star.
To make a long story short do not use AO faster then few HZ. AO units for amateur correct only the mount left over errors, and "fast" noise, not seeing.
At that rate the ONAG should do find for most target and set ups.
Well I would like to post an update to this saga - Gaston has provided for me the adapter to allow the connection of the OPTEC 6.2 reducer which I have shown in the pics below. Thank you Gaston for that part – its like having the key to the Ferrari but no steering wheel
Given the one day of clear skies since it arrived i have not yet got to use it but it will shortly come.
I originally had some concerns over the grub screws not being long enough but being the complete tool that I am I needed to get the right tool . The allen key wasn’t quite right and wasn’t allowing me to nip it up, 10 dollars at Bunning’s got me a new kit and the right key (a ball key) which fitted the job precisely. Its nipped up and will be installed very soon.
I am looking forward to this – in desperation I turned to planetary imaging while I was waiting…… but that’s another story….
The next update will be the unit fitted and working – hopefully.
I might be missing something here, but is there no concern about the lost spectral band from say 750-1100nm.
If you look at the spectral response curves of a typical CCD used for astro imaging eg KAF series, it will show that the chip continues well into the IR
At 800nm the Qe is still around 30% or better, at 900nm its maybe 20% and at 1100nm its dropped off to nearly zero
eg KAF168093 or KAF11002
Of course each CCD has its own curve and some are better and some worse.
DSLRs unless modded, dont see this part of the spectrum anyway so no loss there, but I am referring to chilled astro cameras
That means, at least to me, that the IR end of the spectrum is now being cut off by the OAG passthrough filter/mirror from around 750-800nm upwards
Visually that matters not, but for CCD sensing its a loss if you have any desire to capture that end of the spectrum.
So is it a case of there being nothing but stellar brightness that we are losing here and therefore it doesn't really matter, or are we in fact potentially losing 30% of the useful imaging spectrum ?
I understand how it works well for guiding, but it seems at the expense of IR image capture.
I don't think you're missing much.
99% of the time a UV-IR cut filter is used (400-70nm) for astroshots to reduce the "bloat".
Working in the NIR and above is a completely different ball game...as I've found with the spectroscope!!!
As a matter of fact CCD/CMOS sensors are quite sensitive in IR and this is a problem for visible photography in general.
One shoot color cameras, or LRGB filters for mono-chrome cameras, cut of the UV and NIR light, leaving typical the visible range available from 350nm to 700nm.
The main reason being most optics are well designed and optimized for the visible range, at the expense of the NIR (and UV) unless you are willing to pay much more.
This is especially true for refractors, or reflectors with any corrector lens, like SCT Schmidt’s plate, and FR.
Not to mention that reflective and AR coatings are not designed for NIR either. Optical design is the art of compromising (as elsewhere I guess), if you want well corrected optics for a given wavelength range you would have to give up something elsewhere, like UV, and/or NIR performances.
Even black aluminum anodizing is most of the time transparent above 750nm, which is another significant issue, not very well known but quite common.
This is why the ONAG uses a special optical grade extended range black anodizing, used in aerospace applications, which is much more absorbent in visible (up to 5x) and remains so way above 1000nm. It is more expensive but useful for working in NIR. If you have a monochrome camera, without a built-in NIR filter, you could experience those effects, by taking pictures with and without a NIR filter, and compare the results (sharpness, focus, …). Try also to image a standard black anodized aluminum piece of equipment to see what do you get.
In the past most SLR good lenses used to have a special NIR ruler to correct focus for NIR imaging, this is gone with DSLR since they are built with UV/NIR filters.
Chromatic aberrations and other problems become more pronounced in the NIR for most scopes, leading to larger FWHM (fatter stars with halo) and sometime coma.
Even if they would perform well in NIR the problem remains to image visible + NIR together, most of the time the scope focal plane is significantly offset between visible and NIR. Having a good focus, sharp image, for the all range could be challenging.
well it has been an extremely long journey but tonight I achieved two things - focus of the main dslr camera and the focus of the DMK618k. That took me all of about an hour of fiddling - but essential. Now for the heartache - it got real windy - so much so i have chucked it in - even though its clear. Unfortunately the scope doesn't like the wind . all i basically got was some 30 second shots of no real importance. That aside i have now come to the realisation that i need a real sensitive camera and for that i am open for opinions and suggestions. the dmk cameras i have are not up to the task.
for those interested - i will continue to persevere with the setup, but my limiting factor is the subject of the image needs to have some star nearby to guide with - real close as the field of view is very small with the guide camera. This is a real step up in guiding for me and i am finding it rather challenging
the key i think for me is to get a very sensitive guide camera
David
I've just ordered a new guide camera, the Starlightxpress Lodestar. It was a toss up between that and the Sbig STi. I'll let you know how the lodestar performs and it may be the guider you are after. I'm also interested in the ONAG and may get one down the track.
Allan
David
I've just ordered a new guide camera, the Starlightxpress Lodestar. It was a toss up between that and the Sbig STi. I'll let you know how the lodestar performs and it may be the guider you are after. I'm also interested in the ONAG and may get one down the track.
Allan
i think the principle and quality of the ONAG is terriffic, but its the imaging equipment that is stopping me i think
I would suggest the SX lodestar as well. It is very sensitive with an extended NIR quantum efficient.
A very elegant, compact well-made design, I love mine.
Tests done to compare with the ORION starshoot (a CMOS sensor) have shown a bout 10x more signal for the lodestar versus the ORION (binned 2x2 to match the pixel sizes).
Also the lodestar is a 16 bits camera, not a 8 bits, with very small dark current even at ambient and powered on since a while.
It works perfectly with the ONAG. Have a look to the ONAG's NIR guiding page, the chip featured there is the lodestar one (Sony Exview series, ICX 429AL):
In general the ONAG will have a NIR efficient around 0.6 to 1.0 magnitude less than the full spectrum with such chips.
For instance with my C11 at F/10 I usual can work with 9th magnitude stars at 1 second guider exposure. If you increase the exposure time to 2.5 s you get one more magnitude, or 10th.
Very low noise chip.
P.S: If you select this one you will need a C to T mount adapter since the lodestar uses a C mount thread system:
David
I've just ordered a new guide camera, the Starlightxpress Lodestar. It was a toss up between that and the Sbig STi. I'll let you know how the lodestar performs and it may be the guider you are after. I'm also interested in the ONAG and may get one down the track.
Allan
Allan, you obviously shopped around - how much and where from? do all the cables and adapters come with it?
