I just received the Optolong filters and put them in my second ZWO EFW mini. These filters have a very low profile and should fit in just about any FW. U has a huge filter factor and exposures will be very long even for bright stars.
There is the olde forum comment that James Hamilton was doing scans to confirm significant red leak in the Optolong U filter. Did he do that? Anyone have a link to the report or a plot?
I just did simple testing of the U filter using five red LED flash lights I own. The lowest signal came from a Rigel Systems Starlite mini flash. A 90s exposure barely showed the two red LEDs of the Starlite mini. The other four flash lights showed significant signal in exposures of 10 to 15s. Of course, I have strong suspicions that most of the LEDs torches have emissions in the U band! Difficult to know who is zooming who. I used the Explore Scientific Essentials 127mm w/the 0.7x focal reducer. The OTA may not have very good response in U either.
Anyone have better info the Optolong U filter bandpass?
Next clear night with good transparency I’ll do some standard fields to get a better idea of what the U is doing.
Hi Jim
The red LEDs may be in the 600-700 nm band.
The red leaks tend to be roughly 850 to 1400 nm. (From my poor memory!) People generally cannot see much at wavelengths longer than about 700nm. Things look like a very dull red.
Silicon sensor cameras don’t detect anything above 1020 nm.
So measuring device like this will measure your UBVRI, Sloans, and narrow band filters and tell you if you have a red leak:
UV-5100B UV/VIS Spectrophotometer Auto Setting Wavelength Wavelength Range 190-1000nm
If you just want to see the long end of your U filter, measure all your other filters and check for a red leak, use what I use:
I’m not sure I want to get more technology to play with at this point! The second device isn’t that expensive but both are a single use items for sure. Like cooking in the kitchen I try to get multi-use tools not single use! I think the answer for me at least will be doing some standard fields like my fav. SA 110 with a few of the red extension stars thrown in. The B-V range is like 2.5 or more magnitudes should help elucidate the situation. Now if the early evening clouds will just go away… clear weather is almost here!
First light with the Optolong UBVRcIc filters was interesting. U exposures are long! As Brian mentioned in a previous post the U filters clearly has an infocus central core, presume that is mostly U signal and a much wider illuminated, out-of-focus ring which probably is the red-leak signal. I got a few standard fields last night and will get more hopefully tonight. Quick look the BVRc look pretty “normal”. The Ic filter star psfs look “soft”.
I processed first light using the Optolong UBVRcIc filter set using Landolt SA-107. Skies not the best… still have a fire smoke layer, etc.
The VRcIc coefficients look reasonable and are very small. The B coefficient is +0.243 which looks like the coefficient one might get from an RGB set B filter not a Johnson B passband.
Just one night using one standard field and not the best sky conditions. U filter needs more standard fields, more nights and a larger range of star colors to evaluate. More to come…
Nice work. Actually, it’s not uncommon for the “B” filter of an astronomical set to have a relatively large color term. I’ve just made some rough measurements of the “B” filter in our set (which was made for the Johnson-Cousins UBVRI set, not an RGB set), and it has a coefficient of about 0.23 also.
My reference for comparison are my Astrodon filters which with all my OTAs has a B via B-V transform coefficient near zero (0).
Your phrasing is not clear. You have the Optolong UBVRcIc set? Another brand?
Last night I discovered why the Ic filter psfs were “fuzzy” looking. With the ES127 Essentials and 0.7x focal reducer the filters are not parfocal. U is extremely different, Ic psfs are usable with out refocusing. I don’t use electronic focusers when I observe portable. More complexity, more power usage. This problem is mostly OTA induced. I’ve never been impressed with the ES127 Essentials optics compared to my Esprit 100 for example.
The U exposures are so long with any of my OTAs that the U filter most likely won’t get used at all. Too many uncertainties, too much exposure time and in my old age I can’t stand waiting around for a series of long exposures to finish, fighting mosquitoes, hot humid weather, forest fire smoke, clouds, etc.!
The Optolong filters are my standby backup. My Astrodon’s are primary.
Mine are not parfocal except B and V. I use three filter offsets for BV, I and R (I don’t use U). My B coefficient is about 0.12 (IMX533 on a C11 with Starizona LF reducer).
I think it was a typo… has been fixed. I just read over it… knowing what he meant!
Next question I have is what standard fields are observers using for bright U standards contained in say about a 1-deg. FOV where N is larger than 5 or so? SA-110 gave me 5 stars with good SNRs but I can probably increase that by a few more now that I know the focus is so different.
That Spectrum may be calibrated for sensor response or lack of it.
I found this:
While traditional silicon sensors exhibit a strong decrease in sensitivity beyond approximately 1100 nm, it’s not strictly true that they don’t detect anything above 1020 nm.
Here’s a more accurate picture:
Silicon’s Sensitivity: Silicon sensors are primarily optimized for visible light (400-700 nm) but can detect some near-infrared (NIR) wavelengths up to about 1100 nm.
Declining Sensitivity in NIR: Beyond 700 nm, the sensitivity of silicon sensors decreases. The longer the wavelength, the deeper photons penetrate before generating a signal charge.
Cut-off Point: Generally, silicon sensors lose sensitivity rapidly in the NIR range and have virtually no sensitivity beyond 1100-1200 nm. This is due to the bandgap of silicon.
Infrared (IR) Imaging Applications: Silicon-based cameras are often used for IR imaging (above 700 nm), but they typically have an IR cut filter to block IR wavelengths.
Beyond 1100 nm: For applications requiring detection above 1100 nm, such as short-wave infrared (SWIR) imaging (1000-3000 nm), sensors made of different materials like Indium gallium arsenide (InGaAs) are commonly used.
That is interesting but the thread is focused on the U filter, specifically the Optolong U.
CCD, silicon, characteristics have been known for many decades. Of course, things can be done to change the response, thinning, back-side, front-side illumination. In the olde days some CCDs were coated with a phosphor coating to allow better sensitivity in the U and B regions.
The first CCD conference I attended was in 1989, “CCDs in Astronomy” Tuscon, AZ. State-of-the-art at that time were 2K x 2K CCDs! A few years later we obtained and used a Thomson THX31156 1Kx1K CCD that had a phosphor coating that allowed better U and B sensitivity on that front-illuminated CCD.
Yes it is corrected for instrument response (including the sensor) and atmospheric extinction. The sensitivity of the sensor is low there but it is low within the UV passband too and cool stars are extremely bright beyond 1000nm so checking for IR light leaks beyond 1000nm starts to become important.
While the ultimate cut-off for silicon remains around 1200nm there has been a significant shift in sensitivity of sensors at that end of the spectrum. Compare for example last generation traditional CCD QE curves which had only a few percent sensitivity at 1000nm
with the latest CMOS sensors like the IMX585 which still have 30% sensitivity there
which is a potential issue if filters are only tested for leakage up to 1000nm.
Ultimately, the entire end-to-end imaging system has to be characterized. The OTA glass/mirrors, any intermediate optics, filters and the sensors. Each is a total system. Not easy to do, not impossible but for sure not low-cost. If one has the measured response curves for everything one could convolve it all together to get a pretty good estimate of the total system response.
In photometry that’s what transforms are there to correct for, though ultimately they can only be an approximation.(and cannot cope with out of band light leaks). Spectroscopy (my main area of interest) does not have that problem though as known reference stars can be used to measure the total spectral response including the atmosphere, where all the hard work measuring the star spectrum traceable back to absolute standards has already been done.
SA-110 gave me 5 stars with good SNRs but I can probably increase that by a few more now that I know the focus is so different.
