Affiliation
Variable Stars South (VSS)
Thu, 02/24/2022 - 04:38
Hi All,
CCD photometry of bright stars requires very short exposures, which means atmospheric scintillation can cause significant scatter in measurements. Stopping down the aperture is one way to achieve longer exposures but at the expense of resolving power. Another way is to average many short exposures.
I would be interested to hear if anyone has used a photographic neutral density filter to reduce light intensity while retaining resolution. I appreciate that an ND filter would most likely change the spectral response curve of the system, but it should be possible to determine transformation coefficients to correct this.
Cheers,
Mark
Did you ever get…
Hi Mark,
Did you ever get to try neutral density filters for photometry? I'm contemplating the same approach and would appreciate hearing any lessons learned.
Best regards,
Gary
Hi All,
I am commissioning a William Optics RedCat51 and ZWO ASI2600mm pro CMOS camera for photometry of bright stars. For stars brighter than about V mag 5 exposure times are limited to only a few seconds and scintillation becomes a problem. Since my interests lies mainly in long time series of eclipsing binaries stacking short exposures is not practical.
So, in front of the motorised filter wheel, I have a manual 2" filter draw for a neutral density filter. I purchase two Baader ND filters with transmission of 25% and 12.5%. I've have only tried the 25% filter so far.
However, the weather has been particularly poor in my part of Australia in recent months, so I haven't been able to do much testing. In fact, very strong winds on Christmas night damaged my observatory roof so it will probably be several months before normal operations can resume.
Eventually I will compare transform coefficients with and without the ND filters to see if the ND filter transmission is smooth across the BVI bands of my Chroma filters.
Transformation is not needed for eclipsing binary minima timings, but I plan to also do transformed photometry of other targets. I'll report back here when I have some results. Cheers,
Mark
Keep in mind that "neutral density" filters are not actually neutral - they attenuate different wavelengths by different amounts. See page 271, figure 5 of https://app.aavso.org/jaavso/article/3798/ for an example.
Tom
The variation in transmission with wavelength of the ND filter in the paper quoted by Tom is quite high.
It would be important to use high quality ND filters for photometry. The transmission curves for Hoya ND filters, for example, appear to be much flatter than the example in Fig. 5 of the paper, just a few percent across the bandwidth of a Johnson V filter for example, rather than 30%.
Roy
I'm curious. What might the advantage of using a neutral density filter compared to other techniques? For example, one can stop the scope down or take multiple short exposures and stack them in order to eliminate/minimize the effect of scintillation (as long as the short exposures are in the linear response of the camera). Best regards.
Mike
I take time series through the night. Stacking sets of images would be awkward in my workflow. I can't easily stop down because I image with a ZWO astro camera through a camera lens, and can't control the aperture, which is wide open at f/2.8.
Roy
BSM_NH2 has two Baader neutral density filters in its dual-wheel stack: ND0.9 and ND1.8, basically 12.5% and 1.6% transmissions. If someone wants to experiment with transformation coefficients with/without these filters, it is easy to submit an AAVSOnet proposal.
Neutral density increases the exposure time, very useful to decrease scintillation on short exposures. Other ways of increasing the exposure time are to defocus or to use an engineered diffuser, but those two methods spread the starlight out and make it both more difficult to centroid/measure and increase the chance of contamination from nearby stars (especially in crowded fields). You can decrease the telescope aperture by using a mask at the front with a sub-diameter hole, but that increases scintillation.
I like the engineered diffuser, as you don't lose any of the incoming flux - it gets spread in a precise way that can yield very high precision results. It works well on bright targets in an uncluttered field. I have not looked to see if it modifies the transformation coefficients, but in theory it should have no effect.
You can of course use short exposures and stack. AAVSOnet does this; Cliff Kotnick has written a script that stacks short exposures during the automated process of transferring the images to the AWS Cloud for dark subtraction/flatfielding processing. However, as mentioned here, handling a time series of short exposures can be difficult. Cliff's script does a pretty good job - it collects ~10 seconds of exposures and stacks, and then collects the next ~10 seconds of exposures, etc. We chose 10 seconds to remove the majority of scintillation with the telescope apertures used in AAVSOnet.
Neutral density filters are rarely spectrally flat, especially if they are made with "colored" glass. You could make them with interference layers, but they would look more like the wavy scan shown in the JAAVSO paper mentioned by Tom. You can also make them with a deposited aluminum coating, which should be pretty flat but more expensive. The two ways to test whether the neutral density filter is spectrally flat are to do a transmission scan using a monochromator (strongly preferred), or to look at transformation coefficients. I worry a bit about using a commercial neutral density filter at the Ic bandpass, as most vendors target the RGB cameras and wavelengths outside of this range may have pretty wonky results.
Arne
An interesting article by Jerry Hubbell on the use of an engineered diffuser can be found on pages 54-57 of the June, 2020 issue of ASTRONOMY magazine if you have more questions...well written and illustrated.
Jamey (JENJ)