In the past, I had been imaging with my Borg 77EDII refractor using a Hutech IDAS LPS-P2 filter in front of the optical train, mounted on to the Borg 7870 focal reducer. This was due to excessive light pollution in Gibraltar, benefiting from the excellent light pollution suppression performance of the filter. The Hutech IDAS LPS-P2 filter has the following transmission curve:
In absence of light pollution (as is the case at e-EyE), this kind of filter is not really needed, so I thought I would set up without it. There was a side-effect I had not expected however - the optical train was now being flooded with Ultraviolet (UV) light. This was due to using the very permissive Astrodon E-Series LRGB Gen2 filters:
Since the Borg 77EDII is no Takahashi FSQ, it is not necessarily perfectly colour-corrected for these wavelengths, leading to excessive star bloating in Luminance and Blue (since these allow UV light of 380nm and up through). Please note that this should not happen in a reflecting telescope. The following series of images show a single raw image from each of the LRGB filters, auto-stretched in PixInsight for demonstration purposes (followed by an animated GIF of the same four):
The star bloating is clearly very significant in the above images, which are essentially 5 minute exposures in 1x1 (no binning), captured with a QSI 660wsg-8 CCD camera. This problem has been reported by others using other refracting telescopes, with this popular Cloudy Nights forum thread as a main source. Though the Hutech IDAS LPS-P2 filter helps noticeably, it is not as superbly effective as the Baader UV/IR Cut filter, as reported here.
The Baader UV/IR Cut filter is essentially Baader's Luminance filter from their LRGB set. This filter has 98% transmission between 420nm and 680nm, cutting everything below 420nm and above 680nm. The result is a clean visible spectrum, free from UV and deep IR light. Worthy of note is that this filter does transmit all popular narrowband spectral lines (Hydrogen-Alpha, Hydrogen-Beta, Oxygen-III, Sulphur-II and Nitrogen-II).
I bought the Baader UV/IR Cut filter in 2" format from First Light Optics for about £63 without VAT. This is significantly cheaper than the Hutech IDAS LPS-P2 filter and is supposed to do a much better job of removing star bloating. An added bonus is that since I do not need light pollution suppression at e-EyE, I prefer allowing through the entire visible spectrum (without dips in the transmission curve). I have posted the Baader UV/IR Cut filter to e-EyE so that it is placed in front of my Borg 7870 focal reducer, filtering all that star-bloating UV light. I will convert this blog post into an article once I have results to report back for comparison.
I bought the Baader UV/IR Cut filter in 2" format from First Light Optics for about £63 without VAT. This is significantly cheaper than the Hutech IDAS LPS-P2 filter and is supposed to do a much better job of removing star bloating. An added bonus is that since I do not need light pollution suppression at e-EyE, I prefer allowing through the entire visible spectrum (without dips in the transmission curve). I have posted the Baader UV/IR Cut filter to e-EyE so that it is placed in front of my Borg 7870 focal reducer, filtering all that star-bloating UV light. I will convert this blog post into an article once I have results to report back for comparison.