(This article was written in 2009 and applies to Quantum and Combo Quark filters)
DayStar Filters have been in operation for over 30 years. Tens of thousands of users, observers, photographers and researchers have used Daystar in a wide variety of applications. Below is an attempt to share what we and many of our users have learned through all of our years of experience. If you have something to add, please let us know.
Visual Performance
About Telescope Selection: Often, a client asks us about the kind of telescope we recommend for use with a DayStar rear mounted filter. There is often a misconception that an expensive instrument of a DayStar filter requires an even more expensive telescope in order to perform well. That isn’t true. Most of the expense of high end telescopes comes in the correction of chromatic aberration because different colors’ wavelengths each bend and refract differently. “to get all the colors to line up properly”. But with a narrow bandpass filter this isn’t necessary at all. We are only looking at ONE very narrow wavelength at any given time. A very inexpensive singlet or doublet telescope (with good wavefront properties) would offer just as high quality view as that of an air-spaced triplet apo in our application.
Consider instead, that if all of the expense of a solar telescope is in the filter itself, that one can afford a doublet telescope dedicated for that sole purpose without compromise.
About Focal Length: You may want to explore the question of Focal Length of your telescope as the first part of your visual application. You will want to match your focal length with your eyepiece to your observing expectations. Think about things like set-up time and portability. If you intend to set up your instrumentation each day – or plan to bring your equipment to a remote ‘sidewalk astronomy’ location, then keep your telescope size appropriate to the energy you want to spend for that set-up before you acquire the sun. It just isn’t practical to set up and take down a C-14 for a public afternoon. However, a Megrez 80 would be an outstanding portable, visual instrument.
Aperture is important, but for visual applications it is not likely that the human eye can appreciate seeing achieved with more than 90mm of clear aperture. Further, think about the amount of time you or the intended observer have to explore your view on the Sun. A sidewalk astronomer is going to glance through the eyepiece only for a few seconds, get a first-look impression and after just a few minutes, be moving on. A low power full disk field of view is more appropriate for this application, as the viewer may not understand what he or she is looking at with a view full of surface detail. On the other hand, a dedicated observer who owns his instrument for visual or photographic explorations for years upon years, will grow tired of a small, full disk view month after month. As an observer’s level of understanding grows, then his instrument might also.
DayStar filters, with their F/30 focal length requirement can often arrive at 2000 – 4000mm FL. While this is an excellent way to display subtle surface detail, a sidewalk display will work better using a focal reducer to bring down the power a bit.
The observer also needs to think about mount stability with a 2000 – 4000mm focal length. This is a lot of power for an alt-az mount. Consider a true tracking mount; a german equatorial with a hand controler for a little sanity
About Focal Ratio: We are frequently asked if it’s really necessary to use F/30 light entering the filter; and what happens if the light isn’t F/30. The effect of wider light cone or faster optics is actually a tilt. The shorter light cone has stronger angles that hit the filter, effectively tilting. Tilt causes the wavelength to shift. So light arrives at a variety of angles, so some light is shifted, and other isn’t. The result is a widening of the bandpass. At focal ratios of F/27-F/30, the change is insignificant. At ~F/25, a user might see an increase of 0.1Å, At ~F/20, another 0.1Å wider. Then the curve increases, so by F/15, the filter is about twice as wide.
Users don’t need to observe at rediculous focal lengths, or give up full disk imaging with a DayStar. A standard focal reducer can be applied after the DayStar. The rear thread is 2″ x 24 just like a Celestron SCT. This makes application of the focal reducer a snap. This also makes a brighter image with a faster focal length.
About Barlows, Telecentrics and Powermates: Almost everyone has a 2X barlow. We encourage anyone who would use a DayStar Filter and a barlow lens to please just a TeleVue Powermate. The Powermate was actually designed years ago with the DayStar parallel light cone in mind. At DayStar, we have tested dozens of multiple element barlows and do not see the same true telecentricity in any other manufacturer to allow us to support any other product. * The effect of a non-telecentric barlow is a vignetting of the image at the outside of the field of view. Further, the light from a non-telecentric barlow will enter the light at an angle and cause a hot/cold spot of onband and offband light in the field of view.
We also love the new Powermate PMT adapters. This is a ring which replaces the traditional top half of the barlow (which is unscrewed) with a spaced t-thread. All perfectly spaced to maintain telecentricity, of course, these adapters mate directly to the front of the DayStar. This effectively removes up to 2″ of critical pre-focus distance. As we know, that could translate to 8-10″ in backfocus. The adapter is safer, with a screw-mount attachment to the DayStar and accessories instead of a thumbscrew.
About Eyepieces: Many, if not most eyepieces of late are designed for short focal ratios and correction of chromatic aberration. As DayStar applications are neither short focal nor requires color correction, many of these expensive eyepieces offer a bad view. A bad view might mean poor eye relief and a small exit cone for the user to align to. This is very critical with sidewalk astronomy and first-time observers who may not know what to look for. We recommend as our first choice, the 55mm plossl. The older, erfel and 1-2 element plossls work great, but don’t have the cup that is very important for daytime viewing.
About Seeing: Daytime seeing conditions do not behave the same as nighttime seeing conditions. Daytime seeing is dominated by heating cells and convection. Solar energy heats the ground and that radiation rises. The differences in temperature causes unstable seeing. Most ‘bad’ seeing occurs within 10 feet of the ground. Seeing over blacktop, concrete or rooftops will usually be the worst. Grass or large bodies of water are better.
There are also very important considerations in the upper atmosphere to acknowledge. When we look at the sky some days, we might notice the color of the blue can be milky or a dark blue. This is an indicator of scattering of light in the upper atmosphere. Ice crystals, high cirrus clouds or even the jet stream can cause upper atmosphere scattering that may not be apparent until we try to observe in narrow bandpass. It is always very frustrating to observe in what would seem to be clear skies – only to see no detail because of an overhead jet stream.
Each location woldwide will offer a completely different set of circumstances under which seeing is best. It could be early morning or late afternoon and can sometimes seem counterlogical.
Observers should note that high cirrus clouds and high scattering can also make focus in narrow bandpass impossible. The effect visually resembles an astigmatism or dirty objective. To check for these conditions in your weather forecast, be sure to animations showing jet stream and water vapor layer as they won’t show up on a low level radar. Visual Satellite images are often more accurate.
*If you are a dealer, designer or manufacturer of a barlow and would like us to test your optics for application and recommendation for use with DayStar products, please contact us to arrange a testing opportunity.