Coronado Solar Telescopes Overview
Personal Solar Telescope (PST) or SolarMax?
If you are interested in casual visual observing, a PST is the most economical entry into solar H-alpha observing. The compact design means even most camera tripods will be sufficient mounting. The small BF5 (5mm diameter) filter on the PST means it can be a challenge to get your eye in, as it required looking directly in line into the eyepiece. The larger blocking filters on the SolarMax scopes make this a lot easier. If you are looking to do solar photography, the larger blocking filters and flexibility offered by the SolarMax series will be much more appealing.
Single Stack or Double Stack?
If you are focusing on edge details like prominences, a single-stack h-alpha scope is sufficient for revealing this edge detail. If you are more interested in surface features, then a double-stack system will reveal notably greater contrast.
Adding a double-stack filter after purchase
You can add a double-stacking filter to a single-stack PST or SolarMax telescope. For the PST and SolarMax II scopes, this is easily done by ordering the double-stack filter. For the SolarMax III series with the external etalon design, the two etalons need to be matched at the factory, so you will need to arrange sending in your SolarMax III to Coronado to match a Double stack filter. This is not necessary with the PST and SolarMax II telescopes because of the internal etalon design.
Coronado PST and SolarMax scopes are all compatible with just about any 1.25” eyepiece. Coronado does offer their own CEMAX eyepieces that have coatings optimized for h-alpha viewing and reducing internal reflections, but any preferred eyepiece will work.
Eyepiece field of view is generally not an important consideration when selecting eyepieces for use with h-alpha telescopes. The reason for this is the narrow blocking filter aperture (5mm on the PST, and from 10mm to 15mm on the SMIII 70). Because the diameter of the blocking filter is restricting the field of view, using wider eyepieces doesn’t provide better viewing. In fact, the recommendation is to stick to simpler eyepieces like Plossls (which the Coronado CEMAX eyepieces are a version of) to reduce the number of elements and opportunities for optical reflections.
The wider BF30 filter in some SolarMax III 90 scopes can allow you to take advantage of wider FOV eyepieces, but the same recommendation applies to stick to simpler optical designs with fewer elements and good anti-reflective coatings for the best viewing.
Solar imaging is similar to planetary and lunar imaging, in that the atmosphere plays a big role in clarity from moment to moment, so recording a short video to process the best frames out of is an optimal approach to get sharp solar images.
What Blocking Filter size should I use?
Blocking filter sizing goes by the clear aperture in mm. Thus, a PST with a BF5 has only a 5mm clear aperture. SolarMax scopes will include either a 10mm, 15mm, or 30mm blocking filter depending on the model selected. The BF10 is suitable for imaging with a planetary camera with a small sensor. If you intend to image with a DSLR, Mirrorless Camera, or DSO Astro camera with a larger sensor, then the BF30 filter is the ideal selection to avoid or minimize vignetting. A BF15 is still suitable for an APS-C camera. Even though it will cause vignetting, you will still achieve a full solar disc view.
Which Scope to choose for imaging?
The inexpensive and compact PST is a suitable budget choice for imaging with a small sensor planetary camera. The PST will require the use of a barlow to extend the focal point. Because of this and the small BF5, you will not be able to achieve a full disc view of the sun all at once, and will have to do a mosaic to create a full disc image.
The inexpensive and compact PST is a suitable budget choice for imaging with small sensor planetary cameras that have small pixels, high frame rates, and (ideally) monochrome sensors. Some dedicated planetary cameras can simply be placed into the PST eyepiece holder and reach focus, showing a full disk image. Other planetary cameras may require a low-profile adapter to reach focus. DSLRs will require a 2X barlow T-adapter to reach focus. Because of this and the small BF5, you will not be able to achieve a full disc view of the sun when using a barlow and will need to do a mosaic to create a full disk image.
The BF5 filter in the PST is even less suited to larger camera sensors, so if you want to do solar imaging with anything other than a planetary camera, or achieve full disk images with a DSLR, you will need to move up to the SolarMax II 60mm or SolarMax III 70mm. A BF10 filter will still have only a 10mm dimeter clear aperture, which is perfectly suited to planetary cameras. APS-C sensors are approximately 23x15mm, so a BF10 will producing significant cropping, whereas a BF15 is just wide enough to cover at least most of the height of an APS-C sensor. You can also potentially utilize a barlow to double the magnification and make better use of the sensor in the SolarMax III 70 scopes while still achieving a full solar disc image with the BF15.
The SolarMax 90’s longer focal length means that with the BF15 filter, you will get a fill disk image for an APS-C camera. If you are imaging with a full frame cameras, A BF30 filter on the SolarMax 90 will show the full solar disc on a full frame (36mm x 24mm sensor) camera.
Note that there is a downside to larger aperture scopes for solar viewing, which is that they are more susceptible to atmospheric effects. Having clean steady air is required to get the best results in particular from the larger SolarMax 90mm scopes. In Denver for example where the air tends to be more turbulent, you may achieve a sharper image with a smaller aperture scope.
Focusing & Tuning The Telescope
Unlike with regular telescopes where you focus your image and are ready to go, h-alpha telescopes require tuning in addition to focusing to achieve the best contrast view. This can make h-alpha telescopes seem intimidating at first, but a little practice with allow most people to quickly overcome any hesitations.
Coronado SolarMax III telescopes includes two methods of tuning. The first is called Rich View Tuning, which is done by rotating the large gold ring on the front of the etalon. This adjusts the air space gap between the etalons to optimize the h-alpha bandpass for your current observing conditions. Rotate the ring back and forth to see where you find the highest contrast view. This made be slightly different from one edge of the image to the other.
Next you may notice some reflections in your image. If you see this, you can use the T-Max Tuning (the small gold ring on the edge of the filter) to adjust the angle of the filter to move the reflection out of your view. As you tilt the etalon, the transmission band widens, so less tilt is better. Use just enough tilt to clear reflections out of the way of the solar disc.
For double-stack scopes, start by adjusting your focus, then tune by adjusting the front-most etalon first using the Rich View tuner, then the T-Max (tilt) tuner, if needed to eliminate reflections in the view. Then move down to the primary etalon Rich View tuner to further refine the contrast on the solar surface, and then again move to the T-Max tuner on the first etalon if needed. Once you’ve passed through the tuners from front to back, check your focus again, then you may want to go back to slightly adjust the front-most (Double-stacked) etalon again to further refine the image.
Tip: On the PST, the solar finder is integrated into the body. If a double-stack filter is attached, this may block the solar finder. If it does, you can locate the sun prior to attaching the double-stack filter with, or use the shadow on the scope method to point at the sun.
SolarMax II or SolarMax III?
In 2023, Coronado reintroduced the SolarMax II series for a limited production run leading up to the 2023/2024 solar eclipses. There are a few key differences between the SolarMax II and SolarMax III series. The first difference is the helical focuser on the SolarMax II vs the upgraded two-speed Rack and Pinion focuser on the SolarMax III. The second difference is the etalon design. The SolarMax II uses an internal etalon, while the SolarMax III uses an external etalon. The SolarMax III external etalon has the advantage of richer contrast and a wider environmental operating range. If you will be operating the scope at altitudes above 8,000 feet, Coronado recommends the SolarMax III in this environment to provide better contrast of solar features. A third small difference is that the SolarMax II scopes do not include a dovetail bar, so this will need to be purchased separately, while the SolarMax III scopes include a vixen style dovetail.
One final consideration with the SolarMax II is the internal etalon design is more susceptible to possible banding (a ripple effect) in the image. The SolarMax III external etalons do a better job of avoiding this.
For most people, the SolarMax II series offer a bargain price to get a great H-Alpha solar scope. Serious images who want to shoot with a full frame camera will want the SolarMax III 90 with BF30 filter. (The current SolarMax II 90mm scopes is limited to the BF15.) Similarly, imagers with APS-C cameras may want to opt for the SolarMax III 70mm with 15mm blocking filter vs the BF10 offered on the SolarMax II 60mm. Visually, the SolarMax II 60mm BF10 produces a very pleasing image that doesn’t have the tunnel aspects of the PST that require direct alignment of your eye with the eyepiece.
How do Coronado Scopes compare to competitors?
Lunts and Coronados are generally comparable price for performance. The external etalon design like that found with the SolarMax III series will generally yield the best contrast and detail vs internal etalon designs. The Rich View tuner will perform similarly to the Lunt Pressure tuner. Same idea, different execution.
Daystar is a more ambiguous comparison.
The Daystar Quark etalon is solid and requires heat from electricity to make adjustments. Heating the element takes time, typically about 10 minutes to adjust. There is no waiting after an adjust or cooldown / warmup time with the Coronado scopes.
See the Coronado Solar Glossary for helpful definitions and references. This is relatively concise and dense, and can actually answer a lot of common questions.