Ready for the Solar Eclipse

Okay... I know I said no more astro, but there's a solar eclipse happening on Monday that everyone's excited about, and that I've long been preparing for. After the total solar eclipse on April 8, 2024, the next total solar eclipse that can be seen from the contiguous United States will be on August 23, 2044 (NASA). Unfortunately, I won't be in the path of totality for this one. The maximum eclipse will only be 41.75% from my location, but the forecast promises clear skies!

Solar Eclipse Altitude Chart by Astrospheric

Path of Totality Map by Astrospheric

Forecast by Astrospheric

Lately it's been mostly cloudy and rainy. Which hasn't allowed for much of an opportunity to observe, or to test out gear. We got a break in the weather today, though! Skies were nice and clear. So I got my gear out, and setup for some solar observing.

I'm using a Celestron AVX mount with a small side-by-side dual-mount Vixen dovetail bar:

There are better side-by-side options out there, but these worked out decently well. I originally purchased these saddles to use as wall mounts for the telescopes when not in use, but I hadn't gotten around to installing them yet. So I repurposed them for this side-by-side setup. Which is something I've wanted to try out for a while.

The main scope in this setup is a classic Meade Model 290-P 60mm f/15 achromatic refractor equipped with a Baader AstroSolar ASSF-50 OD 5.0 white light solar filter. I'm pairing the 900mm focal length with a Celestron 15mm eyepiece for a magnification of 60x. At this magnification, the solar disk fills the field of view nearly to the edge, and produces sharp views of sunspots and faculae.

The second scope used in this setup is the Celestron AstroMaster 130, a 5-inch Newtonian equipped with a Celestron EclipSMART Solar Filter. I was aware that the optical quality of the EclipSMART filter would be inferior to that of the Baader AstroSolar filter before purchasing, but the filter itself was more of a bonus feature for me anyway. I purchased it primarily as a dust cap replacement for the scope since mine never came with one. Surprisingly, the filter performed well enough to observe large sunspots, and even a few of the smaller ones. I found the warm orange-yellow color cast of the filter to be appealing, too. The result is similar to that of false color white light images of the Sun.

The Celestron AstroMaster 130 suffers from a number of issues that impact visual quality at higher magnifications. The most damning of which may be that it exhibits significant spherical aberration due to the primary mirror having an elliptical figure with a short focal ratio of f/5. 

Celestron claims that the primary mirror of the AstroMaster 130 has a spherical figure, making it subject to spherical aberration. The effects of spherical aberration are manageable with focal ratios of f/8 and slower. Unfortunately, the AstroMaster 130 has a fast focal ratio of f/5, making the effects of spherical aberration more severe. 

It's also worth noting that spherical mirrors do not produce coma, or comatic aberration. However, long exposures captured with the AstroMaster 130 show that it does produce coma.

ASTAP Aberration Inspector 3x3 grid view of the Orion Nebula, captured with a Celestron AstroMaster 130 and Fuji X-T100. This view is used to inspect the shape of stars for signs of optical aberrations in the corners, along edges, and at the center of an image.

So the primary mirror isn't truly spherical. Additionally, performing a defocused star test shows that the figure is also not parabolic. Therefore, it must be something in between. The primary mirror of the Celestron AstroMaster 130 appears to have an elliptical figure as it produces both coma and spherical aberrations, making it marginally better than a shaving mirror. This is known among astronomy enthusiasts as a hobby killer.

To work within its limitations, I opted to pair the Celestron AstroMaster 130 with a Celestron 25mm eyepiece for a lower magnification of 26x at 650mm focal length. This provides a much wider field of view than that of the Meade 60mm f/15 refractor at 60x magnification.

Aside from having to balance the telescope payload on three axes, one of the more challenging aspects of a side-by-side configuration is aligning the optical axes of the scopes to center a target within the field of view for each scope. Initial results were surprisingly good. Although, the difference in field of view between the two scopes did make this process much more forgiving. With the large disk of the Sun centered in view of the refractor at 60x magnification, the smaller disk of the Sun at 26x magnification was positioned nearly at the edge of the field of view with the Newtonian.

To align the optical axes of the scopes with each other, first center one of the scopes on the disk of the Sun. Then pivot the saddle of the other scope slightly to center the horizontal position of the Sun within the field of view, and adjust the height of one of its tube rings to center the vertical position. The Vixen dovetail of the refractor has set-screws to adjust the height of tube rings. Which are mainly intended for correcting cone error, but can be used in a similar manner to align the optical axes for side-by-side setups as well.