Why You Should Build an Aperture Mask for your Celestron FirstScope.

An aperture mask is a simple cap you place over the telescope to cut down the aperture. The reasons why you might want to do this are not obvious, but in short, they cut down on optical aberrations significantly. This is especially important in a telescope with flawed optics--specifically those with spherical mirrors. This article will be about masking down a FirstScope, but the same principle could apply to any telescope with a spherical mirror, including telescopes that for the price have no right to have one, such as the deplorable PowerSeeker 127EQ and the AstroMaster 114EQ.

The Celestron FirstScope is one of my favorite small telescopes, and it's pretty good for scanning around for bright DSOs, glancing at the Moon, etc. Its stock eyepieces are utter garbage, and when combined with the fast spherical mirror (76mm f/4), the Huygens 20mm is almost unusable.

A few things were brewing that lead up to me deciding to make an aperture mask for the FirstScope. I knew that the Galileoscope, a nice but very cheap refractor, performed much better on planets and the Moon despite a smaller (50mm f/10) objective. I also knew that the FirstScope's huygens performed much better in the Galileoscope than the FirstScope (though still not good).

And of course I knew that the slower the focal ratio of the telescope's primary objective, the less aberrations (of all kinds--chromatic, spherical, coma, eyepiece or objective) you get, to the point that an f/8 or f/9 spherical mirror is pretty decent.

The revelation came when I realized this rule also applied if you reduce the aperture diameter to increase the focal ratio, rather than building the objective with a native longer focal length. This is done in telescope moon caps (the smaller caps on a full-aperture dust cap on many telescopes) to dim the view, but it also reduces aberration. I saw a post by 10 Minute Astronomy where they built an aperture mask for their fast achromatic refractor, and knew I had to try this with the FirstScope.

Finally yesterday I got around to taking the FirstScope inside from my car where it always stays, and making two aperture masks.

The single-aperture mask on the FirstScope.

The first is a poor-man's Bahtinov Mask--simply two circular holes which produce two separate images when out of focus, designed to aid in focusing telescopes. In my case, I wanted it so that I could open the aperture up without dealing with the secondary mirror. Each hole is 1" and the equivalent clear aperture is 1.41" or 36mm. It seems to produce two exit pupils when looking far from the eyepiece.

The second mask, shown on the front, uses a significantly larger hole which provides the same clear aperture accounting for the secondary mirror obstruction. Its diameter is about 1.77", and again its equivalent clear aperture is 1.41".

People have asked why the single-aperture-mask is not an off-axis mask to avoid the secondary mirror? The mirror obstructs so much, surely the image would be useless, lacking contrast. This is in fact necessary, for the size of an off-axis aperture small enough to miss the secondary mirror in this telescope would be too small to be useful. I'm also not convinced an off-axis aperture mask would work as well on a spherical mirror, but I could be mistaken.

The masks are both fastened to the telescope with three yellow plastic screw things, the top notch of which kinda grabs the metal endcap on the open tube to stay in place. I originally used slightly more stable nails, but unfortunately I had to remove them because they were nails.

The diffraction limited magnification with the mask is about 72x, and the SR4mm eyepiece shipped with the FirstScope provides 75x, which matches up pretty nicely by coincidence. Minimum power is 6.37x in theory, but I don't have an eyepiece with the right focal length and you don't NEED an aperture stop to reduce the magnification below the useful lower limit (your iris does that for you).

On the first night there were clouds and rain, but it ended up being rather spectacular the night after (2020-09-13), with a rough start. Clear dark skies and excellent seeing.

The focuser mask didn't end up being useful (it tended to produce double images even when in perfect focus, unless i took my glasses off, and it had a strong rainbow diffraction artifact going along the direction of the two circles. It was also not really easier to focus with it than to just use the single-hole mask. So all of my results will focus on the one-hole mask.

For testing, I used both the H20mm and SR4mm eyepieces which came with the telescope, and the K20mm and K6mm eyepieces which would have come with the Orion FunScope. I would have tried the K10mm, which would have come with the Celestron Cometron, but I forgot and it wouldn't have made much of a difference anyway. I tested planets and double stars (Deep Sky Objects need bright light and don't need sharpness as much, so I didn't bother testing them)

I went out at 8:30 but the sky was cloudy, so I went back home. I drove back out at 9:30 and it was clear and very nice.

The results are as follows:

Jupiter

• H20 Unmasked: Just a white blob, no detail, no clear disk, can barely see moons.

• K20 Unmasked: Moons more clear. Still uncrisp, but a disk is visible.

• SR4 Unmasked: Obvious disk with no detail. Blurry. One of the three visible moons is just barely able to be split into two.

• K6 Unmasked: Disk still surrounded by a blurry halo, but more crisp, with a hint of banding. The two close moons are easily split.

• H20 Masked: Jupiter went from a glowy blob to a disk with streaks/spikes coming from it.

• K20 Masked: View is not perfect, but much improved along similar lines as H20.

• K6 Masked: reveals subtle cloud banding.

Saturn

• H20 Unmasked: elongated, blurry, streaky dot.

• H20 Masked: Faintest hint of ring.

• SR4 Unmasked: Not bad. Ring was easily split from planet but the object was covered in a halo with false color.

• SR4 Masked: Very dim, sharper. Better than K6 unmasked.

• K6 Unmasked: Easier and sharper than SR4 Unmasked.

• K6 Masked: dim and very sharp. Faint hint of detail beyond just "ears" or "rings," a little bit of shading on the disk and hint of shadow of planet on rings.

Alberio

Unmasked

Alberio was hard to find near the zenith even with the red dot finder, and hard to identify in the K20. It is split after a short moment in the K6 as a pale white-yellow and pale blue double. SR4 is just as easy to split and looks surprisingly nice. H20 barely splits alberio. If I didn't know exactly where to look I'd never have seen it in the H20. Difficult to split with the K20 but not as bad as H20.

Masked

H20 easily splits Alberio but Alberio B is very dim and colorless. Ditto for K20.

Alberio is no easier to split with the K6 w/ mask than without. Ditto for SR4.

Double Double (Epsilon Lyrae)

Epsilon Lyr is easily split in two with H20 & K20 without mask. The double double is not split with any eyepiece or mask. I felt as though I could, just barely, tell there was an extremely subtle elongation of the two stars if I really paid attention with the K6 masked, but my guess is there wasn't enough resolution even with a perfect objective.

Polaris

Not so absurd as you might think--I've heard of people succeeding in splitting Polaris with a 3" scopes, and Roger Ivester speculated a 30mm scope should be able to do it. But with this telescope I had no such luck. Maybe the subtlest hint with the unmasked K6, but the masked K6 removed that hint. What was fairly interesting is that it seemed like I could see Airy Disk rings and a bit of extra spokes and streaks with the high power using the SR4 & K6 masked. Seeing seemed to be very good at this point. After I shared with him my initial results, Roger Ivester asked me to make a 66mm mask and try Polaris again. I haven't yet, but will follow up when I do.

Gamma Andromeda (Almach)

Not split with K20 or H20, w/ or w/o mask. Not split with SR4 or K6 without mask. It was split with the SR4 & K6 with the mask, but it was tough and subtle.

Eta Cassiopeiae (Achird)

Not split w/ K20 w/ or w/o mask. Not split with SR4, K6. MAYBE split w/ SR4 plus mask. Not split w/ K6 & mask.

Mars

By this point, at 11:21, Mars had risen to about 25 degrees. The Mask does show an improved disk w/ the K20, but it's still not a perfect circle. But the mask vastly improves the view with the H20, going from unrecognizable blob to a circular disk.

The SR4 shows a disk with a lot of blobby streaks coming out of it. I could see a bright spot on one side, which was consistent with the ice cap, which was quite surprising.

With the mask I could then tell that it was a gibbous phase, with the bright spot still visible, and a dark gradient on the other side. Similar story with the K6.

After that I packed up.

You can see my sketches and scribbled notes here: https://imgur.com/a/ca7iP1y

The Moon

A few days later (tonight as I write this, 2020-09-21 8-9 PM EDT), I had the chance to try it on the Moon. The performance of this scope on the Moon isn't the worst thing in the world. There's this property of the FIrstScope that there's generally a crisp image hidden under a blurry halo, and you just have to pay close attention to pull that crisp image out. But with the stock eyepieces, there are some real issues. The H20 almost never gets the Moon in sharp focus, and the SR4 shows a severe ghostly, slightly colorful halo around the Moon, washing out contrast and making it hard to see.

The aperture mask almost completely solved these issues. The Moon became much crisper in the otherwise heavily distorted H20mm eyepiece, and the blurry halo went away in the SR4. The views in the 20mm & 6mm Kellners and 15mm Goldline eyepieces were also improved, to the point that they rivaled the sharpness of the Galileoscope, and were a joy to look through at medium and high powers.

Here are two pictures of the Moon taken with the FirstScope using a 10mm Kellner eyepiece. Note that these are single frames in very poor seeing conditions, so the masked version doesn't really represent how sharp the masked FirstScope gets, but you can see just how much worse the unmasked FirstScope is. (Note the pictures don't account for image brightness. The unmasked moon is much brighter.)

With Mask

Without Mask

 

 

 

 

 

 

 

 

 

 

 

 

 

Conclusion

As should be expected from going from f/4 to f/8, the views did significantly improve in almost every case where sharpness matters, and the Huygens eyepiece went from almost unusable to just kinda bad. But it wasn't perfect--there's still some blobs and streaks in the view. Still, it was a vast improvement and I think anyone with a FirstScope or one of its clones should fashion an aperture mask of this sort. It absolutely saves the day when it comes to the Moon and planets, and can split some double stars.

This will also work on any other telescope with a spherical mirror, such as the worst telescope ever sold: the PowerSeeker 114EQ. That telescope comes with a sub-aperture-cap on its dust cap to begin with. While it's a pretty bad deal that you have to stop its aperture down from 5" to 1" or 2", it might be something to try if you have that monstrosity.

The FirstScope's included eyepieces (for the default IYA and Moon signature variants) are truly awful and need to be upgraded. But the nature of the FirstScope is it's meant to be incredibly cheap, and if you're on such a shoestring budget you can't afford a better scope, you probably can't afford better eyepieces. Making an aperture mask is incredibly cheap (possibly free) and can drastically improve viewing. I think it'd be great if the FirstScope came with an Aperture Mask built into the dust cap, like on the Orion SkyScanner 100, where it is less needed.

Also for some reason in reviews of the FirstScope, people are very negative about the SR4 eyepiece and forgive the H20. The SR4 is actually the better of the two. Neither of them are good, but the H20 is truly horrific in its distortions.

Next up after a larger 66mm FirstScope mask is to make a mask for the Galileoscope which represents the absolutely puny <1" aperture of Galileo's instruments, so that in combination with the GS's optional galilean eyepiece I can truly experience the awful hell Galileo dealt with in observing. But that'll be another story.

TL;DR, wear your masks!