Why an off-axis stop-down mask?

“Often the effects of poor sky stability can be countered to a certain degree. Larger aperture scopes are adversely affected by atmospheric turbulence more than smaller scopes. The cylinder of light of parallel light rays collected by an 8-inch object is subject to a larger cross section of the atmosphere than is that of a 3-inch. The greater the amplitude of of variation in air parcel boundary layers perceived by the system, the more this refractive aberration manifests itself as image wavering. “For certain deep-sky objects (notably double stars) one needs to trade light gathering ability for turbulence reduction. The aperture of the telescope can be reduced by employing a mask over the light gathering end…..”

Tom Lorenzin “1000+ The Amateur Astronomer’s Field Guide to Deep-Sky Observing”

There is science and calculations as to how large the reduction mask should be, and that it should not extend over the light path of the secondary of a Newtonian. However, in past applications, from my experience, going beyond the path of the secondary has never presented a problem. Now there are limits, as common sense would dictate. So, I’d suggest no more than one-inch over the path of the secondary might be a good rule of thumb. After all, it’s just a small “crescent” cut-out over the path.

So for my 10-inch Newtonian, this time, I’m using a 5 1/8-inch reduction diameter hole. In the past, my 4.25-inch reduction hole, has worked great for some difficult and very close double stars.

I’ve never tried or attempted to see the companion to Sirius, known as Sirius B with a Newtonian reflector, but anxious to give it a try with my 10-inch f/4.5 Newtonian with an off-axis stop-down mask. At the moment, I feel confident, this set-up will allow me to see the companion, with my Newtonian. 🙂

I just need a night of very stable or excellent seeing.

The materials I used for my off-axis mask:

I purchased the following materials from the Dollar Tree. A fairly large sheet of corrugated for only $1.32, and a roll of pre-cut double sided tape tabs was about the same.

So, there you go….all materials to make an 11.875-inch corrugated disc for less than $3.00. And from this, I cut out the off-axis 5.125-inch effective aperture.

November 1st 2023:

I might even try during an early morning observation this week, as Sirius is in very good sky position at 5:00 AM. But this will require that I have everything set-up and ready at 4:00, especially to cool the scope which is critical to achieve this feat.

I already have my sketch card with the “Position Angles” marked, which can be very helpful as knowing the exact position of B, can make it easier and save time.

So my report from this morning…November 2nd 2023:

Beginning at 5:30 AM, with waning gibbous moon, almost 3rd quarter, and still pretty bright. I concluded my session at 7:20, approximately 30 minutes before sunrise. I was able to see the companion, but with difficulty due to poor seeing. I was hoping for a better view of the companion, as it could be seen only intermittently, and Sirius was really ragged and moving.

I kept hoping for that perfect “snapshot” view….one that I can never forget, from the night in March 2016, looking through a friends 102 mm f/8 Takahashi fluorite refractor. Memorable for sure!

Surprisingly, I could still see the companion just as well at 7:20 AM in very “dim daylight” and still with a fairly bright moon, as when I began almost two hours earlier. This would show and prove that double stars can be enjoyed with a moon and light pollution.

The mirror was not cooled down, so I added a fan shortly after beginning, which helped a lot! Consider this, if your telescope being a Newtonian, Makustov or other does not have an internal fan.

My humble set-up to see Sirius B, using my portable mount. Now to bring everything in, change to a larger mount and prepare to observe galaxy IC 10 “again” or the second time, tonight.

If you’ll notice, the companion is near it’s most widest position from Sirius, so if you’ve never seen Sirius B, now is the perfect time, or actually for the next two or more years. So you have plenty of time.

I always keep a small plastic protractor handy, to determine “the Position Angle” for double stars. It’s very versatile, and can work for any type of telescope you’re using. If using a standard Newtonian, just turn the protractor to match your view.

However, if using a Schmidt-Cassegrain, Maksutov or a refractor with a diagonal, which have an uneven number of mirrors….just flip it over and find your actual orientation. A great device for only a couple dollars at Hobby Lobby.

My story of seeing Sirius B in 2012, and then again in 2016, using two separate 102 mm refractors, as following at different locations:

Date: March 14th 2012:

Telescope:  102 mm f/9.8 achromatic refractor, with good seeing and transparency.

I started with a magnification of 83x, but to no avail, and increased to 232x…all the while keeping my eye perfectly still. I used my astro-chair, as it’s very important to be seated for an attempt such as this, or making any deep-sky sketch.  

Knowing the position-angle really helped.  However, I could not hold the companion, or Sirius B constantly.  It was extremely difficult.  But after 40 plus years, I had finally seen the companion to Sirius.  

102 mm Takahashi f/8 fluorite doublet during March 2016

It was actually “very easy” with this scope, and with direct vision. A group of six amateurs took the test using the Takahashi refractor, and what did they see?   

Using the face of a clock, and all without sharing their perceived position of the companion until everyone had the opportunity to observe.  When the last person completed their observation, all correctly identified the position of Sirius B.  

Some were very skilled observers, but a couple or more were not, however, all could “easily” locate and see the companion.   

Observation report by Magda Streicher from South Africa:

Roger, thanks so much for the off-axis mask idea….wonderful.

Sirius B:

After several or should say “many” attempts I spotted Sirius B in May 2020, using averted vision. My eyes were tearing, cloth over my head and let Sirius pass through the eyepiece. However, after 45 minutes of struggle, I saw the companion.

To-Date: I have now found and seen the companion, also known as Sirius B several times, but it’s still never an easy. It is important to have a stable and clear sky. Good seeing is “absolutely critical” if you are considering taking on this task.

Interesting; I found if Sirius is in the west and after sunrise, seeing Sirius B is actually easier, which might be surprising to many. Roger Ivester, mentioned the same in his report.

My notes and sketch are as following: Magda

Much appreciation to Charlotte and Teresa for putting this fabulous event together. Everything was perfect! A memorable class reunion for sure.

If you are reading this, please share some of your favorite photos during the event, and I’ll add them to this post.

Hopefully much more to come in this post…

In the following photo…these five guys, began 1st grade at Polkville in August 1959, and graduated together at Burns. We were the first class at the new school, and went all four years.

However, after the 5th grade, some of us parted ways for three years, some went to Casar and some went to Lawndale (Piedmont)…but back together again in the fall of 1967.

Left to right: Charles Melton, Roger Ivester, Rob Pendleton, Bill Ledford and Don Bridges.

Bortle Dark Sky Scale: The Bortle Dark Sky Scale was developed by John Bortle “based on nearly 50 years of observing experience,” to describe the amount of light pollution in a night sky. It was first published in a 2001 Sky & Telescope article.

Depending on the device you are using, it may be necessary to slide the chart to the right to see in its entirety.

Number Code

Map Color Code

Label

Sky Mag.

Naked Eye Limit Mag.

320mm Limit Mag.

M33 visible?

M31 visible?

Central Galaxy visible?

Zodiacal light visible?

Light Pollution

Clouds

Ground Objects

1

excellent dark sky

22.00–21.99

≥ 7.5

> 17

obvious

.

casts shadows

striking

airglow apparent

.

visible only as silhouettes

2

average dark sky

21.99–21.89

7.0–7.49

16.5

easy with direct vision

.

appears highly structured

bright, faint yellow color

airglow faint

dark everywhere

large near objects vague

3

rural sky

21.89–21.69

6.5–6.99

16.0

easy with averted vision

.

complex structure

obvious

LP on horizon

dark overhead

large distant objects vague

4

rural/suburban transition

21.69–20.49

6.0–6.49

15.5

difficult with averted vision

obvious

only large structures

halfway to zenith

low LP

lit in distance

distant large objects distinct

5

suburban

20.49–19.50

5.5–5.99

14.5–15.0

.

easy with direct vision

washed out

faint

encircling LP

brighter than sky

6

bright suburban

19.50–18.94

5.0–5.49

14.0–14.5

.

easy with averted vision

visible only near zenith

.

LP to 35°

fairly bright

small close objects distinct

7

suburban/urban transition

18.94–18.38

4.5–4.99

14.0

.

difficult with averted vision

invisible

.

LP to zenith

brilliantly lit

.

8

city sky

< 18.38

4.0–4.49

13

.

.

.

.

bright to 35°

.

headlines legible

9

inner city sky

.

≤ 4.0

.

.

.

.

bright at zenith

.

.

My view to the east (last night @ 9:15 PM, November 18, 2023 from my back deck) showing the light pollution at about 15º above the treetops.

I always try to observe deep-sky objects at least one hour or two from the meridian.

It is difficult to see the Pleiades “visually” at this low altitude. I was using an iPhone 14 “handheld” to make a photo for illustrative purposes.

The Pleiades was my first deep-sky object when I was 12 years old. I remember so well, walking out in my front yard in about mid-November and seeing this cluster rising above the treetops.

At that time, I had no clue as to what this group of stars was.

A year or so later I found out it was the Pleiades, M45 and my interest in amateur astronomy began, which continues to this day.

The following photo:

Andromeda is difficult for me visually, and requires averted vision. At the meridian, it can be fairly easy, but averted vision still works best.

The following photo using my phone, presents the galaxy quite a bit brighter than naked-eye.

So, I estimate the Bortle Scale from my backyard to be between 6.5…or between Bortle 6.0 and 7.0.

Meade introduced their Deep-Space telescopes in about the early 1980’s and caught my eye in advertisements.

I was looking for something larger than my 4.5-inch reflector which I was using at that time. My largest telescope previously was a Criterion RV-6, but life got really busy for a period, and sold it.

The Deep-Space series consisted of two telescopes: A 10-inch and 16-inch f/4.5 telescopes with equatorial mounts.

This was at a time when Meade was building their serious Newtonian’s in their Costa Mesa facility, which included grinding and polishing mirrors. All of the hardware, bolts and Allen heads on these telescopes and mounts are SAE (Society of Automotive Engineers) and not metric.

The following is a Meade advertisement from almost 40 years ago, as of 2024.

My story of getting a larger telescope:

At the time, I was contemplating moving up from a 4.5-inch reflector to something larger.  My astronomy club (at that time) had a DS-16 which was a two-person job to set up.  I could take the 16-inch and use at any time, but this thing was a monster, and wouldn’t even fit “effectively” into the back of a van! 

Where would I store this behemoth of a telescope? I didn’t and still don’t have a permanent observatory, and how would I set it up by myself? The 16-inch was definitely not for me!  

The 10-inch seemed to be the perfect larger aperture telescope and with an equatorial mount. I had always liked the simplicity of a Newtonian, and an equatorial mount was a must.

Astronomy Magazine test and evaluation:

Astronomy Magazine tested a 10-inch DS-10A (The 10A was an updated version with a 2-inch focuser, setting circles, and RA drive) with Astronomy giving the test mirror a rating of a B+. 

That settled it…this was the telescope:

So in February 1992, I purchased the 10-inch from Pauli’s Wholesale Optics, in Danbury Connecticut.

And now with over 2,500 hours under the night sky, and over 1,500 deep-sky pencil sketches with even more notes, the telescope is still in pristine condition. I made the right decision and 32 years later as of 2024, I continue to use and enjoy this telescope.

Something for thought: 2,500 hours at the eyepiece over a 32 year period would be equivalent to 63 weeks working a job at 40 hours per week.

I was observing a summertime galaxy with this telescope early today (5:00 AM EDT, Saturday, April 6th 2024) due to the lower humidity and better transparency. The summer’s in North Carolina are often very humid with haze, so an early spring observation is far better.

My other telescopes include a 102 mm refractor, a 6-inch f/6 reflector, an 80 mm f/5 refractor and a couple others.

Shown below is the original mount, which I use when observing deep-sky objects that can’t be observed from my back deck. All objects greater than + 5 north latitude, I can use the Celestron CGE-Pro, as it’s not portable enough to take out into the back yard.

Note the larger 6-inch Parks Optical declination circle, and a handmade brass index pointer.

Sometimes a suburban backyard with unshielded streetlights in close proximity, can make it difficult for 100% star hopping. For efficiency, I use both the RA and Declination circles to get me within 1/2º degree of my target. At this point, I use a star atlas to zero in on my deep-sky object.

The original mount as pictured below is a medium duty mount, and is still fairly heavy and remarkably sturdy, due in-part to the short pedestal.

This mount is perfect for use with an astro-chair, when making observing notes and pencil sketches at the eyepiece.

As for me, it is impossible to observe while standing.

Set up in my backyard using the original mount, as pictured below:

Both mounts, side by side for a comparison:

The Celestron mount is too large and heavy to take in and out of the house, so this Rubbermaid shed on my deck works well for storage. I also cover (inside the shed) the mount with a throw and a piece of auto-marine fabric for extra protection.

The original mount is fairly easy to carry and can be stored in the corner of an extra bedroom.

A sample of a few of my sketches, using only a pencil, and a 5 x 8 blank notecard. My “humble sketches” are all raw sketches, as they appear to me through the eyepiece, without any computer enhancements. I try to make my sketches as accurate as possible, with any embellishing.

NGC 2300 and 2276 – Galaxies in Cepheus

The NGC 3190 Leo Galaxy Group, as following is one of my favorites. I have looked at this galaxy group many times, but never been able to produce a sketch better than the following: March 24th 1995

IC 5146 “The Cocoon Nebula” In Cygnus

NGC 2024 Nebula in Orion

M42: Orion Nebula Complex

M106 Galaxy in Canes Venatici

NGC 3079 Galaxy in Ursa Major

Veil East

Veil West

Messier 20 In Sagittarius

Messier 41 using a 6-inch f/6 reflector

Messier 85 and NGC 4394

Comet Hale-Bopp: Charcoal pencil on black card stock, showing all three-tales; the gas tail, dust and anti-tail. April 1997

Messier 81 with spiral arms

NGC 7479 Galaxy in Pegasus

The Rosette Nebula

M53 Globular Cluster

NGC 5689 Galaxy in Bootes

I also use the Celestron mount with my 6-inch reflector:

Messier 33

More photos of my DS-10A telescope:

Losmandy Dove Tail for adapting to Celestron CGE-Pro Mount.

The brass studs stay if the original Meade Mount is used.

The wing nuts were replaced with the sturdy hand-knobs, and allow the scope to be stored upright.

The telescope can now be set on the mirror end for storage in a closet close to my back deck.

My wife enjoys using the 10-inch to take photos of the moon:

Seeing the companion to Sirius, known as Sirius B or “The Pup” using the 10-inch Newtonian:

Why an off-axis stop-down mask?

“Often the effects of poor sky stability can be countered to a certain degree. Larger aperture scopes are adversely affected by atmospheric turbulence more than smaller scopes. The cylinder of light of parallel light rays collected by an 8-inch object is subject to a larger cross section of the atmosphere than is that of a 3-inch. The greater the amplitude of of variation in air parcel boundary layers perceived by the system, the more this refractive aberration manifests itself as image wavering. “For certain deep-sky objects (notably double stars) one needs to trade light gathering ability for turbulence reduction. The aperture of the telescope can be reduced by employing a mask over the light gathering end…..”

Tom Lorenzin “1000+ The Amateur Astronomer’s Field Guide to Deep-Sky Observing”

There is science and calculations as to how large the reduction mask should be, and that it should not extend over the light path of the secondary of a Newtonian. However, in past applications, from my experience, going beyond the path of the secondary has never presented a problem. Now there are limits, as common sense would dictate. So, I’d suggest no more than one-inch over the path of the secondary might be a good rule of thumb. After all, it’s just a small “crescent” cut-out over the path.

So for my 10-inch Newtonian, this time, I’m using a 5 1/8-inch reduction diameter hole. In the past, my 4.25-inch reduction hole, has worked great for some difficult and very close double stars.

I’ve never tried or attempted to see the companion to Sirius, known as Sirius B with a Newtonian reflector, but anxious to give it a try with my 10-inch f/4.5 Newtonian with an off-axis stop-down mask. At the moment, I feel confident, this set-up will allow me to see the companion, with my Newtonian. 🙂

I just need a night of very stable or excellent seeing.

November 1st 2023:

I might even try during an early morning observation this week, as Sirius is in very good sky position at 5:00 AM. But this will require that I have everything set-up and ready at 4:00, especially to cool the scope which is critical to achieve this feat.

I already have my sketch card with the “Position Angles” marked, which can be very helpful as knowing the exact position of B, can make it easier and save time.

So my report from this morning…November 2nd 2023:

Beginning at 5:30 AM, with a waning gibbous moon, almost 3rd quarter, and still pretty bright. I concluded my session at 7:20, approximately 30 minutes before sunrise. I was able to see the companion, but with difficulty due to poor seeing. I was hoping for a better view of the companion, as it could be seen only intermittently, and Sirius was really ragged and moving.

I kept hoping for that perfect “snapshot” view….one that I can never forget, from the night in March 2016, looking through a friends 102 mm f/8 Takahashi fluorite refractor. Memorable for sure!

Surprisingly, I could still see the companion just as well at 7:20 AM in very “dim daylight” and still with a fairly bright moon, as when I began almost two hours earlier. This would show and prove that double stars can be enjoyed with a moon and light pollution.

The mirror was not cooled down, so I added a fan shortly after beginning, which helped a lot! Consider this, if your telescope being a Newtonian, Makustov or other does not have an internal fan.

My humble set-up to see Sirius B, using my portable mount.

The companion is near it’s most widest position from Sirius, so if you’ve never seen Sirius B, now is the perfect time, or actually for the next two or more years…

I continue to ride, but “absolutely certain” I’ll not be able to do it again. My rides in recent years are quite a bit shorter and much slower, but more fun.

Date: October 9th 2023:

I’ve went from an 80mm f/5 refractor to this 10-inch Newtonian in just a matter of days. So, I’m proud of the progress I’ve make, from never using a camera with a telescope, to “at least” an image of the moon. And all within a week!

I changed some settings on my camera, something I’d never done before. It had been set on ISO 100, for my everyday snapshots, and changed to ISO 400 this morning for this moon shot. I was amazed how much brighter the moon was when focusing through the camera using this 10-inch versus the 80mm refractor.

For sure a lot easier to focus with the brighter view due to the size of the telescope.

The field using this telescope is really narrow and small as this photo shows. Now I’ve got to figure out how to set the “exposure time” for some small and bright deep-sky objects.

I’m actually amazed that I could do this...and hopefully more and better to come. Debbie

Telescope: 80mm: 400mm Focal Length: f/5

Camera: Nikon D3300

Nikon T-Adaptor

Camera Setting: Manual

Vixen GP Equatorial Mount

The following photo was made after sunrise

Nikon T-Adaptor