AstroNotes 2020 April Vol: 59 issue 04

Editor’s Message . President’s Report . Ottawa Skies . FLO Star Parties . Monthly Challenge Objects . Estelle’s Pick of the Month . Announcements . Carp Star Parties . Next Meeting . Centre Information



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Editor’s Message

Wow! It is almost the middle of April. It feels like I have been living in a cave without human contact for a month. Oh wait, I have been! I hope your Covid-19 isolation is going well and all are healthy.

I have to hand it to our meeting chair, Dave Chisholm and Chris Teron for finding a way to go ahead with the April meeting despite the Aviation Museum being closed and nobody being allow out to play. The presenters all did a great job and what few problems presented themselves were almost seamlessly handle by Chris and Dave. Attendance at our virtual meeting was about the same as our usual turnout at the Museum. Well done! It is likely that the May meeting will be another virtual meeting on Zoom. Watch your email for detail.

Throughout this issue, wherever there is a scheduled gathering for an outreach event or a star party, I have left the dates in place. The organizers of the event will send out an email with updates as we get closer to each date so if you would normally attend any of these, please keep an eye on your in box. There is actually some discussion about trying the Astronomy Day event in a similar fashion to our April meeting.

In this issue our feature article is Part One of Jim Thompson’s wonder presentation on astronomical filters. I felt this was important information that many of you would like to have a record of so you wouldn’t have to rely on what you remember from his presentation. If your memory is … anyway, Part Two will be in next month AstroNotes.

Oscar continues to present us with his Monthly Challenge which I know many of you enjoy. If you have taken the trouble to hunt them down, I would love to have your sketches or photos to publish here. Please share them. I know they would be a great addition to this publication.

Stay safe. Clear skies


President’s Report

Hi Everyone, I hope you are doing well in these challenging times and staying healthy.

I bet you have heard the phrase ‘adversity breeds creativity’. We are certainly seeing creativity in spades with people being forced to do things in different ways with the restrictions on travelling outdoors. One example which we can all relate to is the virtual monthly meeting that we held recently, where the meeting was held entirely online. While we have streamed meetings online in the past, we have never in our 114-year Ottawa Centre history held a meeting entirely online. Thanks to Dave Chisholm, our meeting chair, and Chris Teron, our Centre Secretary and meeting tech expert, for making the meeting a success. Jenna Hinds, the National RASC Youth Outreach coordinator deserves thanks as well. She was a driving force, offering training and support on the use of the ‘Zoom’ collaborative tool.

There was an email sent from RASC National office recently to all of us that is noteworthy. It described a new online program consisting of webinars and other posted resources. There are a variety of interesting offerings including ‘Self-isolation star parties’ and another series on how to use the RASC Observer’s Handbook where the authors of the Handbook chapters speak. Please go to for more info.

There are a lot more interesting and creative online offerings that are currently being planned. They will be announced shortly. I’m sure we will learn from the challenges we faced during the pandemic and will adopt these ideas for future use after the pandemic.

I’d also like to share with you some exciting news about our association with the Canada Aviation and Space Museum (CASM). For the last year, we have had informal talks with Jesse Rogerson, the Science Advisor at CASM, about the possibility of building an observatory onsite at CASM. This observatory would be used by the RASC Ottawa Centre and it would support public astronomy programs at CASM. Since it is centrally located in Ottawa, it would be accessible to many. We expect it will be a big draw for star parties, daytime solar observing events and whenever there an astronomy event in the news. Jesse is very excited about it, as I am. Recently, we discussed the topic of the observatory with senior management at CASM, who are also excited and motivated. It is still early days for this idea, and we are looking for funds to build the observatory. One day there will be an observatory at CASM, and it will be a source of joy and pride for all of us. I’ll keep you posted on developments.

Stay well, and please keep in touch.

Mike Moghadam, President
Ottawa RASC Centre

Ottawa Skies

By Dave Chisholm

Full super moon on April 8. This full moon was known by early Native American tribes as the Full Pink Moon because it marked the appearance of the moss pink, or wild ground phlox, which is one of the first spring flowers. This moon has also been known as the Sprouting Grass Moon, the Growing Moon, and the Egg Moon. Many coastal tribes called it the Full Fish Moon because this was the time that the shad swam upstream to spawn. This is also the third of four supermoons for 2020.

Comet PanSTARRS C/2017 T2 will be visible in the evening skies. Its perihelion is on May 8th at which point it should be magnitude 8.

Comet C/2019 Y4 (ATLAS) will come closest to Earth on May 23, 2020. Its perihelion or closest approach to the sun will occur on May 31, 2020.

If predictions are correct, Comet ATLAS might reach a visual magnitude of +5 around May 1, 2020. That is theoretically bright enough to be see with the eye, but the fuzziness of faint comets can make them harder to spot than comparably bright stars.

The Lyrids is an average shower, usually producing about 20 meteors per hour at its peak. It is produced by dust particles left behind by comet C/1861 G1 Thatcher, which was discovered in 1861. The shower runs annually from April 16-25. It peaks this year on the night of the night of the 22nd and morning of the 23rd. These meteors can sometimes produce bright dust trails that last for several seconds. The nearly new moon will ensure dark skies for what should be a good show this year. Best viewing will be from a dark location after midnight. Meteors will radiate from the constellation Lyra but can appear anywhere in the sky.


Not Visible

Rise/Set 06:04/17:02 -> 05:48/19:36



Visible early evening.

Rise/Set 08:17/23:44 -> 07:27/23:44



Visible before sunrise.

Rise/Set 04:06/13:04 -> 03:11/12:55



Visible before sunrise.

Rise/Set 03:40/12:32 -> 01:54/10:50



Visible before sunrise.

Rise/Set 04:00/13:04 -> 02:10/11:16



Visible early part of the evening.

Rise/Set 07:40/21:24 -> 05:50/19:38



Visible later in the month before sunrise.

Rise/Set 06:05/17:20 -> 04:13/15:31

Understanding Astronomical Filters
Part I: What Are They?

Jim Thompson

In this article I will discuss what astronomical filters are. More specifically I will talk about the following topics: what do they do, the different types, how they work, and some common filter nomenclature. In part 2, I will discuss how to choose and use astronomical filters.

To begin let us look at the question: “What do filters do?” The image presented here is of a lovely mountain valley with a glacial lake in the foreground. The colour and sharpness of the image is very good, with lots of detail visible. This is what one might call a perfect image.

Unfortunately, more often than not, something we don’t want to see puts itself between us and the beautiful scene we are trying to observe or image. This is the situation where filters can help. Filters block light that we don’t want to see. By doing so they can improve contrast and sharpness, help to emphasize features, and allow us to see faint details.

Astronomical filters in practical terms are fairly simple. They consist of a thin piece of glass held in a machined aluminum housing. This assembly can be screwed onto the end of your eyepiece or camera nosepiece before you insert it into your focuser. The glass itself has been treated to darken the particular wavelengths of light we don’t want to see. By darkening what we don’t want to see it makes what we want to see easier to see…not brighter, just easier to see. The darkening is achieved by giving the glass one of, or a combination of two principle properties: absorption and/or reflection.

To help understand how filters work consider this image of Mars that was captured by the Hubble Space Telescope. There are many detail features visible in the image including the South Polar Cap and extensive clouds. I have edited the image to look as it would when observed from the Earth’s surface. Looking at Mars through our atmosphere washes out the contrast of the original HST image, making details harder to see. By adding different colour filters, it is possible to highlight certain features. For example: a blue filter makes the polar cap and clouds easier to see, and a red filter increases the contrast of surface features.

Another example of what can be achieved using a filter can be seen in this image I captured of the Cygnus Wall, part of the North American Nebula. I captured it from my backyard in central Ottawa where the light pollution is very bad. With no filter the contrast in the image is poor but adding a 7nm hydrogen alpha filter greatly increases the contrast between the glowing hydrogen and dark dusty sections. Without the use of filters, observing or imaging of nebulae from the city would not be possible.

As I mentioned earlier there are two mechanisms that filters use to block light: absorption and reflection. As it happens, we can broadly divide the available astronomical filters into two categories based on what mechanism they are primarily using. Absorption type filters are more commonly known as Colour filters, or sometimes Planetary filters. They are the simplest to make and least expensive filters available. If you have some in your astronomy kit already, you may have noticed that they tend to have numbers on them; #38A blue, #11 green, #29 red, etc. These are called Wratten numbers which I will tell you more about later. The other main filter category is Reflection type filters, which are also known by the name Deepsky, Nebula, Light Pollution, or Interference filters. These filters are significantly more complex to manufacture as I will explain shortly.

Of course, as with any rule there are always exceptions. There are also filters available that combine both absorption and reflection to perform special tasks. These include special filters for planetary observing, chromatic aberration correction, solar observing, or UV/IR blocking. For now, we will ignore these special filters and just focus on the two main types.

First let’s look a bit closer at absorption type filters. These filters have been used for over 100 years, mostly for conventional terrestrial photography. Absorption type filters started out being made from die infused gel. Eventually these gels were sandwiched between two thin layers of glass to make them more durable and heat resistant. This type of sandwich construction is still in use today by cheaper filter manufacturers, but filters constructed this way don’t tend to stand up very well in humid conditions. If given a choice one should go with more modern and durable die infused glass, which can be found today at a reasonable price. The use of dies to produce different colours has been going on for hundreds of years. Essentially the molecular structure of the die material selectively absorbs certain wavelengths of light, while allowing other wavelengths to pass. Absorption rate can be varied by either changing the concentration of die, or by increasing the thickness of the filter. Absorption type filters are not very precise, producing wide band passes and gradual cut-offs. The most commonly found absorption filters for amateur astronomy are Wratten filters. Although they are made today by many different manufacturers, they all refer back to the recipes that were created by English inventor Frederick Wratten. He came up with a standard reproducible series of filters for photographic application back at the start of the 20th century. He eventually sold his filter standard to Eastman Kodak in 1912, a company that still makes Wratten filters for photography today.

Reflection or “interference” type filters operate based on the wave property of light. These filters are constructed by putting many very thin layers of different refractory materials on a glass substrate. By alternating the refractive index of the material from one layer to the next, the filter sets up a series of interfaces at which the incoming light is partly transmitted and partly reflected. This is much like when you look at the surface of a calm lake; the lake’s surface is a boundary between two mediums with different refractive indexes (ie. air and water), so some light is reflected at the boundary and some is transmitted. In the case of the interference filter, the thickness of each layer is carefully selected by the manufacturer so that all the transmitted and reflected light passing through the filter is in phase with itself for the desired wavelengths, and out of phase for the wavelengths they are trying to block. Adding up all of the in-phase light on the other side of the filter results in very little attenuation of the desired wavelengths. However, all the out of phase light tends to cancel itself out when it is collected on the other side of the filter, thus making very high blocking rates possible for the undesired wavelengths. As you might imagine, fabricating this type of filter is very complicated. The thickness of each layer is on the order of ¼ of the wavelength of light being passed by the filter, and it is very important that these layers are uniformly applied. As a result, this type of filter is generally much more expensive to buy than absorption type filters.

When talking about filters and how they perform there is a basic list of terms that is often referred to. The last couple of slides of my presentation define some of the main filter properties to be aware of. First off are the basic types of band passes. The simplest filter is something we call a “low pass”. This filter is defined by a single cut-off wavelength, where below the cut-off there is very little attenuation and above there is a lot of attenuation. Related and equally simple is the “high pass” filter, which is essentially the opposite of a low pass filter, little attenuation above the cut-off, and a lot of attenuation below the cut-off. Yellow, orange, and red colour filters are all high pass filters. Now if we combine a low-pass and a high-pass we get something called a “band pass” filter. Only light between the two cut-off wavelengths gets through the filter, everything outside the cut-offs is blocked. The term used to describe the width of the band pass is called the “Full Width Half Maximum” or FWHM, and it is defined as the difference in wavelength between the points on either side of the band where the transmission value is ½ the peak in-band transmission. Most of the filters that you will encounter in amateur astronomy are band pass filters. Some filters have wide pass bands, some filters have narrow pass bands. Some have two or three pass bands in one filter, and others have multiple bands. I will discuss all these variations in more detail in Part 2.

The final filter nomenclature item to discuss is “Luminous Transmissivity”. It is a number ranging from zero to 100% that describes in general terms how much light a filter blocks. A filter with a %LT of 100% is perfectly clear, while one with a %LT near 0% is essentially opaque. A filter’s %LT is calculated spectrally by taking the average filter transmission weighted by the corresponding response of the detector of interest. Thus, a filter has a different %LT value depending on what you are using the filter with, your eyes or a camera. This is a common misconception that even filter manufacturers don’t seem to understand. For example, if you happen to look online for %LT values quoted by manufacturers for Wratten filters, the number quoted is that for the filter used with the human eye during the daytime specifically, not nighttime. Since the spectral response of our eyes changes significantly between light adapted (photopic) and dark adapted (scotopic) conditions, the perceived transmission properties of filters will also change significantly.

This brings us to the end of Part 1. Hopefully you all now have at least a basic understanding of what astronomical filters are and how they work. Although I did not say it explicitly during my presentation, it should be clear that filters are a very useful tool for improving our amateur astronomy experience. Next time I will discuss what filters you should think about adding to your kit, and how to use them.

Monthly Challenge Objects

Oscar Echeverri



Estelle’s Pick of the Month

Library Closed


International Astronomy Day is coming (maybe)!! Make your calendars.


  • Saturday, May 2 – watch for email updates


  • Saturday, September 26

Carp Star Parties

  • Here is the schedule for our Public Star Parties for the summer. Thanks Paul.

    • Friday, May 22 – maybe, watch for email updates

    • Friday June 19

    • Friday, July 17

    • Friday August 15

    • Friday, September 12

    • Saturday, October 10

FLO Star Party Dates for 2020

Our Ottawa Centre’s Members’ Star Parties at the FLO will continue this summer. If you haven’t attended before, be sure to mark at least one of these dates on your calendar. You are welcome to bring family members or a guest. The GO/NO GO call will be made on the Centre mailing list, about noon the day of the star party


  • Saturday January 25 – Waxing Crescent Moon, 1.1% illumination NO GO

  • Friday February 21 – Waning Crescent Moon, 2.8% illumination - GO

  • Friday March 20 – Waning Crescent Moon, 10.8% illumination- canceled due to COVID 19 & clouds

  • Saturday April 25 – Waxing Crescent Moon, 7.3% illumination – watch for email updates!!

  • Saturday May 23 – Waxing Crescent Moon, 1.4% illumination

  • Saturday June 20 – Day before New Moon, 0.2% illumination

Next Meeting

7:30 PM Friday May 1, 2020 THIS WILL BE A VIRTUAL MEETING ON ZOOM. Watch for email updates. Note there will be no $4.00 parking fee. The meeting runs until 9:30 pm

PLUS: all our regular meeting features: Ottawa Skies, 10-minute Astronomy News Update, Observation Reports and, sadly, no Door Prizes!

All RASC monthly meetings are free and open to members and non-members alike. Refreshments will be available, and this will be a wonderful opportunity to meet new friends who share a common interest and chat in a relaxed, stimulating and fun environment.  Please join us!

Centre Information

To subscribe (or unsubscribe) to our members-only discussion list ( ) please contact .

The Ottawa Centre 2020 Council

President: Mike Moghadam (
Vice President: Stephen Nourse
Secretary: Chris Teron (
Treasurer: David Parfett (
Centre Meeting Chair: Dave Chisholm (
Councillors: Carmen Rush, Gerry Shewan, Jim Sofia
National Council Representatives: Karen Finstad, OPEN
Past President: Tim Cole

2020 Appointed Positions

Membership: Art Fraser
Star Parties: Paul Sadler
Fred Lossing Observatory: Rick Scholes (
Light Pollution Abatement: OPEN
Public Outreach Coordinator: Jean-Sebastien (JS) Gaudet
Hospitality: Art & Anne Fraser
Stan Mott Astronomy Library: Estelle Rother
Ted Bean Telescope Library: Darren Weatherall
Webmaster: Mick Wilson (
AstroNotes Editors: Gordon Webster & Douglas Fleming (