pointing with a radio telescope?

[dan_adi]

Hello,

How do you point and track an object with a radio telescope?

With optical scope we have plate solving, auto guiding , but what about radio scopes?

[SteveBz]

Hi,

It depends on your design.  On mine the Earth is my RA and I only change DEC.  I can't track. 

The width of your beam is 1.22 * lambda/width - in radians,  or about 10 degrees for my dish, which is 1.3 m wide.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/Raylei.html 

Then I use an inclinometer to track DEC:

https://www.amazon.co.uk/Trend-DLB-Digital-Level-Black/dp/B01HF9FJJU/ref=asc_df_B01HF9FJJU/?tag=googshopuk-21&linkCode=df0&hvadid=232122755890&hvpos=&hvnetw=g&hvrand=10905520700690284117&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006661&hvtargid=pla-421592053330&th=1

Because of my resolution I sky-scan and adjust by 10 degrees per 24 hours.  In reality the inclination gives Altitude, so I need to adjust it.  Overhead is my latitude so DEC = INCL - (90 -LAT).  You can see it's right because if INCL = 90, then DEC = LAT, and that seems right.

Post your design here and let's look.

Kind regards

Steve.

 

 

[dan_adi]

1 hour ago, SteveBz said:

Hi,

It depends on your design.  On mine the Earth is my RA and I only change DEC.  I can't track. 

The width of your beam is 1.22 * lambda/width - in radians,  or about 10 degrees for my dish, which is 1.3 m wide.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/Raylei.html 

Then I use an inclinometer to track DEC:

https://www.amazon.co.uk/Trend-DLB-Digital-Level-Black/dp/B01HF9FJJU/ref=asc_df_B01HF9FJJU/?tag=googshopuk-21&linkCode=df0&hvadid=232122755890&hvpos=&hvnetw=g&hvrand=10905520700690284117&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006661&hvtargid=pla-421592053330&th=1

Because of my resolution I sky-scan and adjust by 10 degrees per 24 hours.  In reality the inclination gives Altitude, so I need to adjust it.  Overhead is my latitude so DEC = INCL - (90 -LAT).  You can see it's right because if INCL = 90, then DEC = LAT, and that seems right.

Post your design here and let's look.

Kind regards

Steve.

 

 

I came across this site https://www.radio2space.com/product/spider-230c-compact-radio-telescope/ and I was wondering how pointing and tracking is achieved.

[ShinyTwelve]

36 minutes ago, dan_adi said:

I came across this site https://www.radio2space.com/product/spider-230c-compact-radio-telescope/ and I was wondering how pointing and tracking is achieved.

In the box below the main image it lists the kit contents. The software is annotated as being for control/acquisition.

"RadioUniversePRO control and acquisition software for Windows 10/11, designed to control all devices of SPIDER radio telescope and to collect radio astronomy data."

[SteveBz]

I didn't see that version, but a previous or maybe the original, used an EQ6.  I love their products.  I just don't have that sort of money.  21k!!!  The one in my profile picture used an EQ5, and star alignment was indeed a problem.  That's why I switched.  At one stage I thought about mounting a finder scope on it.

I probably spent £100 on mine and then spent a month or two plotting the spiral arms of the milky way.  Each observation is an entire project, for me at least.

But if you are planning to buy one of these, it would be amazing.

Good luck, and it doesn't even matter if it's cloudy.

Kind regards

Steve.

Edited April 8, 2023 by SteveBz

[robin_astro]

5 hours ago, dan_adi said:

With optical scope we have plate solving, auto guiding , but what about radio scopes?

Unless you have an array of linked radio telescopes several km apart, radio images are very fuzzy compared with optical images (because the wavelength is so much longer) so you don't need the same accuracy of pointing and tracking. For example with the setup you linked to (a 2.3m dish operating at 1.4GHz) a point radio source would look like an approximately 5 degree diameter blob so a degree or so out on pointing does not matter.

Cheers

Robin

Edited April 8, 2023 by robin_astro

[SteveBz]

If your beam width is a few degrees, you can probably polar align to within 1 degree anyway and then do 1-star alignment with the moon or sun's shadow of the central antenna on the dish.

[dan_adi]

11 hours ago, robin_astro said:

Unless you have an array of linked radio telescopes several km apart, radio images are very fuzzy compared with optical images (because the wavelength is so much longer) so you don't need the same accuracy of pointing and tracking. For example with the setup you linked to (a 2.3m dish operating at 1.4GHz) a point radio source would look like an approximately 5 degree diameter blob so a degree or so out on pointing does not matter.

Cheers

Robin

I see, thanks for the explanation.

[Zermelo]

https://astronomy.stackexchange.com/questions/48397/how-are-radio-telescopes-pointed

[Ed astro]

I'll second Robin's comment. At 1420 MHz (hydrogen line frequency, wavelength ~21 cm) the half power beam width of a 3 metre dish is 5 degrees- that means that if you are trying to detect a point source and your pointing is+-2.5 degrees off you receive half of the power of that source. Also note that the hydrogen clouds we typically observe at 1420 MHz are very diffuse and span several degrees in the sky, and are not really point sources even at very low (several degree) angular resolution. An accuracy of around 1 degree is therefore more than adequate for this purpose. For observations at the 1420 MHz hydrogen line with the 3 metre dish I made a rather crude and simple system for positioning using two degree arcs to indicate azimuth and elevation. Before starting an observation I calibrate these by pointing the dish at the sun.

However, the radio spectrum extends way beyond 1420 MHz... Last year I started doing radio astronomy at 22.235 GHz (spectral line of water) with a 1 metre solid dish. At 22 GHz (wavelength of 13.5 mm) accurate pointing is much more critical, and the beam width of even the 1 metre dish is around 1 degree. Pointing deviations of as little as 15 arcmin already have a noticeable effect. I have therefore mounted the 1 metre dish on a HEQ5 mount and use a single point alignment on the Sun, Moon or the brightest radio source at 22GHz (W49). I have also attached a finder to the side of the dish and aligned it with where the dish is pointing using the Moon as a reference point, so I can also use bright stars for alignment of the mount at night.

In summary, methods for pointing a radio telescope may vary greatly depending on the desired pointig accuracy, which in turn depends on the telescope's size and operating wavelength.

[Alpha Adhito]

As previously mentioned, for amateur radio astronomy (esp. on the H-Line), you don't actually need the extreme precision of optical astronomy do as most dish would give you few degrees of field of view. I'm a complete beginner in the subject so I'm using whatever available on the observatory where I work. Here you can see I'm using a Celestron CGX for mounting a 2.3 m dish (as the mount was the least used by the students) and mostly I'm using it as an extremely fancy tripod and don't even switch on the mount, then used the Mk. I eyeball for pointing the dish 😅 (sometimes I just pointed the dish up and let the sky drift overhead). The dish, although really huge weighs just over 7 kg so I still have a lot of room for the mount. For calibration you can point it to the sun and match the feedhorn shadow to the center of the dish and click align to the sun in the software. 

In the future I might build an Alt-Az mount using antenna rotator or some gearbox and stepper motor then coupled it with OnStep. Satellite tracking is the next subject I'm interested, the 1420 MHz feed is adequate for most L-Band satellite reception🙂.

Edited December 4, 2023 by Alpha Adhito

[athornett]

You have had lot of detailed responses above.

Simplistically, the options for small radio telescopes are:

1. You point the telescope in one direction at particular declination then leave the rotation of the Earth ro bring different parts of the sky into field of view of the telescope. The declination can then be varied to cover more of sky. I just point it and then use inclinometer to find out what angle I have got. I use this method on my dipole array see www.astronomy.me.uk

2. Small dishes can also sit on EQ6 mounts as long as they are light. Ideal for msh dishes (ie not solid ones) due to weight and effect of wind blowing on solid dishes. Bear in mind relatively large efeect of small weights far away from fulcrum. Also problem here is that radio telescopes left out all time and astrophotography mounts do not tend to like rain amd snow on them!

3. Purpose built rotators and directional finders, difficult to buy over the counter, so usually hand-built by amateur themselves.

Andy

[athornett]

NB Although previous posts have commented on large field of view of radio telescopes, I have noted that I can see gaps on my Milky Way hydrogen map with my 86 x 86cm array (equivalent to roughly 86cm dish so not big) when my changes in declination are > 5 degrees. However this is question of neatness of the map - I don't need this much detail to either map spiral arms of Milky Way or to demonstrate existence of dark matter, both those observations being quite resilient to variations in equipment used (some people have even used a tin can as their aerial!)

Andy