Focal length and Focal ratio - test my sanity

[rogerg (Roger)]

I'd really rather keep the thread to observations in general, not specific to occultations or a particular occlation. My original clarification was only simplifiedto "a star" because I thought there wouldn't be any difference as to how a star vs a larger object made up of billions of stars would be treated. Still a little confused on that one

To my way of reading there are 4 TNO's going directly over our locations in the next month, and another 3 or so which are worth observing because the path is close enough that an outside observation would be of use. And I'm aiming for time to observe one or two

[Blue Skies (Jacquie)]

Originally Posted by mithrandir

You might want to check on http://occsec.wellington.net.nz/plan.../pluto2010.htm because you may be outside the track.

I've tried CdC and Stellarium an their plots both look like Hobart might see it but Sydney and Perth are much closer to the edge.

The last one that was attempted that I know of the track shifted quite a few hundred km north, and Perth ended up being on the edge. Even a miss is a positive result in this game!

[StellariuS (Jayden)]

Does this dodgy image work? it might put us back on track.

All stars are the same size no matter the aperture or focal length, just brightness and scales change.

This is how I've come to understand the works of it. but i have only seen through one telescope. I'm fairly new to starwatching so please correct me if I'm wrong.

Go paint lol

[rogerg (Roger)]

Originally Posted by StellariuS

Does this dodgy image work? it might put us back on track.

All stars are the same size no matter the aperture or focal length, just brightness and scales change.

This is how I've come to understand the works of it. but i have only seen through one telescope. I'm fairly new to starwatching so please correct me if I'm wrong.

Go paint lol

Thanks for your effrots to help it might help I'm not sure

Image scale: I don't have any problem there, I think we're all in agreeance that longer focal length = smaller FOV = smaller area of sky = larger view of an object = smaller scale (each pixel covers fewer arc seconds of sky).

Star brightness: Your graphic doesn't indicate if 12" F/7 or 6" F/3 is brighter, and that's really the critical question I'm getting at

Also just to clarify again I'm only interested in photographically thinking - not visual.


Roger.

[Terry B]

Originally Posted by rogerg

Thanks for your effrots to help it might help I'm not sure

Image scale: I don't have any problem there, I think we're all in agreeance that longer focal length = smaller FOV = smaller area of sky = larger view of an object = smaller scale (each pixel covers fewer arc seconds of sky).

Star brightness: Your graphic doesn't indicate if 12" F/7 or 6" F/3 is brighter, and that's really the critical question I'm getting at

Also just to clarify again I'm only interested in photographically thinking - not visual.


Roger.

The graphic isn't really true. The 12" f7 and the 12" f3 should have the stars at the same brightness. This holds for all except the very bright stars that will saturate the sensor and spread out over lots of pixels. The 12" scope will also show dim stars that won't be visible in the 6" scope.
For my scope I have taken images of stars for photometry with and without a reducer. The limiting magnitude I can detect with a 5 minute exposure is the same at f8 and f6.4. To improve my ability to measure dimmer stars I need a bigger scope not a bigger focal reducer.

[tonybarry (Tony)]

Hello Jacqui,

I also have an 8" scope (LX90) and a GStar which I can use to observe the occultation. What do the knowledgeable people say about this combination? Will it suffice? Or is the limiting magnitude of the optical / CCD system too great?

One thing is that the GStar will need to integrate to some extent to obtain the necessary light collection. This will reduce the time accuracy. Will this allow the observations to be still useful?

Regards,
Tony Barry

[Blue Skies (Jacquie)]

Sorry, Tony, only just noticed your questions.

I'm a beginner in this game as well (hence the questions in this thread) and can't tell you much more until I can get together with Roger to do some tests. But I'm told that only one or two integrations is enough - you need to have a frame rate of 25/sec being recorded, so as long as you can achieve that and see the target star it should be right.

[AstroJunk (Jonathan)]

Tony, I happen to have the rather pointless and unofficial world record for observing the faintest ever asteroidal occultation of a 14.5th mag star. I mention this because your 8" and GSTAR combo could have made this observation too.

Yes, you would have had to use a higher level of integration which would have reduced your acuracy. But would this have still be useful? Yes, absolutely. There is an enourmous scientific payload of every extra observation, no matter how inacurate. Just getting the duration of an event with a stopwatch is massively helpful.

Hope you get involved,

Jonathan

Originally Posted by tonybarry

Hello Jacqui,

I also have an 8" scope (LX90) and a GStar which I can use to observe the occultation. What do the knowledgeable people say about this combination? Will it suffice? Or is the limiting magnitude of the optical / CCD system too great?

One thing is that the GStar will need to integrate to some extent to obtain the necessary light collection. This will reduce the time accuracy. Will this allow the observations to be still useful?

Regards,
Tony Barry

[DaveGee (Dave Gault)]

Originally Posted by Blue Skies

We really want to know... is whether an 8" is enough aperture to pick up this star using a focal reducer and my gstar camera

Hi Jacqui,

Short answer... YES!

Slightly longer answer... Yes, an 8" f10 SCT with a f3.3 focal reducer and a Gstar camera is enough for you to detect the mag. 15 target star and mag. 14 Pluto.

Longer answer... Yes, an 8"... blar blar blar... I use my 8" LX90 and WAT-120N to observe asteroid occultations of faint stars. I prefer my 10" newtonian as it's already set up and lives in my skyshed ready to go with the minimum of fuss, but the 8" SCT is good enough. The 10" gives a slightly better Signal to Noise video but not by much. A 12" 14" or 20" will give you even better S/N @ a faster frame rate but in the end, if you use what you have to the best of your ability, you can't go wrong.

Remember, the mag. 14.0 Pluto will merge with the mag. 15.2 target star and the pair will brighten to mag. 13.7 and if/when the star disappears you will see a mag. 0.3 drop in brightness.

Bottom line. In the days before the event, practise observing Pluto appulsing (approaching) the star to determine the fastest frame rate you can achieve to "only just" detect the target star. This is best done at a similar time of the event so that the targets are at a similar altitude and so atmospheric extinction will be the same as that on event night. This, in the trade is called a mocultation... mock-occultation.... get it...

[Blue Skies (Jacquie)]

Originally Posted by DaveGee

Remember, the mag. 14.0 Pluto will merge with the mag. 15.2 target star and the pair will brighten to mag. 13.7 and if/when the star disappears you will see a mag. 0.3 drop in brightness.

Bottom line. In the days before the event, practise observing Pluto appulsing (approaching) the star to determine the fastest frame rate you can achieve to "only just" detect the target star. This is best done at a similar time of the event so that the targets are at a similar altitude and so atmospheric extinction will be the same as that on event night.

Now this is exactly the info I was looking for. I'm still trying to get my head around combined magnitudes, though. I had been wondering what rate of integration was needed but you've answered that - it varies with each situation. I really should have asked you first...

This, in the trade is called a mocultation... mock-occultation.... get it...

...mocultation...! Its hard to find jokes in astronomy but that's a good one!

P.S. pm coming your way!

[tonybarry (Tony)]

Longer answer... Yes, an 8"... blar blar blar... I use my 8" LX90 and WAT-120N to observe asteroid occultations of faint stars. I prefer my 10" newtonian as it's already set up and lives in my skyshed ready to go with the minimum of fuss, but the 8" SCT is good enough. The 10" gives a slightly better Signal to Noise video but not by much. A 12" 14" or 20" will give you even better S/N @ a faster frame rate but in the end, if you use what you have to the best of your ability, you can't go wrong.

Remember, the mag. 14.0 Pluto will merge with the mag. 15.2 target star and the pair will brighten to mag. 13.7 and if/when the star disappears you will see a mag. 0.3 drop in brightness.

Bottom line. In the days before the event, practise observing Pluto appulsing (approaching) the star to determine the fastest frame rate you can achieve to "only just" detect the target star. This is best done at a similar time of the event so that the targets are at a similar altitude and so atmospheric extinction will be the same as that on event night.

Hello Dave,

Many thanks for this answer, which is exactly what I was hoping for.

I have both an f/6.3 and an f/3.3 focal reducer for my LX90-8" - which would be better for this purpose?

From Massey & Quirk's book (Deep Sky Video Astronomy, p14) I have the native (f/10) field of view of an 8" SCT with a GStar at prime focus as 11' x 8' and a limiting magnitude of 15.5 at 128x accumulation (2.56 seconds per frame).

Putting a f/6.3 reducer on the scope should increase the field of view (helpful in the case of locating an object with few distinguishing features).
If I understand things correctly, this will make the apparent disc of the planet on the CCD occupy fewer pixels, dumping a little more light on each pixel, which for Pluto (at mag 14.0) is probably a good thing.

Now the less pleasant thoughts I have. According to your web page, the apparent diameter of Pluto is about 0.1", but I understand that the seeing in average conditions blurs starlike objects out to about 2". The resolving power of an 8" SCT (Dawes limit) is about 0.6" and the GStar has about 0.87" / pixel at f/10 of 2000mm.

So at f/10, the 8" SCT / GStar combination should produce an image of Pluto of about 2.3 pixels diameter (due entirely to the seeing conditions). Putting the f/6.3 reducer in the imaging train should reduce the image down to 1.4 pixels diameter. The f/3.3 reducer lowers that to less than one pixel on the CCD ... which for me is a little problematic. Would you prefer the f/3.3 over the f/6.3 reducer?

The next question is regarding hot pixels, which the GStar has plenty of (at 128x accumulation). They are not obvious at 1x accumulation. How did you deal with hot pixels?

Regards,
Tony Barry

[DaveGee (Dave Gault)]

Originally Posted by tonybarry

I have both an f/6.3 and an f/3.3 focal reducer for my LX90-8" - which would be better for this purpose?
<snip> Putting the f/6.3 reducer in the imaging train should reduce the image down to 1.4 pixels diameter. The f/3.3 reducer lowers that to less than one pixel on the CCD ... which for me is a little problematic.
Would you prefer the f/3.3 over the f/6.3 reducer?

The next question is regarding hot pixels, which the GStar has plenty of (at 128x accumulation). They are not obvious at 1x accumulation. How did you deal with hot pixels?

Hi Tony,
Sorry for the delay in answering... I must check the forums more often...

I always use a f3.3 reducer with my 8"LX90 for asteroid events. I'm always seeking a larger FOV to help IDing the field and to make sure I have enough comparison stars. I've never had the conditions good enough to get star images on one pixel, and if that were to happen, I'd simply defocus a little to smear the light onto more pixels.

I have plenty of hot pixels on my WAT120N. I've been tempted to use black paint on the monitor to see if that helps.... Sadly there is not much you can do except to move the target to avoid them, or chill the camera...

Good luck!