Is the moon too close

[Craig123]

Hi all

At the moment the forecast says clear tonight here ! I was thinking of imaging  with 7 nm Ha filter but is this target too close to the moon or would i be ok. Is it better to go for something further away even with Ha.

 

Many thanks

 

[xtreemchaos]

it wont be not affected but you should be able to get a result. charl.

[MarsG76]

Give it a go.. there have been time when I though that the moon was too close to my NB imaging, and the subs turn out great.. even being 25-20 minutes long exposure per sub.... so the only real way to determine if its too close or if there will be a good result is a test exposure.

 

[JOC]

It sounds like a line from the latest disaster movie!

[Craig123]

 

9 minutes ago, MarsG76 said:

Give it a go.. there have been time when I though that the moon was too close to my NB imaging, and the subs turn out great.. even being 25-20 minutes long exposure per sub.... so the only real way to determine if its too close or if there will be a good result is a test exposure.

 

Being pretty new to it and having 'wasted' 3 nights on focus issues this month what should I look out for in the test sub (Apart from blurry image?.. I'm really going to look closely at the first sub instead of letting it click away all night. I guess it will be more washed out than the ones I did last week). 

[Craig123]

3 minutes ago, JOC said:

It sounds like a line from the latest disaster movie!

Ha. Yes, indeed. A disaster movie is what I'm hoping to avoid tonight for once. 

[MarsG76]

Just now, Craig123 said:

Than

Being pretty new to it and having 'wasted' 3 nights on focus issues this month what should I look out for in the test sub (Apart from blurry image?.. I'm really going to look closely at the first sub instead of letting it click away all night. I guess it will be more washed out than the ones I did last week). 

It's not really a waste if you took something from the night that will help you to succeed in future sessions.....

As a test... this is what I do... I determine/decide how long exposure I want to spend on the sub in ISO800.... than I up the ISO to 3200 (highest in my astro modded canon 40D) and exposure for a quarter of the time.... eg. a 30 minute planned sub becomes a 7.5 minute exposure.... this will show me if there is any drift, since 7.5 mins will show up as slightly oval stars, and anything but perfect points in the middle of the frame is unacceptable, as well as whether the sub is over or under exposed, and adjust my exposure time accordingly if in needs to be... the test sub is quite noisy but the ISO800 is generally the same amount of "stuff" but with a lot less noise....

[MarsG76]

Replace my "ISO" statement with what ever your camera/CCD uses, like for example "Gain"...

[Craig123]

26 minutes ago, MarsG76 said:

It's not really a waste if you took something from the night that will help you to succeed in future sessions.....

As a test... this is what I do... I determine/decide how long exposure I want to spend on the sub in ISO800.... than I up the ISO to 3200 (highest in my astro modded canon 40D) and exposure for a quarter of the time.... eg. a 30 minute planned sub becomes a 7.5 minute exposure.... this will show me if there is any drift, since 7.5 mins will show up as slightly oval stars, and anything but perfect points in the middle of the frame is unacceptable, as well as whether the sub is over or under exposed, and adjust my exposure time accordingly if in needs to be... the test sub is quite noisy but the ISO800 is generally the same amount of "stuff" but with a lot less noise....

Thanks. That is a good way of keeping a check on it before just letting it all run after a taking few second subs... and gives an idea of the overall image as it comes out stretched when it downloads.I think I need to keep monitoring and adjust it if need be. Maybe stick to the usual 20 mins instead of 30.

[MarsG76]

7 minutes ago, Craig123 said:

Thanks. That is a good way of keeping a check on it before just letting it all run after a taking few second subs... and gives an idea of the overall image as it comes out stretched when it downloads.I think I need to keep monitoring and adjust it if need be. Maybe stick to the usual 20 mins instead of 30.

Takes some of the guess work and chance of nasty surprises out of the equation.

[AndyUK]

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... It's not really a waste if you took something from the night that will help you to succeed in future sessions.....

I 100% agree with this!  I always set myself a couple of aims for each testing session - If I achieve the first, it's a result.  If I achieve both... (well, I'll let you know!). 

You might get a bit of a gradient, but for testing, any night without clouds will do .  Silly question (and I don't want to teach you to suck eggs) but as you've mentioned focusing issues, can I ask how you focus? 

I used to use a Bahtinov mask with Bahtinov grabber (I think there's now something similar in APT?).  If using a 7nm Ha filter, I used bin 3x3 and looped 10s exposures on a nice fat star (say Altair in your screenshot?).  That usually pegged the focus down to within a few microns... (but then of course there were other potential gremlins to watch out for, namely guiding, spacing/tilt and drift/rotation - I could probably write a book on spacing/tilt and the evils of tilt adjusters!)

 

[Craig123]

Luckily I've never come across tilt.. I don't like the sound of it ! It was too hazy and cloudy last night to get a good run going but I tried out Bhatiinov grabber. I got it in critical focus but it seemed to jump back out without touching anything quite often.. I guess because of the conditions.Locking focus also had a greater than expected effect on the grabber readings. If I got 3 or 4 downloads in ctitical focus then I went with that. After that I tried to get the best fwhm reading I could.

Not a wasted night that one but certainly frustrating at times . ??

[AndyUK]

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Not a wasted night that one but certainly frustrating at times . ??

(Ah... If only I'd had a pound for every night I've had like that...!) 

The focus point with Bahtinov Grabber does shift between focus frames, but with careful tweaks you can usually get it fluctuating within the +/- critical focus point.  You're right that tightening the focuser does have an impact though - It's a bit of a trial and error thing, but once you know roughly what the shift is, and which way, you can then then compensate. 

I know some people are very happy to focus on FWHM values and everyone has their own preference, but I just found Bahtinov grabber satisfied my OCD's when seeing the concentric rings appear on the screen

 

[Craig123]

25 minutes ago, AndyUK said:

(Ah... If only I'd had a pound for every night I've had like that...!) 

The focus point with Bahtinov Grabber does shift between focus frames, but with careful tweaks you can usually get it fluctuating within the +/- critical focus point.  You're right that tightening the focuser does have an impact though - It's a bit of a trial and error thing, but once you know roughly what the shift is, and which way, you can then then compensate. 

I know some people are very happy to focus on FWHM values and everyone has their own preference, but I just found Bahtinov grabber satisfied my OCD's when seeing the concentric rings appear on the screen

 

Yeah.. I was worried you might say that opening phrase.. Does it get better?? . Lol. I seem to move forward a few paces then stop moving forward all of a sudden. 

When I had the circles on with the grabber there was a point where the numbers just went back and forth without touching the focuser somewhat manically which seemed to be when I was closest. Once it said 0.00 pixels out. That was the highlight last night but it was pretty cloudy so hopefully easier with no cloud, no moon and no ha filter. 

 

[AndyUK]

Quote

I was worried you might say that opening phrase.. Does it get better?? . Lol.

I'll let you know...

I can't say I can recall ever having seen a value of 0.00 when using Bahtinov Grabber (maybe it happened but I wasn't looking?!).  I used to be very happy if I could achieve a focus error of +/- 10 microns (I'm not sure how that calculates but that would definitely be sub-pixel). 

It'll obviously be much easier with better transparency, and admittedly it's a little more difficult with narrowband filters. (I now have 3nm filters - that was the reason I decided to go for an auto-focuser!).  If you wanted to practice with Bahtinov Grabber (to see if you like it), maybe try Luminance filter instead - The "whiskers" are MUCH easier to see and you can use shorter exposures .