AP beyond scope's max recommended mag power

[mulder85]

Hello all,

Disclaimer for all: I am still a newbie so good chance this is a bad question, but I was hoping for an explanation anyway.

I've got a SkyWatcher 8" (1200mm FL) Dobsonian (manual, no GOTO). So far, I am mostly interested in visual and AP planets. I have recently started gradually upgrading my equipment. Got a ZWO ASI 224MC, and a TV Powermate 2.5x (hoping to get an HEQ5 mount soon too). Here is where the baffling happens. According to math, my scope's potential magnification is 400x (200mm aperture x2). Obviously that  is supposed to be theoretical, true only for nights with perfect seeing, and in the past, using a typical 2x barlow and a 6mm plossl, I could touch that 400x (1200x2 /6 = 400) , and my mileage would vary according to seeing, so it verified the hypothesis. Along came my new equipment, the camera and the powermate. The maths for this produce a magnification of 491x (1200x2.5 / 6.1mm) which by far exceed  the theoretical maximum. I was, however, able to produce the attached planetary pics, which I think are pretty decent for a manual dobsonian, considering I have still not fully mastered the art of stacking and processing, and the fact that they were taken from within the city (Thessaloniki, Greece). I have also noticed that visually, when using the Powermate and the 6mm eyepiece (500x zoom), it still looks as clear as, if not clearer than the common 2x barlow and 6mm eyepiece combo(400x). So I guess these are my questions:

1. When doing AP, is the maximum zoom we can use higher than for visual, because of stacking, or not?

2. Is there a chance that the TV Powermate can implicitly increase the theoretical max magnification, because of its superior quality?

3. Would adding a motorized mount, enhance the results I get, or have I pretty much reached the absolute max quality shots I can get with the specific OTA?

 

Thanks a lot in advance,

Alex

[JamesF]

The considerations are different for imaging.  The telescope has a maximum resolution based on its aperture (and the wavelength of light, in fact).  You can increase or decrease the focal length by using barlows or reducers, but the resolution is fixed.  What changing the focal length does is change how much of the image covers any given area of the sensor.  If one pixel of the sensor has a greater area than the smallest thing you can resolve given your aperture then you're effectively throwing information away that you might otherwise be able to capture.  If multiple pixels cover the smallest thing you can resolve then you're not actually getting a better image, just a larger one.

Of course there are other issues too.  Spreading the light over more pixels results in a dimmer image (less light per unit area) and may require longer exposure times to get good data.

(It's actually more complex than this, but it'll do for starters.)

Magnification is effectively the ratio of the sizes of the objective and the exit pupil (if I recall correctly).  As there's no exit pupil with a camera, magnification doesn't come into play.

James

[Cornelius Varley]

There is no magnification in imaging, so comparing the practical maximum visual magnification and the theoretical imaging "magnification" is immaterial . Change the camera to one with a larger sensor and the image would appear to be less "magnified".

[mulder85]

Thanks a lot for both your explanations. Apparently I had it all wrong, thinking that the same rules apply. Is there a formula that helps calculate that sweet spot of pixel density for a specific target(Jupiter or Saturn for example)?

[vlaiv]

As stated above - magnification for imaging makes no sense.

However, there is something called limiting resolution that is for imaging the same as maximum magnification for visual - going further brings in no additional detail.

Good news is that your powermate x2.5 and ASI224 bring you spot on for limiting resolution (3.75um pixels at F/15). Some people use even higher resolutions, because it makes things easy for them to process, but according to your images you seem to manage your processing just fine.

Motorized tracking will improve your images - it will let you take longer sequences of frames and get better stacking results. 

Barlow / powermate will not affect maximum resolution and can only degrade image if it is not of adequate quality.

For planetary imaging to get best possible results - aim for better seeing, learn what exposures work best (try 5-20ms range), improve your processing skills. Also make sure your collimation is perfect, and keep target centered on optical axis (sensor center).

Learn to do additional processing if you aren't doing it already (dark/bias removal, flat frames). Use at least 16bit math if not 32 (you don't really have choice here, just explore software that gives you best results, like Pipp, AS3, Regitax, AstraImage, ...).

[Stub Mandrel]

I was just typing that @vlaiv is the best person to answer your query!

With 'lucky imaging' and a pixel scale that is about half the resolution of your scope or a bit less will enable you to achieve optimum results when the seeing is good.

 

[mulder85]

Thanks @vlaiv, lots of useful ideas and suggestions there. Three things to note:

1) I thought dark/bias/flats) are not required for planetary work, but I suppose a perfectionist would do these too.

2) Never thought about that sensor center targeting. Because I don't have a GOTO, currently I just find the target and let it slip from one side of the screen to the other, then repeat again and again(PIPP takes care of it afterwards). I suppose all this is taking its toll both in vibration (from moving the scope) and not being centered on the sensor.

3) I think I keep forgetting about how big seeing factor is. I am also confident that if i take this equipment back to my little village which is ten times darker, it will have an impact, even if it is just planetary photo.

Thanks again

 

[Stub Mandrel]

8 minutes ago, mulder85 said:

Never thought about that sensor center targeting. Because I don't have a GOTO, currently I just find the target and let it slip from one side of the screen to the other, then repeat again and again(PIPP takes care of it afterwards). I suppose all this is taking its toll both in vibration (from moving the scope) and not being centered on the sensor.

The only benefit is being able to stack more frames, but the approach you are using should be OK.

[vlaiv]

5 minutes ago, mulder85 said:

Thanks @vlaiv, lots of useful ideas and suggestions there. Three things to note:

1) I thought dark/bias/flats) are not required for planetary work, but I suppose a perfectionist would do these too.

2) Never thought about that sensor center targeting. Because I don't have a GOTO, currently I just find the target and let it slip from one side of the screen to the other, then repeat again and again(PIPP takes care of it afterwards). I suppose all this is taking its toll both in vibration (from moving the scope) and not being centered on the sensor.

3) I think I keep forgetting about how big seeing factor is. I am also confident that if i take this equipment back to my little village which is ten times darker, it will have an impact, even if it is just planetary photo.

Thanks again

 

1) Yes, sort of perfectionist approach, for best results, learn to do those as well - it might not have very big impact on final result, but I think cleaner subs help software to do alignment and decide which frames are good and which ones are bad.

2) Centering of target will help you if your collimation is good - you are using Newtonian scope (very good for this purpose, I have exact same scope and love the views it gives) and there is going to be coma off axis even if your collimation is spot on, so it's best to keep your target centered. This of course you will be able to do if you get EQ mount (I also have HEQ5 and have mounted that OTA, it is good for planetary but not best choice for DSO). Alternatively, if you don't want to get EQ mount straight away - have a look at EQ platforms. It will not be precise as EQ mount but it will help you take 3-4 minute sequences.

One more thing, EQ tracking does not need to be perfect, you can have a small drift, it is even better if you do - it provides kind of "natural" dither and helps produce better image. With small drift you can use bayer drizzle algorithm in AS!3.

3) Look into optimizing planetary observing - same things apply for imaging. Dark skies help a bit, but I've done some planetary imaging in twilight - atmosphere tends to be more stable then, so full darkness is not that important. Basic things are: cool your ota to ambient, put rig on grass not on concrete - that can warm up and produce thermal currents. Aim over grass or trees, avoid houses or anything "solid" that can soak heat during the day and release that in the evening / night ...

[mulder85]

6 minutes ago, vlaiv said:

1) Yes, sort of perfectionist approach, for best results, learn to do those as well - it might not have very big impact on final result, but I think cleaner subs help software to do alignment and decide which frames are good and which ones are bad.

2) Centering of target will help you if your collimation is good - you are using Newtonian scope (very good for this purpose, I have exact same scope and love the views it gives) and there is going to be coma off axis even if your collimation is spot on, so it's best to keep your target centered. This of course you will be able to do if you get EQ mount (I also have HEQ5 and have mounted that OTA, it is good for planetary but not best choice for DSO). Alternatively, if you don't want to get EQ mount straight away - have a look at EQ platforms. It will not be precise as EQ mount but it will help you take 3-4 minute sequences.

One more thing, EQ tracking does not need to be perfect, you can have a small drift, it is even better if you do - it provides kind of "natural" dither and helps produce better image. With small drift you can use bayer drizzle algorithm in AS!3.

3) Look into optimizing planetary observing - same things apply for imaging. Dark skies help a bit, but I've done some planetary imaging in twilight - atmosphere tends to be more stable then, so full darkness is not that important. Basic things are: cool your ota to ambient, put rig on grass not on concrete - that can warm up and produce thermal currents. Aim over grass or trees, avoid houses or anything "solid" that can soak heat during the day and release that in the evening / night ...

Perfect, all my questions on this are covered now. Thank you @vlaiv and anyone else who contributed.

[JamesF]

3 hours ago, mulder85 said:

Is there a formula that helps calculate that sweet spot of pixel density for a specific target(Jupiter or Saturn for example)?

It's not target-dependent, but yes, you can calculate this sort of thing.  However, for planetary imaging a lot of people use a rule of thumb that the focal ratio of the optical train should be five to six times the pixel size of the camera in thousandths of a millimetre.

So, with your 1200m focal length 200m aperture dob and a camera with 3.75um pixels you might aim for a focal ratio in the region of 18 to 22.  From the native f/6 you could do that with a 3x barlow or even use a 2.5x or 2x barlow with a spacer between the barlow and camera to increase the multiplication factor (working out how big a spacer is generally done by trial and error).  It is only an approximation to start with however, so if you're not spot on it's no big deal, particularly with a colour camera where the optimal arrangement varies across the whole spectrum.

Of course with longer focal lengths and no tracking the target is on the sensor for less time so you get fewer frames to stack.  You might therefore decide that until you have a tracking system it's much less hassle (and in fact might give better results) if you stick with shorter focal lengths.

James