Beginning planetary imaging with a 150mm Newtonian (F5) - which barlow or powermate for planetary?

[Altair8389]

Hi SGL,

I am based in London, and have started to get an urge to image the main planets. I have a Skywatcher 150mm PDS (D=150mm, and FL=750mm). I have struggled with collimation, and found that a laser collimator may be causing calibration issues.

Anyway, I have read on forums, that to image Jupiter, you need about F20 for best results in average seeing conditions.

 

Does this mean I need a x4 powermate. What would be the difference between a moderate priced x4 barlow. I see Powermates are very expensive.

Would Explore Scieintific or even Celestron barlows or equivalent Powermates be ok ?

 

I am looking at Jupiter, Mars and Saturn for the present time.

 

Should I get x3, x4 and x5 powermates as a set ?? (very expensive) = 700 pounds !

 

Thanks

 

Magnus

[ArmyAirForce]

Laser collimaters usually need collimating before use, otherwise you're just mis-aligning your optics with a mis-aligned device. Adjustment screws are often hidden under black silicone plugs in the body of the collimater. I put mine in a metal 'V' block, pointing at the wall about 10 feet away. It was slowly rotated and the red dot on the wall scribed a 3 inch circle - so not aligned. It was adjusted until the dot stayed still.

I've got the Skywatcher 200PDS ( 8in mirror ) f5 scope for planets. On an average night, I can only get away with a x2 barlow, which depending on camera position, gives between x2 and x3 magnification ( f10 to f15 ). On nights of really good seeing, I can get away with the Televue x3, giving x3 to x4 magnification ( f15 to f20 ).

For a six inch scope, I think x4 or higher is probably too much. A good x2 or x2.5 will probably give better results.

Don't forget that the planets, particularly Saturn are very low in the UK, meaning you are looking through a lot more atmosphere than is ideal. No amount of magnification is going to help that. This was the best I could manage on Saturn, using an Infra-red pass filter on a mono camera. Saturn was only about 18 degrees above the horizon. Below that is a shot from 2014 when Saturn was about 30 degrees high. The detail is much cleaner on the higher image.

[Varavall]

23 minutes ago, Altair8389 said:

I have a Skywatcher 150mm PDS

Hi, 

I have a 200pds and I have used a 2.5x Powermate and obtained some pleasing results, but I sold that and bought a secondhand 5x Powermate for 150 euros (about £125). Keep am eye on the sale ads! You can stack the Powermates so maybe a 2x and 3x could be an option giving you 2x, 3x and 6x. Of course you can start out with a cheaper Barlow and see how it performs. If you say what camera you are using, then I am sure there are plenty here with experience of Powermate/Barlow with that camera.

29 minutes ago, Altair8389 said:

I have struggled with collimation

I just use a cheshire and I have no problem collimating the scope. Just remember that when the locking screws are tightened, the collimation changes, so there is a bit of locking and unlocking to do during the adjustment. Have patience and do it slowly!

[vlaiv]

30 minutes ago, Altair8389 said:

Anyway, I have read on forums, that to image Jupiter, you need about F20 for best results in average seeing conditions.

That really depends on pixel size.

There is no difference between average seeing conditions and best possible seeing conditions. You choose F/ratio to match pixel size for "perfect" conditions. Point of lucky imaging technique is to try to capture those short moments of "perfect" seeing, so you want sampling rate to be matched to what the scope is capable without worrying about seeing part.

In any case - here is formula

F/ratio = pixel size * 2 / wavelength_ of_light

(here pixel size and wavelength of light is in same units, and for regular imaging 500nm is often used so wavelength is 0.5um)

For regular imaging (not narrow band filters) above simplifies to

F/ratio = pixel size * 4

If you have camera like ASI178 with 2.4um pixel size, then optimum F/ratio is 2.4 * 4 = F/9.6

For 3.75 (ASI224, ASI120 and alike) you get 3.75 * 4 = F/15

For F/20 you would need 5um pixel size, and I'm not sure if I know camera with that pixel size to be honest (closest that I've heard of is 5.2um).

 

[Altair8389]

1 hour ago, vlaiv said:

That really depends on pixel size.

There is no difference between average seeing conditions and best possible seeing conditions. You choose F/ratio to match pixel size for "perfect" conditions. Point of lucky imaging technique is to try to capture those short moments of "perfect" seeing, so you want sampling rate to be matched to what the scope is capable without worrying about seeing part.

In any case - here is formula

F/ratio = pixel size * 2 / wavelength_ of_light

(here pixel size and wavelength of light is in same units, and for regular imaging 500nm is often used so wavelength is 0.5um)

For regular imaging (not narrow band filters) above simplifies to

F/ratio = pixel size * 4

If you have camera like ASI178 with 2.4um pixel size, then optimum F/ratio is 2.4 * 4 = F/9.6

For 3.75 (ASI224, ASI120 and alike) you get 3.75 * 4 = F/15

For F/20 you would need 5um pixel size, and I'm not sure if I know camera with that pixel size to be honest (closest that I've heard of is 5.2um).

 

I have the ASI 462MC (2.9 micron), and ASI 120mm-s (3.75 micron). 

So the F20 rule is wrong then (I think the author was talking about native F10 telescopes). I saw some highly detailed Jupiter images with the SW 150pds, not sure how common that is though.

Even bigger scopes seem to get average images. 
I will test out my x2 Orion shorty barlow, and x3 barlow on Jupiter and see what you think.

 

[vlaiv]

4 minutes ago, Altair8389 said:

I saw some highly detailed Jupiter images with the SW 150pds, not sure how common that is though.

Planetary imaging is really game of SNR vs speed tradeoff.

You need to use very short exposures in order to freeze the seeing - to be able to select the best subs. You need to keep single exposure at about 5ms or less (regardless what people often say about - histogram and such - that is not good way to set exposure length). Individual subs will be very noisy at those exposure times and you really don't want higher F/ratio than you need as that further lowers SNR per exposure.

150mm is enough aperture for some good planetary images.

This was captured with 130mm of aperture:

and this with only 100mm:

[Altair8389]

On 10/08/2022 at 18:10, vlaiv said:

Planetary imaging is really game of SNR vs speed tradeoff.

You need to use very short exposures in order to freeze the seeing - to be able to select the best subs. You need to keep single exposure at about 5ms or less (regardless what people often say about - histogram and such - that is not good way to set exposure length). Individual subs will be very noisy at those exposure times and you really don't want higher F/ratio than you need as that further lowers SNR per exposure.

150mm is enough aperture for some good planetary images.

This was captured with 130mm of aperture:

and this with only 100mm:

Hi vlaiv, that is very good imaging ! Please can you tell me how you captured the Jupiter with 130mm.

Did you use ADC and/or barlow. Which telescope (native focal length/aperture) and reflector ? The seeing looks good and altitude seems high ?

Fantastic work !!

Magnus

[vlaiv]

18 minutes ago, Altair8389 said:

Hi vlaiv, that is very good imaging ! Please can you tell me how you captured the Jupiter with 130mm.

Did you use ADC and/or barlow. Which telescope (native focal length/aperture) and reflector ? The seeing looks good and altitude seems high ?

Fantastic work !!

Magnus

It was taken with rather modest equipment.

SkyWatcher 130/900 newtonian on EQ2 mount. Camera used was QHY5IILc (same sensor as ASI120c USB2.0) and crappy Celeron laptop

Image was taken around opposition in spring 2015. Jupiter at that time was at altitude of about 60 degrees from my location (so pretty decent).

I used simple GSO x2.0 barlow lens (one with removable barlow element).

Trick is to use very short exposures - like 5ms, regardless of histogram (which is useful to detect clipping but not much else). Recording is often faint - but after stacking it can be processed to look good.

Capture in raw format and use ROI so you can get good frame rate.

Trick is to capture planet in moments of good seeing and seeing needs to be "frozen".

I don't have recording of that Jupiter image, but here is Saturn with same equipment:

Good frame (not debayered):

Poor frame:

Result after frame selection, stacking in (AS!2 at the time) and Registax 6 sharpening:

 

[johnturley]

On 10/08/2022 at 18:00, Altair8389 said:

I have the ASI 462MC (2.9 micron), and ASI 120mm-s (3.75 micron). 

So the F20 rule is wrong then (I think the author was talking about native F10 telescopes). I saw some highly detailed Jupiter images with the SW 150pds, not sure how common that is though.

Even bigger scopes seem to get average images. 
I will test out my x2 Orion shorty barlow, and x3 barlow on Jupiter and see what you think.

 

The focal ratio required to get a decent sized image, and the optimum focal ratio for your camera based on pixel size can be quite different.

With a Skywatcher 150 PSD, focal length 750 mm, you will need around f20 (4 x Barlow) to get a decent sized image, at f10 (2 x Barlow) the image size will be very small. 

To get good sized images of Jupiter and Saturn I think you need an effective focal length of around 3000 - 4000 mm, this would explain why long focal length SCT's such as the C11 and C14 are popular with many  planetary imagers, as they have this order of focal length without requiring any amplification with a Barlow, and have focal ratios close to the optimum for most planetary cameras. 

John 

 

Edited August 23, 2022 by johnturley

[vlaiv]

2 minutes ago, johnturley said:

The focal ratio required to get a decent sized image, and the optimum focal ratio for your camera based on pixel size can be quite different.

With a Skywatcher 150 PSD, focal length 750 mm, you will need around f20 (4 x Barlow) to get a decent sized image, at f10 (2 x Barlow) the image size will be very small. 

John 

 

Another way to achieve the same effect is to sample at F/10 and then simply enlarge finished image in software.

This way you will get the same level of detail (you simply can't capture more than your telescope is able to resolve), but at the time of capture you won't spread light over more pixels than necessary thus reducing SNR and making it harder for stacking software to recognize sharp from soft frames, or in worse case - too dim image forces you to extend exposure to get bright image (sometimes it is hard to fight this urge although you should set exposure based on coherence time rather than on brightness) - and you start getting softer recordings because of atmosphere induced motion blur is affecting more and more subs.

[michael.h.f.wilkinson]

I have found I get a bit more detail going for 5x pixel size than 4x pixel size. Seeing wasn't great, but at F/10 on my 2.4 micron pixel camera I got this:

whereas using a 1.3x Siebert Optics tele-centric Barlow (similar optics to PowerMate), at roughly 5x pixel size I get this:

The Cassini division is much more clearly seen in this case. Siebert Optics makes a series of tele-centric Barlows at  a range of magnifications. They can be found here:

https://www.siebertoptics.com/SiebertOptics-barlows.html

(scroll a bit further down the page to find the tele-centrics)

 

[Pixies]

On 10/08/2022 at 16:25, vlaiv said:

That really depends on pixel size.

There is no difference between average seeing conditions and best possible seeing conditions. You choose F/ratio to match pixel size for "perfect" conditions. Point of lucky imaging technique is to try to capture those short moments of "perfect" seeing, so you want sampling rate to be matched to what the scope is capable without worrying about seeing part.

In any case - here is formula

F/ratio = pixel size * 2 / wavelength_ of_light

(here pixel size and wavelength of light is in same units, and for regular imaging 500nm is often used so wavelength is 0.5um)

For regular imaging (not narrow band filters) above simplifies to

F/ratio = pixel size * 4

If you have camera like ASI178 with 2.4um pixel size, then optimum F/ratio is 2.4 * 4 = F/9.6

For 3.75 (ASI224, ASI120 and alike) you get 3.75 * 4 = F/15

For F/20 you would need 5um pixel size, and I'm not sure if I know camera with that pixel size to be honest (closest that I've heard of is 5.2um).

 

Sorry to jump in. 

@vlaiv, does the above apply to a mono camera only? What about OSC?

You see, I was thinking about a ASI678MC for EEVA/EAA with an F5 400mm scope. But if I wanted to use it for planetary, what focal ratio would I need (assuming a different scope with larger aperture)

[vlaiv]

6 minutes ago, michael.h.f.wilkinson said:

whereas using a 1.3x Siebert Optics tele-centric Barlow (similar optics to PowerMate), at roughly 5x pixel size I get this:

One of bottom two images was made by enlarging scaled down to 80% version (top row). Can you tell which one?

If not - then all the data is contained in 80% sized version - or x4 is enough and difference in your example is due to seeing and not due to sampling.

 

[vlaiv]

6 minutes ago, Pixies said:

Sorry to jump in. 

@vlaiv, does the above apply to a mono camera only? What about OSC?

You see, I was thinking about a ASI678MC for EEVA/EAA with an F5 400mm scope. But if I wanted to use it for planetary, what focal ratio would I need (assuming a different scope with larger aperture)

In this particular case - it is the same as AS!3 uses special debayering algorithm - bayer drizzle that restores original resolution even from OSC sensor.

So formula works for both mono and OSC.

[Pixies]

6 minutes ago, vlaiv said:

In this particular case - it is the same as AS!3 uses special debayering algorithm - bayer drizzle that restores original resolution even from OSC sensor.

So formula works for both mono and OSC.

So as this camera has 2um pixels, I'd be best with an f8 scope for planetary?

My options are an F6 200mm newt or an F10 105mm achro. Perhaps I'll need to rethink and get something with 2.5um or 3um pixels (F10 or F6 with x2 barlow)?

[michael.h.f.wilkinson]

11 minutes ago, vlaiv said:

One of bottom two images was made by enlarging scaled down to 80% version (top row). Can you tell which one?

If not - then all the data is contained in 80% sized version - or x4 is enough and difference in your example is due to seeing and not due to sampling.

 

The right-hand lower row seems to have the slightly higher contrast in the Cassini division. However, this is a comparison on the stacked images. I consistently got better results after stacking when switching to the 1.3x Barlow, using the same telescope, on the same night, while seeing stayed the same.

[johnturley]

2 hours ago, vlaiv said:

Another way to achieve the same effect is to sample at F/10 and then simply enlarge finished image in software.

 

Several people have stated this or similar, but I've yet to find any satisfactory software to enlarge the finished image.

Yes, you can enlarge the image  in processing programs such as PIPP and Registax, or enlarge the image when imaging at the telescope by reducing the capture area, but when you save the final image after processing back to your PC as a JPEG file, it still ends up EXACTLY the same physical size.

The only things that seems to effect the size of the saved image, are the size of the sensor of the camera, and the effective focal length of the telescope. 

John 

 

Edited August 23, 2022 by johnturley

[CraigT82]

21 minutes ago, johnturley said:

Several people have stated this or similar, but I've yet to find any satisfactory software to enlarge the finished image.

Yes, you can enlarge the image  in processing programs such as PIPP and Registax, or enlarge the image when photographing by reducing the capture area, but when you save the final image after processing back to your PC as a JPEG file, it still ends up EXACTLY the same physical size.

The only things that seems to effect the size of the saved image, are the size of the sensor of the camera, and the effective focal length of the telescope. 

John 

 

John the best way to enlarge an image is to do it right at the end of the processing, once you have aligned, stacked and sharpened the image. 
 

I don’t know what you use for image finishing (photoshop?) but in GIMP you can enlarge for shrink the image by any factor using the ‘Scale Image’ tool under the ‘Image’ tab (shown below). You just enter the new image size in pixels that you want, so for a 2x enlargement of this particular image I would enter 3,292 into the circled box (1646*2) and then hit the  ‘Scale’ button. 


 

Edit: Should probably add a note on the interpolation method for the scaling. Choosing no interpolation method will result in a blocky ‘pixelated’ appearance. In GIMP the Cubic method gives a nice smooth interpolation. Here is an 800% image of the two methods, ‘cubic’ top and ‘none’ bottom:

Edited August 23, 2022 by CraigT82

[vlaiv]

1 hour ago, Pixies said:

My options are an F6 200mm newt or an F10 105mm achro. Perhaps I'll need to rethink and get something with 2.5um or 3um pixels (F10 or F6 with x2 barlow)?

You can always adjust barlow magnification by changing sensor to barlow element distance.

It is best to get barlow with detachable element - that way you can use variable extension to dial in needed magnification and F/ratio.

Increasing distance between the two increases magnification and vice verse.

If you know focal length of barlow element - you can calculate needed distance by using:

magnification = 1 + distance / barlow_focal_length

[vlaiv]

47 minutes ago, johnturley said:

Several people have stated this or similar, but I've yet to find any satisfactory software to enlarge the finished image.

Yes, you can enlarge the image  in processing programs such as PIPP and Registax, or enlarge the image when photographing by reducing the capture area, but when you save the final image after processing back to your PC as a JPEG file, it still ends up EXACTLY the same physical size.

The only things that seems to effect the size of the saved image, are the size of the sensor of the camera, and the effective focal length of the telescope. 

John 

 

It seems that there misunderstanding on different image sizes.

I'll try to explain several aspects of image size and how things (usually) work.

First is physical / recorded image size.

Second is viewing size.

It is very often the case that viewing size is adjusted to suit display device and might not be actual recorded (or resampled) image size.

Many people use APS-C sized sensor to create images and often end up with images that are 6000x4000 or similar in pixels. Very few display devices (if any) have that much pixels to be able to display image fully.

If you look at that image on your FullHD computer screen or mobile phone of certain resolution - it will be resampled for viewing purposes - to fit the display area.

Take for example another image - this one recorded at 3000x2000 - so x4 smaller then previous. This one is still larger than display device and will also be scaled down for viewing. If you view them on same computer screens side by side - they will look like they are of the same size - but in reality they are not.

You are just viewing them scaled to say 15% and 30% of their size respectively. This happens automatically by software used to display things.

If you want to appreciate all recorded information - always enlarge image to 100% scale. This will often make it larger than screen and you'll have to scroll and pan around to see every bit of it.

Planetary images are almost always smaller than display device. In majority of cases - such images are not scaled up to fill the screen but are left as they are (at least on computer screens - not sure about mobile phones).

With these images you don't need to go 100% unless they are automatically scaled up.

In any case - when we talk about image size - we are most often talking about pixel count used to record the image and not file size, nor FOV / display size on the screen.

 

[vlaiv]

1 hour ago, michael.h.f.wilkinson said:

I consistently got better results after stacking when switching to the 1.3x Barlow, using the same telescope, on the same night, while seeing stayed the same.

There are quite a few parameters that can impact result of stacking.

Of the top of my head - alignment point size will change its relative size compared to features if you change pixel scale. Shooting at two different scales and not adjusting alignment point size can have effect on stacking result even if you have exactly the same level of detail in both recordings.

In any case, not sure if there is any point discussing this further, you know what works best for you.

[johnturley]

45 minutes ago, CraigT82 said:

John the best way to enlarge an image is to do it right at the end of the processing, once you have aligned, stacked and sharpened the image. 
 

I don’t know what you use for image finishing (photoshop?) but in GIMP you can enlarge for shrink the image by any factor using the ‘Scale Image’ tool under the ‘Image’ tab (shown below). You just enter the new image size in pixels that you want, so for a 2x enlargement of this particular image I would enter 3,292 into the circled box (1646*2) and then hit the  ‘Scale’ button. 


 

Edit: Should probably add a note on the interpolation method for the scaling. Choosing no interpolation method will result in a blocky ‘pixelated’ appearance. In GIMP the Cubic method gives a nice smooth interpolation. Here is an 800% image of the two methods, ‘cubic’ top and ‘none’ bottom:

 

I don't have GIMP, just Lightroom (which doesn't allow me the option of increasing the size of the saved image, or if it does I haven't figured out how to do it.

Is GIMP available as a free download, or do you require a subscription.

John 

[CraigT82]

4 minutes ago, johnturley said:

I don't have GIMP, just Lightroom (which doesn't allow me the option of increasing the size of the saved image, or if it does I haven't figured out how to do it.

Is GIMP available as a free download, or do you require a subscription.

John 

Gimp is completely free John 👍🏼

[Altair8389]

8 hours ago, johnturley said:

The focal ratio required to get a decent sized image, and the optimum focal ratio for your camera based on pixel size can be quite different.

With a Skywatcher 150 PSD, focal length 750 mm, you will need around f20 (4 x Barlow) to get a decent sized image, at f10 (2 x Barlow) the image size will be very small. 

To get good sized images of Jupiter and Saturn I think you need an effective focal length of around 3000 - 4000 mm, this would explain why long focal length SCT's such as the C11 and C14 are popular with many  planetary imagers, as they have this order of focal length without requiring any amplification with a Barlow, and have focal ratios close to the optimum for most planetary cameras. 

John 

 

Thank you John.

Thats very informative. At the moment, I do not envisage spending big on a heavy mount which can support a C14 telescope which is also quite expensive and I do not have the room in my home at present for such a large scope (too heavy) and big.

But maybe in the future (I would perhaps need a helper to setup my C14)...

 

Magnus

[Altair8389]

9 hours ago, vlaiv said:

It was taken with rather modest equipment.

SkyWatcher 130/900 newtonian on EQ2 mount. Camera used was QHY5IILc (same sensor as ASI120c USB2.0) and crappy Celeron laptop

Image was taken around opposition in spring 2015. Jupiter at that time was at altitude of about 60 degrees from my location (so pretty decent).

I used simple GSO x2.0 barlow lens (one with removable barlow element).

Trick is to use very short exposures - like 5ms, regardless of histogram (which is useful to detect clipping but not much else). Recording is often faint - but after stacking it can be processed to look good.

Capture in raw format and use ROI so you can get good frame rate.

Trick is to capture planet in moments of good seeing and seeing needs to be "frozen".

I don't have recording of that Jupiter image, but here is Saturn with same equipment:

Good frame (not debayered):

Poor frame:

Result after frame selection, stacking in (AS!2 at the time) and Registax 6 sharpening:

 

Super work there vlaiv, Saturn looks well defined for a modest telescope and equipment. I hope I can do something similar or better.

Thanks for your input.

Magnus