To Barlow Or Not To Barlow When Imaging Saturn.

[Grotemobile]

These were taken with my CPC 800 HD.The first one

is a 2X barlow.The second one,no barlow.The third one

with the barlow lense, screwed on the end of the camera,

giving 1.5x.It seems more detail, with no barlow,but smaller image.

Steve

[Pete Presland]

the 2nd image is clearly sharper, and brighter though all 3 are good images. Saturn is a pretty tough target at the best of times, made even harder by its position presently. 

there are no hard and fast rules when it comes to barlowing or not, other if the "seeing" allows do it! your barlow makes your scope F20 which is not unrealistic at all, i would always try my barlow and see what i get on any given night.

what webcam are using and are you using a filter?

[JamesF]

Ideally you should choose your barlow to match the camera and OTA.  That doesn't necessarily mean the seeing will always play ball though

It's also possible that the smaller image may look more detailed and better defined purely because it's smaller.  Part of that may be due to the fact that without a barlow more light is arriving at each photosite.  The result may look different if you also varied the exposure time so the histogram was equally full for all three images.

If you're using an ASI120 or QHY5L-II then I'd agree with Pete that f/20 is about the right place to be.  On average nights that may not seem to do you many favours, but on really good nights you'll reap the benefit.

James

[Pete Presland]

some of the nights this year even F10 has been a struggle

[FarSide]

Steve, forgive me for my ignorance but what tube are you using? 

[Grotemobile]

the 2nd image is clearly sharper, and brighter though all 3 are good images. Saturn is a pretty tough target at the best of times, made even harder by its position presently. 

there are no hard and fast rules when it comes to barlowing or not, other if the "seeing" allows do it! your barlow makes your scope F20 which is not unrealistic at all, i would always try my barlow and see what i get on any given night.

what webcam are using and are you using a filter?

 

http://www.modernastronomy.com/camerasPlanetary.html#IMG132E

This is the camera I am using. The IMG132E.

It has a built in I/R filter.

[Grotemobile]

Steve, forgive me for my ignorance but what tube are you using? 

The scope is the CPC800 HD.I presume this is what

you mean by tube.

[FarSide]

So essentially a Celestron C8? 

[Grotemobile]

So essentially a Celestron C8?

 

It is a 8" scope.

http://www.firstlightoptics.com/cpc-deluxe-hd/celestron-cpc-deluxe-800-hd.html

[FarSide]

Basically, Steve, i'd say you are 'over clocking' what you can perform with the 8" focal limits. For Saturn i use a 2.5X powermate on my C11 and i am limited at F20 - i am also limted at F20 for Jupiter. More often than not TOO much power destroys resolution rather than achieving more. Personally i'd say the 8" is on the limits at F10/F15 for Saturn - anymore and you are killing your resolution in favor of non-native magnification. 

PS: This also all balances on seeing conditions. The better seeing conditions the more you can push your f/ratio.

[JamesF]

The IMG132E pixel size is a smidge less than the ASI120 or QHY5L-II but only by about 3%, so I don't think f/20 is at all unreasonable to be honest.

The idea is to match the resolution of the camera to the resolution of the telescope.  As it turns out (and somewhat unexpectedly for me when I worked through all the maths) that pretty much comes down to achieving a given focal ratio with a given camera regardless of what telescope you're using.

James

[DrRobin]

Hi,

I wouldn't have thought that there would be any difference between a C11 at F/20 and a C8 at F/20, both should be equally achievable under the same conditions and with a similar camera.  The C8 will have a shorter focal length (and hence a lower magnification), but the F number should be the same in both scopes.

I have struggled this year to go to F/20 on my C11 on Saturn and had much better success at F/10.  I did however, get F/30 on Jupiter without too many problems, perhaps with Jupiter been higher in the sky helped.

Before my kit was permanently set up I imaged both Jupiter and Saturn at F/10, mostly to assess conditions and to get something in the bag.  If it looked good, put a 2x and then 3x barlow in and tried at the higher F number.  Now that it is permanently set up changing the barlow is slightly more involved, so I tend to go with whatever is in place and if time allows change it.

Robin

[FarSide]

My apologies guys i was mixing seeing conditions/barlow choice with telescope/barlow choice - dunce.

Essentially you are fighting with seeing conditions  with what barlow you choose to use - the worse the seeing conditions the smaller the barlow choice should be. Generally F20 on Jupiter and Saturn are acceptable for better than average seeing; you'll want to lower that the worse the conditions are.

[JamesF]

Yes, seeing will always place a limit on what can be achieved and one may have to lower one's ambitions should the night turn out to be less than would be desired.  I also agree that there's little point pushing the effective focal ratio beyond that at which the resolution of the telescope and the resolution of the camera are matched.  What surprised me when I worked through the maths to find that point was how exactly it panned out.

For  the OP:

There are a few wrinkles along the way because the resolution of a telescope is different at different wavelengths and we're almost always talking about working with a range of wavelengths (a large range in the case of colour cameras), but what falls out of the maths is that the camera and OTA are matched when a given focal ratio is achieved that pretty much depends only on the pixel size of the camera.  Initially it seems counter-intuitive, but that's how it works out.  It doesn't matter what telescope you put the camera in; if you want to match the resolution of the two, all you have to do is make the effective focal length the right value by choosing an appropriate barlow to get the required focal ratio.

For my SPC900 the desirable focal ratio turns out to be around f/35 and I did manage that even with my 127 Mak.  The same is true of the DFK21 and DMK21 because they have the same pixel size.  When I started using an ASI120 I didn't have to work anywhere near as hard because the much smaller pixel size means the required focal ratio drops to around f/20.  The IMG132E will be pretty much the same.

But other factors do come into play.  The seeing sometimes just won't give you enough to make that kind of focal ratio worthwhile.  Even keeping an image on the sensor is a challenge when you're working at a focal length of nearly four metres with a 127 Mak on an old EQ3-2  And you do need to balance the brightness of the image against image scale, gain and exposure time to achieve a good result.  That's probably far easier when the target is high in the sky than when it's scooting along the horizon.

So I think the best you can do is be aware of the ideal and attempt to work towards it, but accept that some days you just can't get there.

As an aside, I find it's also worth working backwards from the image to find the actual focal length/focal ratio in use at the time.  Firecapture will attempt this for you and write it in the text file accompanying the capture if it has enough information.  SCTs and Maks only have the stated focal length at one focuser position and the variation can be quite surprising, so knowing what you're actually getting when your kit is all set up is worthwhile.  That's one of the reasons I'm now looking for a decent 1.5x-ish barlow to use with my C9.25 and ASI120.  With the external focuser and filter wheel the focal length when the camera is in focus is really a bit too much for even a 2x barlow.

James

[Grotemobile]

Basically, Steve, i'd say you are 'over clocking' what you can perform with the 8" focal limits. For Saturn i use a 2.5X powermate on my C11 and i am limited at F20 - i am also limted at F20 for Jupiter. More often than not TOO much power destroys resolution rather than achieving more. Personally i'd say the 8" is on the limits at F10/F15 for Saturn - anymore and you are killing your resolution in favor of non-native magnification. 

 

PS: This also all balances on seeing conditions. The better seeing conditions the more you can push your f/ratio.

 

I do seem to get better results at F10 with Saturn.

F15 seems to give good results on Jupiter.

Practice makes perfect.More mag, seems to give

less, sometimes.

Steve

[Pete Presland]

there are so many variables, seeing conditions, local heat sources, postion in the sky, optics the list is endless. the more you get out there, the better chance of getting some decent seeing conditions you have. it is also worth keeping an eye on the weather conditions, position of jet stream, high pressure ect.. no guarantees but it can only help.

when i image i always start off at F10 and work my way up to see how far i can push the conditions, in this digital age you are only a "delete" button away from binning any poor data, nothing ventured nothing gained as they say.

what camera and filter are using?

[FarSide]

Steve, this will be primarily because of seeing - please ignore my babble! James is utterly correct. 

If seeing conditions are above average F20 on Saturn is perfectly achievable but this depends on seeing. Sometimes i drop back to F10/F15 if seeing conditions are bad but F20 is acceptable for most of your imaging sessions of Jupiter and Saturn. The reason why you were getting better images at F10 rather than F20 is because the seeing conditions just simply wern't up to it - when seeing conditions are right you will be able to push your focal ratio up higher.

Mars, Uranus and Neptune are utterly different beasts. Uranus and Neptune particularly are heavily reliant on almost perfect seeing conditions and very long imaging times to achieve any level of detail (if any) and Mars is particularly reliant of perihelion, good seeing and the right barlow at the right time, usually for me X2.5/X3. Every planet is different under different seeing conditions with different barlows used. Experience really counts. 

Other factors, especially with SCTs (because they are enclosed) is cooling your tube and mirror - my C11 takes at least an hour and a half to cool to ambient before optimal visuals come through, collimation too!

[Grotemobile]

there are so many variables, seeing conditions, local heat sources, postion in the sky, optics the list is endless. the more you get out there, the better chance of getting some decent seeing conditions you have. it is also worth keeping an eye on the weather conditions, position of jet stream, high pressure ect.. no guarantees but it can only help.

when i image i always start off at F10 and work my way up to see how far i can push the conditions, in this digital age you are only a "delete" button away from binning any poor data, nothing ventured nothing gained as they say.

what camera and filter are using?

http://www.modernastronomy.com/camerasPlanetary.html#IMG132E

This is the camera I am using.

Its got a built in I/R filter.

[Pete Presland]

thats a nice set up, more than capable of producing some very good images given the right conditions.

[Grotemobile]

thats a nice set up, more than capable of producing some very good images given the right conditions.

I have  had some good results out of it

over the years. Been a good investment.

Not a lot of people no about this camera.

[cutepetgroomer]

Back home I use my Barlow when there is no breeze. It makes for a better image over all. Also my scope I use the Barlow on is the 70/300 since it is a 3x Barlow I have. When I acquire a 2x Barlow then I can give it a go in my other scopes as well.. But yes it better to use a Barlow when seeing conditions are near perfect in my opinion.

[Mr Bergman]

Barlow or not to, should, in scientific terms, also be a question of resolving power if I am not wrong, and a decision whether to undersample or oversample, if you don´t are lucky and have the exact barlow config to meet your correct samplecriteria. We have to take our pixelsize into consideration, with regards to what size of detail you want to record.

 

This explains it; 

 

http://www.astropix.com/wp/2011/03/01/sampling-and-pixel-size/  

and then a quick calculation here; 

 

http://celestialwonders.com/tools/imageScaleCalc.html

 

According to this, I need a 3x barlow to be able to catch the finest detail with my qhy5ii-colour in a Skywatcher 190MN.  And that was interesting, since that only gives me an f-ratio of about 16....usually the recommended f-value for planetary work is 25-30. 

 

Any ideas? 

 

 

/Erik

 

[JamesF]

According to this, I need a 3x barlow to be able to catch the finest detail with my qhy5ii-colour in a Skywatcher 190MN.  And that was interesting, since that only gives me an f-ratio of about 16....usually the recommended f-value for planetary work is 25-30. 

It's not an exact calculation really.  Especially with a colour camera, though the same is true of mono to a lesser extent.  Resolution depends on wavelength.  If you have "ideal" sampling at one end of the spectrum you'll be over- or under-sampling at the other.  It also depends on whether you decide that the sampling rate should be based on the "edge" length of the camera pixels, or the diagonal.

I'd say that for a camera with the pixel size of the QHY5L-II, 25 to 30 is a little high.  I try to aim for somewhere between 20 and 25, though some people like to go a little lower than that.  The limiting factor however is probably what you can comfortably achieve with your optical train.  With the MN190 f/16 is easy with a 3x barlow and there are some reasonably-priced 3x barlows of good quality to be had.  A 4x barlow would give you just over f/21 which would be my preference, but there aren't that many 4x options about.  Using 5x for f/26.5 seems extreme to me.

James

[Mr Bergman]

It's not an exact calculation really.  Especially with a colour camera, though the same is true of mono to a lesser extent.  Resolution depends on wavelength.  If you have "ideal" sampling at one end of the spectrum you'll be over- or under-sampling at the other.  It also depends on whether you decide that the sampling rate should be based on the "edge" length of the camera pixels, or the diagonal.

I'd say that for a camera with the pixel size of the QHY5L-II, 25 to 30 is a little high.  I try to aim for somewhere between 20 and 25, though some people like to go a little lower than that.  The limiting factor however is probably what you can comfortably achieve with your optical train.  With the MN190 f/16 is easy with a 3x barlow and there are some reasonably-priced 3x barlows of good quality to be had.  A 4x barlow would give you just over f/21 which would be my preference, but there aren't that many 4x options about.  Using 5x for f/26.5 seems extreme to me.

James

Agree James, youre totally right, and also, these calculations assume a perfect seeing, more or less which never occurred to me at least.

 Yes, f25-30 is a bit high for me. I have been looking for a 4x barlow actually, but the only one that I found was the  2" Televue 4x powermate, which is too expensive. A 3x would be perfect I think! And I will not be too far from the correct sampling rate in any dimensions. I have to look for some good 3x`s right now! ( Sorry for hijacking the thread Grotemobile ).

[JamesF]

There's the 3x TeleVue barlow.  That comes up second hand every so often.  Or you could try the Revelation Astro 2.5x barlow.  It has a good reputation and with an extension of somewhere around 30mm between the barlow and camera I think that would be about 3x.  It also has the bonus of allowing you to drop back a little on the image scale if the seeing isn't really good enough though I'd have to admit that I tend not to do that.  If the seeing isn't good enough I just take it as an opportunity to practise.

James