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gnomus

Atik 460Ex or 383L?

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I did install Artemis but never used it for imaging as I did not have an Atik guidecam. I have heard that this may be changing in the future and if it does I'd cinsider it bit then again I dont use an Atik FW. I went ahead and bought APT and really like it as it was pretty similar to BYEOS. If your already using APT no reason to change right now, I would suggest you try to change as little as possible as you learn the new equipment.

The CCD version of APT will easily set up sequences for all your filters, run your ASCOM FW, etc...

I think I took 15 BIAS, I'm sure those with more experience can advise also.

Edited by nightster

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Hi Gnomus,

Well after a few days of head scratching and emails to QSI about there statement of reducing the back focus due to the addition of a filter, I have come up with I hope the correct answer. My physics originally said that the back focus should be increased due to a filter being incorporated into the imaging train. But QSI website clearly states that you remove some back focus. Like an idiot I accepted their information after all they are supposed to be the experts especially as I use their cameras.

I have sent an email to them and received an answer today saying yes you remove some back focus ???? I have return emailed them again as I am not satisfied with the off the cuff answer.

This is their answer unsigned by any individual. Just  QSI customer service. I will await the next  answer to my query of this  statement !

Quote "Back focus in this context is the distance from the end of the telescope or other optical system to the focal plane. The speed of light varies depending on the medium it is traveling through. If you put a glass filter in the optical path, it will move the prime focus towards the telescope, thus reducing the total back focus. The amount it moves can be estimated by taking one third of the thickness of the filter glass. That figure is then subtracted from the original back focus distance to yield the correct back focus with the filter in the optical path."

I have since worked out what I think happens to a light ray at 50 degrees to the optical axis It will extend the focal point away from the lens by about 0.4mm for a 1.1 mm thick filter of n = 1.48 

I apologise for the incorrect information, I should not have just accepted a manufacturers data without checking it myself.

I also have searched the web for confirmation before I posted this.

I came across this explanation on the Precise Parts Website, written by them.

Quote "

Q. How can I figure out the adapter length required?
A. The Adapter Effective Length is the net distance the custom adapter will add to the optical path.
If you are designing a SLR or DSLR lens adapter for an astronomical camera, please see this question first.
If you need an adapter consuming the minimum possible amount of back focus, just enter 0 for the Adapter Effective Length to let Build-An-Adapter calculate the minimum possible effective length for this particular adapter. The calculated length value will be displayed in the length field in either inch or mm depending on the unit selected. The calculated value may or may not actually be zero; the minimum length is constrained by the particular features required for that adapter.
Always check the 2D drawing displayed on the Build page to verify where the effective length is measured from.
It is important to note when designing a custom adapter for a photographic lens, for a field flattener, or for a reducer, that a minimum length adapter will almost certainly be inadequate. To reach focus or produce a flat image field, a flattener, reducer, or lens must normally be located at a precise Optical Distance in front of the camera CCD.
To determine the Adapter Effective Length for such optical system, the Image Train Back Focus needs to be calculated precisely and subtracted from this Optical Distance:
• Adapter Effective Length = Optical Distance - Image Train Back focus

For example, if you have a flattener that requires an Optical Distance of 55 mm and you want to connect a camera having 17 mm of back-focus to that flattener, your Image Train Back Focus is simply 17 mm and you will need a custom adapter with an Effective Length of 38 mm (55 minus 17) to end up with the correct spacing for that flattener.
The recommended Optical Distance for a reducer/flattener is also known as the Metal Back Distance and the value is generally supplied by its manufacturer.
If there is a filter wheel and/or other accessory in our image train between the reducer/flattener and the camera, their thicknesses should also be taken into account in our calculations and we need to determine the total Image Train Back focus, which is the sum of the image train components thicknesses and back-focus, then as before we use the formula above and subtract this total Image Train Back focus, from our Optical Distance.
Additionally, if there is any glass filter installed between the reducer/flattener and the CCD (perhaps inside a filter wheel), the effective length may need a small correction to take into account light diffraction through the filter glass: each 3 mm of glass thickness INCREASES the Optical Distance by about 1 mm so the correction must be ADDED in the formula, therefore increasing the Adapter Effective Length by the same amount to compensate for this correction.
In our sample image train above with the 17 mm back-focus camera, if we install a filter wheel that has a thickness of 25 mm equipped with 3 mm thick glass filters, our total Image Train Back-Focus sums up to 42 mm (17 plus 25) and the Adapter Effective Length will need to be 14 mm (55 minus 42 PLUS 1 mm for the filter correction).
Please contact us if you are not sure what the distances and back-focus values are for your particular image train. Describe your image train with as much details as possible, listing all the components and accessories in the optical path including any imaging filters. Although we do have a fairly comprehensive database of recommended distances and back-focus please note that we can only offer suggestions and each device manufacturer has the last word regarding recommended distances and back-focus values. We always suggest double checking distance values with the respective device manufacturer."

Hope this makes up for my mistake in believing the QSI website without question. The rest of what I said about the light being slowed down in the filter is quite correct.

Regards,

Derek

Edited by Physopto
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As an addendum to the above.

It is possible that what QSI really mean is that when a filter is added, say 3mm thick.  You add the 3 mm to the back focus then remove approximately  1/3 of this so in effect just add 2mm to the back focus in total.

What ever,  their method of explanation sucks!.

I followed their instructions and even with emails to Astronomik this last point was missed. I now have to look at my imaging train again I reckon I'm about 8mm out possibly. Too short now. It has not shown up in my images though !!!

Derek

Edited by Physopto

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Hi Gnomus,

Well after a few days of head scratching and emails to QSI about there statement of reducing the back focus due to the addition of a filter,................

Derek

Thank you very much for this detailed explanation, which, I think, sorts the matter out once and for all.  In short (assuming I understand correctly) introducing filters between the reducer and the sensor will reduce 'Backfocus', and therefore, require a corresponding increase in the 'Optical Distance'.  

Thank goodness for that because I just bet on "increase" (and ordered the required extension tube) yesterday!

With the allowable 'tolerances' in the various components of the image train (reducer; wheel; adapters; extension tubes; filters; possible OAG; and camera), I wonder just how precise one needs to be with all of this, or indeed how precise one can be.

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.....

I like the fact that you can sequence L>R>G>B x N in Artemis. I'm not sure if I can set up such a sequence in APT yet. The camera certainly works in APT (and I can dither).

This is very easy to do in APT. You just check the box for a 'Vertical Plan' execution.

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I'm glad Derek has gone through this as well because, whatever I try to do with the physics, I can only make a filter extend the back focus. Ho hum.

Let me give you a few Artemis scraps based on 10 years of using it and liking it;

Use the white point value in the Display window to choose the right exposure for your flats. I go for about 23000. The difference between the Black and White gives you an idea of the vignetting in your system. A difference of 4000 counts between the two might seem alarming but isn't. Flats will sort it.

The Sequencer over-rides most of the settings in the capture window but not the sub frame selection. Once you have used the sub frame and FWHM to focus you must restore the full frame option. If you don't, you will capture a sequence of sub frames. Very, very annoying.

I would keep file names short. I don't name the object in the subs captures, for instance. I name the object only in the file devoted to it. This is because your stacking software may display only the first part of the name and this can be irritating if all your subs start the same way. Much stacking software will balk at files with the same name, so I have a system in which I add N1, N2, N3 etc to the file names, meaning Night 1, Night 2 etc etc.

I don't use the Camera Rotate 180 option because you can get in a pickle with upside down darks and flats. If I want to reframe after the flip I open the pre-flip image in whatever software and rotate it so I frame the flip to that. 

I would take at least 50 bias and as short as possible. Build in a delay of a couple of seconds when shooting bias and flats because they warm up the chip.

I would also use the nice metal cover Atik supply for shooting darks and bias. I am amazed by how much light leakage there is in what you might think to be a closed scope. However, you also have a shutter, which I don't, so maybe this matters less for you.

Olly

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I'm glad Derek has gone through this as well because, whatever I try to do with the physics, I can only make a filter extend the back focus. Ho hum.

Let me give you a few Artemis scraps based on 10 years of using it and liking it;

....

Use the white point value in the Display window to choose the right exposure for your flats. I go for about 23000. The difference between the Black and White gives you an idea of the vignetting in your system. A difference of 4000 counts between the two might seem alarming but isn't. Flats will sort it

...

Olly

Olly

Thank you very much for these pointers. I will get straight onto producing a set of biases and will work on a Darks library.

Steve

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Hi Olly  and Gnomus well everyone reading really.

I just got another reply to my question from QSI, this time signed. I'm afraid it is wrong........................

Quote.

"A reduction in backfocus is published because it's based upon using our internal filter wheel. In that configuration, the distance between the filter and the sensor is fixed. So the question becomes, where do you position the camera so that the light cone from the filter to the image plane is the same as when there is no filter in place. The answer is that you must move the camera closer to the telescope reference by about 1mm for a 3mm filter. Hence the subtraction rather than addition.                                       

For another discussion on this matter you can check out this discussion on the cloudy night message board: http://www.cloudynights.com/topic/386437-back-focus-distance-of-astrodon-filters/

Rich

"

This is a post off Cloudy Nights

Quote

"Posted 21 August 2012 - 10:44 AM

Listen to Konihlav!!

here is a quote from Don Goldman posted on the ccd-newastro yahoo news group awhile ago...he knows a thing or two about filters:

>>
Re: Filters & Back Focus Distance

Camera manufacturers show a smaller optical backfocus than mechanical backfocus
because they are measuring the distance from the imaging CCD focal plane. This
includes detector chamber window and sometimes coverslips on the detector.

HOWEVER, that is not where WE measure backfocus from. We want to know how much
space we need to add from the metal back of a telescope or from a field
corrector/reducer to the imaging focal plane. This is how we select spacers,
etc. So, our starting point is the scope.

THEREFORE, the addition of a 3 mm filter ADDS 1 mm [t * (n-1)/n] of backfocus
between the scope and your camera (t is the filter thickness and n is the
refractive index of the substrate of the filter - typically 1.5) So it ends up
being t / 3. 3/3 = 1mm in this discussion. You need to ADD 1mm of space between
your scope and your camera.

This is often confusing. When you place a filter in a beam of light that
converges from left to right, the focus is extended FURTHER right INCREASING the
backfocus distance as measured from the scope.

All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED.
<<

you can find the full thread here:

http://tech.dir.grou...o/message/67993

"

You can now guess who I believe.

Derek

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...

Use the white point value in the Display window to choose the right exposure for your flats. I go for about 23000. The difference between the Black and White gives you an idea of the vignetting in your system. A difference of 4000 counts between the two might seem alarming but isn't. Flats will sort it.

...

Sorry to keep bothering folks with questions, but can I just clarify what the above means?

Firstly, in the Display Window I should have 'Autostretch' ticked.  Then, I take a series of exposures, adjusting length of exposure until the 'autostretched' displayed value for 'White' is around 23000.  Then  I'm all set.

If I have understood correctly, this seems quite straightforward (and is a marvellous tip - thanks again, Olly).

Steve

Edited by gnomus

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I haven't read every post, but I gather your bought the Atik383.  In case no-one has warned you, because of the shutter you can't have your flats too short or you will get shutter shadow.  Took me a while to work out why this was happening when I got mine.

I think you need to take a flat of not less than something like 1.5 - 2secs (you will be able to tell by whether you get a shadow or not).  This means if the light source is too bright as these lengths, you'll have to dim the light source down a bit.  I found this a bit of a pain as using different filters meant different length subs and numbers of sheets of paper to dim the light, though the same applies to any mono camera I guess.   

I always found an ADU of 22000 worked the best.

Carole 

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I use -15C...any lower an the camera struggles on warmer nights. -15C means I can use the same darks over and over.

Artemis? Personally I don't use it. It may work perfectly, but one look at the interface convinced me that it was written by someone who's skills hadn't progressed beyond 1996 freeware applications. I now use Sequence Generator Pro to control the camera, and I can't recommend it highly enough. There's lots of alternatives (Nebulosity, for example) but SGP is hard to beat for bangs per buck. Its worth the £50 for the plate solving alone.

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If you find that getting the flats is a problem because of too much intensity, you can get some neutral density film and put it over the scope lens dew shield. It will be out of focus and averages out. It is also important to have the camera at the same angle to the optics used for each image, as the doughnuts will move around as the camera is turned if they come from a field Flattener or the objective. Olly will know more as he has done loads of this sort of thing. You can get different densities of neutral density film. A couple of sheets of 2x probably will do you.

It may be possible to lock the shutter open by software. If so then you will not get the shutter effect and will not need the neutral density film. It is important to do your flats at the same focus as you have done your imaging as far as I am aware. By doing this all the dust particles in the light train will be of the correct sizes i.e. to be at the same out of focus size. It should not matter about stretching the flat, unless you particularly want to see if doughnuts are present (dust), you just hover the mouse pointer over the images and find the highest ADU count. Depending on software, there is possibly a window that will tell you the ADU ranges for each flat, not sure of that. I use MaximDL only.

Derek

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...In case no-one has warned you, because of the shutter you can't have your flats too short or you will get shutter shadow.  Took me a while to work out why this was happening when I got mine.

I think you need to take a flat of not less than something like 1.5 - 2secs (you will be able to tell by whether you get a shadow or not).  ...

I always found an ADU of 22000 worked the best.

Carole 

Thanks Carole

I have tried a few frames and, using my flatfield panel (with an ND filter over the panel), I am finding that an exposure of 1.3 seconds gives a White ADU value of ~22500.  I assume that is close enough?

Steve

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I use -15C...any lower an the camera struggles on warmer nights. -15C means I can use the same darks over and over.

Artemis? Personally I don't use it. It may work perfectly, but one look at the interface convinced me that it was written by someone who's skills hadn't progressed beyond 1996 freeware applications. I now use Sequence Generator Pro to control the camera, and I can't recommend it highly enough. There's lots of alternatives (Nebulosity, for example) but SGP is hard to beat for bangs per buck. Its worth the £50 for the plate solving alone.

I see there's a highly limited function version. I may download that and see what I think. Thanks for the tip.

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The trial for SGP is 45 days long and totally unlimited. If you choose not to buy after the 45 days, then it reverts to the Lite version. I bought it after 1 night.

http://www.mainsequencesoftware.com/Products/SGProCompare

Yes I found that out.  I currently have it running a sequence to take 50 bias frames at -15C.

Does SGP have any sort of 'Looped' exposure mode that will allow me to take a sequence of unsaved, short exposures for the purpose of manual focussing? 

Edited by gnomus
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Yes I found that out.  I currently have it running a sequence to take 50 bias frames at -15C.

Does SGP have any sort of 'Looped' exposure mode that will allow me to take a sequence of unsaved, short exposures for the purpose of manual focussing? 

Yes. You can use the focusing tab or the frame tab.

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Thanks again Stephen. I have now taken an image of a distant rooftop!!!

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SGP does get a good press, I must say. As for the retro aspect of Artemis, well, maybe that's why I like it so much!

Note Carole's point about the shutter. Flats do need to be long enough to avoid imaging the wipe of the blades. I don't have any experience of using this camera hands on so I don't know how long they have to be.

Olly

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Shutters usually rotate around so the wipe is not a problem.

/per

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SGP does get a good press, I must say. As for the retro aspect of Artemis, well, maybe that's why I like it so much!

Note Carole's point about the shutter. Flats do need to be long enough to avoid imaging the wipe of the blades. I don't have any experience of using this camera hands on so I don't know how long they have to be.

Olly

OK, I've thought about this a little longer, and I have a better understanding of the flats issue. Indeed, looking back through some of the 1.3 second flats I took earlier, I can convince myself that I can see a 'wiper blade' effect in some of them (possibly all of them). I have one of these Aurora panels. The flats I took were with the panel in the constant-on position. There are two other settings that provide a slow and a slightly faster strobe type effect. It occurs to me that this would provide less overall light and thus longer exposures. I can go down this route or I could get more ND film. I have a 1.2 and a 0.6. The 1.3 second exposures were taken with just the 1.2. If I add in the 0.6, I think this would take me to around 5.2 secs (which might be enough). I wonder if the strobe effect might be the better way to go, however.

What do folks reckon?

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I answered my own question.  Adding in the 0.6 ND filter meant that I had to go to 7 seconds to get a 24000 ADU.  Putting the Aurora onto slow strobe resulted in 3 seconds for ~24000 ADU.  Looks like I need to use both the 1.2 and the 0.6 with panel full-on.  I have to say I didn't see the wiper on the 3 second exposure though, but better safe than sorry. 

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A sheet or two of ordinary printer paper will dim the light enough.

Another plug for SGP...it has a flat wizard. Tell it the target ADU and it will start looping exposures until it finds the correct exposure length. And it can do this for each filter if taking flats per filter is your thang.

http://mainsequencesoftware.com/Content/SGPHelp/SequenceGeneratorPro.html?FlatsCalibrationWizard.html

It can also automatically add flats into the imaging sequence per target.

http://mainsequencesoftware.com/Content/SGPHelp/SequenceGeneratorPro.html?FlatsCalibrationWizard.html

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I answered my own question.  Adding in the 0.6 ND filter meant that I had to go to 7 seconds to get a 24000 ADU.  Putting the Aurora onto slow strobe resulted in 3 seconds for ~24000 ADU.  Looks like I need to use both the 1.2 and the 0.6 with panel full-on.  I have to say I didn't see the wiper on the 3 second exposure though, but better safe than sorry. 

Where are you adding the ND filter? if its in the filterwheel, then it will introduce it's own aberrations (dust bunnies and vignetting?)

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