Flats in Maxim DL

[pvelez]

At the moment I am dabbling in photometry - climbing the light(learning) curve. One point that its constantly stressed in the literature is the need to calibrate with flat frames.

I have been particularly unsuccesful in my efforts to date.

To give you an idea of my issue, I've attached 3 images. The first shows an uncalibrated image taken with Maxim DL. The vignetting is reasonably apparent.

The second is a copy of the flat frame I took using a t shirt at twilight on Friday.

The third is the first image calibrated in Maxim DL. As you can see, something has gone awry in the processing.

Now the flat image looks quite bright. However, when I open it in ImagesPlus and review the histogram, the curve peaks quite nicely around the 30% level. I suspect that Maxim auto-stretches the image for display.

My suspicion is that the auto-stretch is also applied when calibrating the image so that rather than removing the vignetting, the program overcompensates.

My next thought is that there is some serious reflection going on inside my scope or the extension tube of the focuser. I am using a fast schmidt-newt - f4- so I have a MPCC in front of the nosepiece to avoid coma. I am using a SBIG ST-8300 which has a big sensor.

If you look carefully at the flat, you will see that the bright patch at the centre is a narrow elipse extending almost to the top and bottom of the frame but not to the side. I suspect internal reflections are to blame.

But if that's the case, why does it only show up in the flats and not on the CCD?

Or perhaps my images are still overexposed.

I am most puzzled. Any tips would be gratefully received.

Pete

[TheDecepticon]

Taking flats is quite tricky until you work it out, then it is quite easy. I tried to use T shirt flats too, but had a real hard time getting them right. I ended up I bought a light box from Exfso which gave me a very stable light source. As I use a QHY9, which has the same chip as the 8300, you might have to use a potentiometer in the power supply to dim the light box off a bit to get the right length flats, etc.

http://www.iceinspace.com.au/forum/showthread.php?t=49736

In MaxIm, I take a single exposure and set screen stretch to "range" so it is an unclipped display. Using the "info" dialouge box, I experiment with the exposure time until I get between 20 and 30000 in the centre of the image on the brightness scale at the left of the info box information. If that looks ok, I save it and open it in Phototshop, adjust the black slider until it is nearly on the left hand side of the histogram and make sure it looks nice and gray and smooth in transition from centre to edges.

I also believe that you will find a "how to" here as well, if that is any help.

http://www.iceinspace.com.au/63-211-0-0-1-0.html

I can do some screen shots of how I do it, but I don't think I could get that done today. I'm also thinking that if you search the forum, this has been discussed before and should provide some good info as well.:thumbsup:

[bmitchell82]

i too had a bit of issues taking flats with maxim untill i figured one thing out...

That is making sure you know what the dynamic range of your CCD is, if its 16bit, youll have 65000 adu approximately. Now i use the 14bit 40d, which is considerably less than 65000, so i was aiming for about 20-25k adu.... wrong!

So follow what decepti said, but just bare in mind also about your dynamic range.

Oh for a really cheap and good quality light box i built one out of a Broccoli box, 2 Cold cathode 300mm fluros, a piece of white foam board from office works and a bit of duct tape. all up set me back $35 bucks and i never looked back.

[bert]

Since you are using maxim, try ccd autopilot for flats (try the 1 month trial).
I also use the tshirt/sky flat method.

In ccdap you can simply give it a target range in adu and press take flats. It will adjust to make the exposure time right for the current sky brightness. I get perfect flats every time.

Brett

[bmitchell82]

i had a look at ap, but it wanted some programs like pinpoint ect ect. so it was a waste of time. unless i had the coin to get the program i was out of luck :(

[bert]

Brendan, Not so. Pinpoint is required if you want ccdap to do plate solves to check for slew errors, and for calibrating your guiding. It is an option, not a requirement. But it is very handy.

Brett

[jase]

Save your money on CCDAP. Whilst a good tool for automation, if all you want to do is flats download the free MaximDL SkyFlatAssistant plug-in - http://winfij.homeip.net/maximdl/skyflats.html

Don't be fooled by the name. You don't need to use the tool exclusively for sky flats. It can also be used for tshirt or lightbox flats. Similarly to CCDAP it will measure the target ADU to ensure exposure lengths are optimal based on light conditions. You set the target ADU at the start of the run. Clearly, the exposure will vary even if the light flux remains the same as astronomic filters have different bandpasses. Narrowband filters will require a longer exposure time to reach the target ADU.

For lightbox flats and similar the light conditions don't change, thus exposure times would remain fairly constant (for a single filter that is). Sky flats however are a different story given the light alters dramatically (and rapidly) during the small dawn and dusk time windows, hence the need for measuring the target ADU and dynamically adjusting the exposure time to compensate is key to success.

What CCDAP will however provide is the ability to shift the mount ever so slightly to move any stars that maybe (likely) to be picked up during a sky flat acquisition run. As the stars are placed on different pixels across the CCD array, they are easily recognised as outlier pixels and rejected via the conventional data rejection algorithms.

So, you'll end up with accurate flats...but at different exposure times... How the hell do you calibrated the data then... easy... scale your dark frames to compensate. Take a series of 10min darks, then bias frames. Produce masters of both, then use MaximDL or CCDStack to subtract the bias from the dark to give you a thermal frame that can be scaled to match the duration of the flat frame.

Personally, I don't use CCDAP anymore. Good tool, but what ACP delivers is far more flexible in automating sky flats, plus it has a web interface. Anyway, I'll keep on topic as comparisons may lead to greater discussions (different thread).

[bert]

Jase,

Could you start that thread as threatened?:)

Very interested in your comments on acp.

Brett

[jase]

Brett, I think I've contributed my opinionated views on the two products here before. A search may reveal an insight.

For those wanting additional reading material on sky flats (don't be afraid of them as they're not as complex as you think), you can download a white paper on the topic.

The Flat Sky: Calibration and Background Uniformity in Wide Field Astronomical Images (http://adswww.harvard.edu/cgi-bin/nph-bib_query?1996PASP..108..944C&db_key=AST&nosetcookie=1)

Download the high resolution pdf and enjoy!

[pvelez]

Thanks to all for the suggestions.

Having just started using Maxim DL, I still have the trainer wheels on.

When I have a chance to do some processing, I'll give it all a whirl. The SkFlatAssistant plug-in sounds promising as my main issue is getting the exposure length right.

I'm off to buy a lightbox (figuratively speaking) from Exsfo now.

Pete

[Moon]

Jase - another great brain dump - thanks.

I do find the ADU values reported by screen stretch window in Maxim can be quirky at times.

James

[jase]

James, the screen stretch function is just that - a screen stretch. It is by no means the best method in judging the accuracy of a flat. FITS files are numerical. When processing data, it helps to understand the figures.

In MaximDL, on the status bar to the lower right you'll find an item 'I: xx,xxx'. Where ever the mouse pointer hovers on the image, the 'I' value (Intensity) displays the pixel's numerical value. Towards the center of the flat you should be hitting the target ADU. +/- 1,000 isn't an issue btw, doesn't need to be dead on.

If you want to get truly analytical on the flats, use MaximDL's line tool and draw a diagonal line across the frame. This will present a graph showing values along the line. There are a few other ways to make an assessment.

[pvelez]

Jase

thanks for this. I've downloaded and installed the plug-in.

Now for the dumb question - how do I drive it? I can't find anything on the web that tells me how to initiate it.

I am sure there is an easy answer - just dunno what it is.

Pete

[pvelez]

Brett

some tips would be handy here.

I've downloaded CCD autopilot and gave it a run with my lightbox. t took 2 flats - 7.6 sec and 3.4 sec and then decided it was too bright and refused to play ball any more.

Am I missing something? I assumed that it would run until it hit the target ADU setting.

Pete

[pvelez]

I'm open to any feedback now.

I have tried to run the Sky Flat Assistant plug in in Maxim DL. All I have is a greyed out menu item in the Plug Ins listing. It won't run or even make itself available.

Am I missing something?

I am taking somequite nice shots but have a devil of a time getting flats to adjust for colour imbalance across the field. Flats with my lightbox that match the target ADU still give me an unusually light strip acros my images. You can see this with the flats I've included in my first post.

Pete

[Moon]

You need to take one image manually and have it open. Then the menu becomes active. It's a very cool little tool :-)
James

[pvelez]

Thanks James - thats good progress.

Now I have an error message that says MSSTDFML.dll is missing. A quick google search yields lots of sites urging me to do a system scan with their software - which gives me the willies.

any ideas from the computer gurus?

Pete

[pvelez]

More progress - I have installed the missing dll file and I have it up and running.

I'll give it a whirl now.

Pete

[Bassnut]

In yr original post, you had stars in the flat, thats not good at all, thats going to mess with the stars in the light (probably eliminate them, as in yr final image). Do you still have stars in yr flats?.

[pvelez]

Hi Fred

only the last frame was a flat - the other 2 were examples of before and after applying my "flats".

I've now fired up Sky Flats Assistant and am having a processing run to see if I have a better result.

Pete

[Terry B]

Pete
Looking at your unclabrated frame there is some vignetting in the left upper corner. It looks like frosting on the corner of the frame.
As for flats, they are tricky to get correct. I use CCDSoft rather than maxim but the process is the same.
I have a white board on the wall of my observatory and aim the scope at it. It is lit either by a light aiming at the opposite wall or just on an overcast day with the even illumination that that creates in my observatory.
If you use a T shirt you have to be carefull that the front of the scope is evenly lit and nothing partly shaddows the T shirt like a dew shield.

[pvelez]

Thanks Terry

Yes, the vignetting was a function of the dew shield I used to take those shots. I've since upgraded to a light box and a RC8 which makes for a better match than the old LXD-75.

I've now experimented with Sky Flats Assistant which seems quite intuitive once you get it running. Now all I need to do is capture some lights and new flats and apply them. I tried to apply today's flats to some shots I took a month ago - urgh! not good. The old shots had already been calibrated with the dodgy flats so I ended up with a very nasty colour gradient. The right half of the screen has a delightful green hue while the centre left has a wonderful magenta look, fading to green at the extremities. I won't share those with you all - it could put you off your dinner.

Looking at the new flats, it seems they are similar to those I took but with a shorter rather than longer exposure time with ADU around 22,000 across the board.

So I'll let uyou all know how I go - probably tomorrow night

Pete

[pvelez]

Ok - it must be me.

I took a few test shots of an attractive group of galaxies in - Pavo I think - last night. I then fired up the light box (thanks Exfso) and took a series of flats using the Sky Flat Assistant plug in with Maxim. Took darks at the correct temperature and a bias or 2 for good measure so the darks would be scaled and applied to the flats.

All good.

I then used the Calibration Wizard to point to the relevant folder and made sure all the cal files were there. They were.

To the combine routine - open Stack, add folder and select Auto-Calibrate. Measure, align and combine files into R, G, B and L groups. Then colour combine.

Am I missing something?

I have an image with a very green hue in the background to the right of the target and about a third to the left. Down the middle there is a strong magenta tone.

So I conclude its my flats. I take a single image - through a green filter as its the most sensitive - looks like some vignetting to me as the centre of the image is noticeably brighter than the edges. I then apply the flat from the same filter - similar look in terms of brightness - wham! Maxim has overdone it. I have effectively an inversion of the light with too much brightness at the outside and not enough in the centre.

Now my weighting of colours is around 1.23-R 1-G and 1.89-B, so when I do a combine, I have too much green for all the image except the centre. Where there is insufficient green, its a delightful magenta.

Quite arty but a poor astrophoto. My missus thinks I'm like Michelangelo painting the ceiling in the Sistene Chapel - "Michael, when are you going to finish??". The cold nights never produce anything worth looking at let alone printing.

So.....how do I reconcile this?

1. Is my green filter sensitive to light at different intensities so it passes more light when the light is brighter than it should - so my flat is not really flat?

2. Or is there a problem with my calibration routine? I am no Maxim DL expert - just a novice.

3. Or is it not vignetting but rather something else rather odd? I had a SN f4 and had a similar issue - you can see that from the shots in the original post. I now use an RC8 with the SBIG 8300. Is this some other optical aberration eg do I need a flattener?

4. Or am I just lousy at flats? I reviewed each of the flats and found that they stayed well within the ADU range - coming in at around 20k - 22k ADU. The EL flats as well as the T-shirt flats were pretty good too.

5. Or maybe its just me. Eminently possible but in this case I suspect not.

If anyone can put me out of my misery, I'd be grateful.

My suspicion is that number 2 is the culprit. Perhaps I am double or triple calibrating so the images are being progressively lightened at the margins with each iteration. Luckily I have kept a back up of uncalibrated images from last night so I can try and manually calibrate them first and then stack, align and combine.

Incidentally, I read somewhere that the FITS header for images taken with Maxim DL include a field identifying whether calibration has been completed. For the life of me I can't find it. Am I making this up?

If I can get to the computer tonight, I'll post some images so all the above makes sense.

Cheers

Pete

[Bassnut]

How many flats are you stacking for each filter, or are you takeing just one flat for each?.

[pvelez]

Stack of 10 Fred - median combine

Pete

[Bassnut]

Well, its got me buggered. Can you post the flats, darks (stacked) and lights FITS on an FTP server for us to fiddle with?.

[pvelez]

Fred

it would be my pleasure - though I don't have access to a FTP server. Is there a public-accessable site you know of I can use?

I'm testing a combine at the moment. Will post a sample shortly so you can see what's going on - or not as the case may be

Pete

[pvelez]

So here are a few shots to show you what I am wrestling with.

The first is an uncalibrated 5 minute image. It is a median combine of 10 images with a median combine. I have used the default auto-stretch in Maxim. Yes, I know there is heaps of dust about!

The second is a master flat taken through a green filter. Same deal with the stretch.

The third is after calibration.

Hope you can see my dilemma. Comments happily accepted.

Pete

[Bassnut]

Here is the same problem as with the original image sequence, could you look at your post and confirm the order, as in what the 1st 2nd and 3rd images are.

[pvelez]

Sorry Fred

first is the master flat, second is the uncalibrated image, third is the "calibrated" image.

Pete

[Bassnut]

OK, well, the flat looks reasonably normal, if not a bit mishaped, and the uncalibrated light looks like a fair match. The calibrated light looks sort of "over calibrated"". Ive seen a similar situation before, with a missmatch between light and flat filters. Are you absolutely sure the light and flat filters are the same?, check the filter allocation is correct in DL set up, or there isnt a mix up after capture. Check the FITs header filter data for each, DL auto calibrate relies on correct fits header data. Try manual flat selection for a light to confirm.

[Bassnut]

Oh, and I hope you are saveing darks and flat stacks in DL with auto stretch DISABLED ?.

[pvelez]

Thanks Fred

agree that the light and flat look ok - its the calibration that is odd.

Using Sky Flat Assistant, I have been manually setting the filter before capturing the flat so it should match. The FITS header simply reads from Maxim DL so it should all correlate.

Pete

[pvelez]

Sounds an interesting one - how do I check this? I still have training wheels on with Maxim.

Looking at the FITS header it reads under CSSTRETCH "Range" / Initial display stretch mode.

CBLACK is 18574 and CWHITE is 23297

Not sure if that is good or bad

Pete

[pvelez]

Here is a LRGB - curves and levels with a bit of a saturation boost to show the colour gradient.

(Only 1 image so the order is right!)

Pete

[pvelez]

I'm trying a new tack tonight - I'll take a few shots with another imaging program and see if they calibrate better. I believe I have CCDSoft lurking on my computer at home.

Any suggestions for Maxim are welcome. I am keen to use as few software packages as possible and until this arose Maxim looked the goods.

Pete

[jase]

Pete,

Make sure you've got everything checked in MaximDL as per the attached.
In particular if your scaling your darks, click on the dark frame group to determine what scaling you are doing. auto-scale is the conventional method, auto-optimize is RMS noise based similar to that used in CCDStack. You may find you'll obtain a better result with the latter - experiment. I typically build the bias and dark frame masters before processing the flats.

Looking at the images in post 28, your flats are over correcting the light frames hence the reason why you have a dark band down the center. You need to ensure your flats are accurate as possible. Ideally, what you should be left with is no system characteristics, i.e. donuts and huge gradient shifts, but more the typical sky glow and smoother gradients caused by light pollution. Looking at the raw light frame and the flat, I would suggest the flat isn't matching the system characteristics.

[pvelez]

Thanks for taking the time to post the screenshot.

My version of Maxim DL has a different layout for this screen but teh settings are the same.

I've tried to image with CCDSoft - same result

I've also used AIP4WIN to calibrate - again, same deal.

I'm now playing with shorter and longer exposures to get the ADU count away from the 20,000 threshold I've been using so far.

Will keep you posted

Pete

[pvelez]

If I am binnng at 2x, do I need to target the ADU for a flat at half what I target for 1x binning? So if my target is 20,000 ADU, do I reduce that to 10,000 if I am binning 2x?

Pete

[pvelez]

OK, I think I'll leave you all n eace for a bit on this one

I've experimented with a range of settings. Sky Flat Assistant allows you to set the target ADU for your flat and then gives you an error margin. The default is 10% which I had been blindly using. Dropping it to 1% made the calibration a lot neater. Its still not perfect - I am sure there is some dark magic involved that I don't comrehend, but its a lot better.

Incidentally, I suspect that the key is not so much the target ADU setting as consistency between the images. I didn't see that much difference between 18,000, 19,000 and 20,000 ADU. But there was a big difference if the allowable error is 10% rather than 1% - even with a median combine.

Does that make sense?

Pete

[jase]

Pete,

Where did you get a target ADU of 20,000 from? The full well depth of the SBIG 8300 is only 25,500! This may explain the over correction of the light frame data as I previously noted.

Go on the 1/3rds principle (~30%);
25,500 / 30% = 7650 target ADU, so round it up to 8000 and accept a difference of +/- 1000 ADU. When you combine a series of flat subs, data rejection with clip the highs and lows (extremes) and level the data (assuming you don't use sum!!). Average or median combines will work well, if you want to get fancy, CCDStack's clip min/max or poison sigma reject are great. If you're finding that its not correcting the frames well enough, increase the principle to 35%. Just set the target in SkyFlat assistant and let it work out the optimal exposure time. Too easy.



No, target ADU remains the same, its just that the exposure time to reach it will be shorter.

Cheers

[pvelez]

Jase

I think there is a difference in nomenclature here. The full well capacity of the ST 8300 chip is about 25,500 e- which I understand converts to about 65,000 ADU. The Sky Flat Assistant sets by reference to ADU and I had thought that Maxim's graph function did the same. Feel free to correct me if I'm wrong on this.

Last night I tested flats with target ADU counts from 8,000 (co-incidentally) up to 30,000. It was late when I twigged to the error factor and dropped from 10% down to 2% and then 1%. Flats at 8,000 ADU yielded a very nasty vertical dark stripe down the image. The sweet spot (though still not that sweet) was at around 19,000 to 20,000 with a 1% error setting.

Its still not perfect - I am using 5 minute subs. I suspect that the uneven light will be less an issue at shorter subs. If so, applying the same flats will be worse rather than better I guess. Thats logical so its probably wrong.



This bit is more of a challenge for me. I've set Calibration Wizard to the correct file and its identified in Set Calibration the relevant dark, bias and flat files. I've then created a master flat from the flats in that folder using median combine. The settings are as per your earlier post (albeit under a tab marked Advanced). I then open the light and calibrate it from the Process tab. I'm not sure how to level the data as you suggest. Is this an alternative to median combine?



Understood - thanks

Pete

[jase]

For 16bit cameras the maximum pixel value is indeed 65,535...however you're not calculating the flat saturation value from this figure!!! In doing so it would mean that every 16bit camera would have the same saturation value regardless of the CCD chips characteristics = incorrect flats!!! You need to calculate the value from the CCD chip's full well depth - 25,500 for the ST8300. Forget about the 16bit ADU for the time being.



The vertical stripe you note is probably characteristics of the camera. Short exposures can exhibit this. Do your bias frames show a similar feature in the image? It is normally corrected during the bias/dark frame application to the flat subs prior to being combined.



5min flat subs! Crazy. Long subs will work if your flats are controlled, i.e. you're using a lightbox or similar, but reaching the target ADU faster (shorter subs) is preferred especially if you're doing real sky flats as the longer the exposures the more changes you'll pick up stars in the dawn/dust sky and your time window to acquire the flats are short given the sky luminosity changes quickly at dawn and dusk. As a reference, the longest I'll go for sky flats is 120secs. I do reach this value when acquiring flats through the 3nm bandpass SII and OIII filters as the quantity of energy being passed is small compared to LRGB broadband filters. Long or short, you need to ensure you're scaling your dark frames to match the exposure times. Flats work best when the exposure time is altered to reach the target ADU - unless you have a way of controlling the luminosity of the FOV. Subsequently, I don't take flats shorter than 10 seconds. This is due to the Apogee camera's Melles Griot shutter leafs causing a star fish shape at the edge of the flat field. SBIG camera's use a different shutter mechanism, hence you wont have problems going shorter than 10 seconds for your flats.



Yes, we're using different versions of MaximDL. I'm running 4.6, not version 5 hence the screen capture differences however the fundamentals are the same. You don't level the data manually, the algorithm used for data rejection is doing this for you. Its is determining which pixels should be considered outlier and rejecting them. Median combine is more than ample to produce calibration frame masters. Try getting around 8 flat subs per filter, but 3 will work. You want to ensure that you're not adding noise to the light frame subs as a consequence of bad flats.

Hope that clarifies.

[pvelez]

Thanks again for your time Jase



OK, so I aim for about 8,500 as the target ADU in Sky Flat Assistant. I'll give that a go again.



Until recently I didn't take bias frames. Now I do so that Maxim can autoscale the darks before subtracting them from the flats. To be honest I haven't looked too closely at them. I'll check this out too.




It would be! I was only referring to 5 minute lights. My thinking was that the uneveness of illumination on the sensor becomes more pronounced the longer my light subs go. If my flats are too bright - the contrast with a shorter light sub will be more pronounced than if I applied the same flat to a long sub.




Now this may be an interesting avenue too. I've been using a light box - luckily it has a potentiometer so I can step down the intensity. Using a target ADU count of 20,000, my flat subs are less than a second - in fact for the green filter its about 0.18 seconds - quite close to the spec minimum of 0.1s seconds. I can turn the intensity down a bit more and with the reduced ADU count I can probably increase the exposure time a bit. I wonder if the flat is affected by the shutter sweeping across the sensor so that I don't have even illumination across the field.

So I'll turn down the pot and take the subs as long as possible - the 8,000 target will make this easier.



OK - got that (I think). I've been taking 10 flats and median combining them. I've relied on Maxim autoscaling the darks and subtracting them so I might try doing that manually.

Looks like I need to get the kids to bed early tonight so I have a chance to sort this out - all part of the learning curve.

Thanks again

Pete

[pvelez]

I think I have this all sorted now. I spent a fair amount of time over the last few days playing with flats. Here is a summary of the results:

1. Error 1 - I found a few times that the FW moved to the wrong filter – usually to the Ha filter which is in the first position in the wheel rather than to the luminence filter as instructed. This doubtless happened enough to throw the results.

2. Error 2 - I was testing flats against lights taken a few days earlier. I couldn’t work out why all the dust donuts obvious on the flats were not taken out of the lights. The answer was that the lights didn’t have the dust on them at all. I suspect the dust accumulated on the filters between lights and flats. The most persistent of the donuts was in a dark section of the light. Had it been brighter in that section of the image, I might have realised that there was no dust there in the first place.

3. Error 3 – I was testing my flats against a single exposure taken through a green filter. I didn’t realise that the calibrated flat was displayed with a different stretch than the uncalibrated image. So in flicking between the calibrated and uncalibrated image using the undo button, I was actually seeing not only the effect of calibration, there was also a different stretch which gave greater prominence to the fainter sections of the image.

4. Error 4 – I don’t think I was properly applying darks to the flats. I now set up master frames so they are dark subtracted.

I suspect it was a combination of all of these things – as well as pilot error more generally – that had me in a tangle.

A couple of other points:

5. Shutter artefacts don’t seem to be an issue. Generally, the shape of the flat doesn’t change from a sub-1 second exposure to a 30 second exposure. I played with twilight flats over the weekend and so managed flats out to 30+ seconds. The pattern of illumination was the same.

6. I couldn’t work out why I had a strange colour gradient. The sky background at the left of the image always had a blue hue while to the right it was quite green. Looking at the blue filtered images, it was clear that there was a lot more blue in the background on the left side. However, when I upped the target ADU for the blue filter from 20,000 to 28,000 the difference was much less stark. To be honest I can’t work out why this might be the case – other than perhaps that the limited sensitivity of the ST8300 to blue light (and the cut-off of the blue filter) means that the response of the sensor is not linear at that wavelength (though that sounds v odd) or that Sky Flat Assistant responds too quickly to the peak intensity so it cuts short the exposure before there are enough photons registered by the sensor to give a truly flat field. Anyway, the result is that I take blues at 28,000 ADU and R, G and L at 20,000. I have yet to work out Ha – my Ha is not parfocal with the other filters and the cable for my electric focuser has yet to arrive so I’ll sort that out later.

I could be wrong about all this – however I managed to take about 1.5 hours of the Triffid last night and it looks great – without colour gradient or obvious uneven illumination. Maybe its not a drama as the target is quite bright. I’ll only know when I head back to galaxies.

Thanks to everyone who contributed to solving this. I suspect almost all of the errors were spotted by other contributors to this thread before I did. Goes to show there are some v smart (and experienced) imagers out there.

Cheers

Pete

[Exfso]

I am glad we put the pot on the box pete.:)

[pvelez]

You betcha

Pete