Seeing? Optics, Focus?, ???

[PRejto]

I could use some advice. I'm kind of stumped. I never quite ran into this problem when I used my KAF8300 CCD on my TEC140. But, my Trius is giving me fits (and not such good ones at that!).

As you can see from the attached screen shot of CCDInspector, I had reasonable seeing (I presume) given all the FWHM values in the 2s. I used FocusMax and have a large sample of V curves. To my eyes blue looks shockingly poor, but, I cannot focus any better. The FWHM values make me think the images should look better. The .jpgs are in order of R G B and are at 160%. 5 min exposures with the blues showing very good aspect ratios. Aligned and stacked in CCDStack.

Is this seeing, optics, poor focus (but why if poor focus? I even ran v curves just with the blue filter (Baader) and cannot do better)?

I know Greg Bradley has complained about "bloated" stars using the Trius on his TEC180. Perhaps this is something similar. If so, it seems to be due to blue for some reason.

[JohnH]

I am no expert here but had this issue with my 127ED, which, granted, is not a true APO, I had to re-focus for each (parfocal Astronomics type II) filter. It became tedious so I ended up automating the process using offsets from multiple calibration runs. I also note that the BLUE wavelength is more subject to atmospheric distortions and thus, ideally, should be shot with the target at the highest point, also a corrector or reducer in the imaging train may make things worse.

I could never remove this effect completely, so I settled for focusing the R and G to match the B star sizes rather than optimise all, I then got the best L frames possible - after all that is where the image resolution comes from, RGB just add colour.

It seems a limit of the refractive optics that the spot size is smaller for a longer wavelengths, I am not sure if that is what is dominating overall though, guess that depends on the arc secs per pixel.

[RickS]

If you look at the CCDI data you'll see that, on average, the blue subs are less bright than the red and green subs (Background).

When the subs get stretched the blue ones need to be stretched further and the blue channel stars bloat giving a blue halo. It's the same reason that Hubble palette images have magenta star halos.

Cheers,
Rick.

[PRejto]

Rick,

I appreciate that you pointed this out. No doubt this is part of the equation. However, it doesn't exactly explain why the blue looks so much worse than anything I previously got with my KAF8300 CCD camera. Nor would it explain quite bloated bright stars in luminance.

I'm beginning to think that it might be due to the fact that TEC colour correction goes from 436 nm - 1000 nm, but the Baader transmits to ca 380 nm. Together with the ca 40% QE of the Sony chip at 380 nm might this be the reason?

Many thanks,

Peter

[RickS]

Probably a combination of factors, Peter, as you say. The relative dimness of the blue subs will be down to factors such as camera QE, filter characteristics, etc. Atmospheric scattering has a bigger effect at the blue end of the spectrum as well. I attempted to help answer the question you asked: why your blue subs seem poor by comparison to the red and green. You didn't previously mention bloated stars in Luminance so I didn't attempt to address that issue...

Cheers,
Rick.

[PRejto]

You are correct! I did mention Greg's experience but forgot to say that it was with luminance. He privately pointed me to an image in luminance where the large halo effect is quite evident.

Thanks!!

Peter

[Shiraz]

that would be my guess. The APO could be spraying out a fair bit of a halo in the violet and still be in spec. Since the FWHM are pretty consistent, it is not likely to be seeing or focus.

The Trius could have more violet sensitivity than the 8300 for a variety of reasons - window material, AR coatings etc. as well as the high QE. Might be worth trying a minus-violet filter or fringe killer in conjunction with LRGB, which is a bit of a pain.

[ericwbenson]

Hi Peter,
I think you can safely discard focus or seeing as a cause, simply due to the FWHM results you listed, you can confirm this yourself by looking at the FWHMs for the three stacked channels, I'll bet they are very comparable.

The listing you posted did not show the altitude of the individual frames, but again you can easily assure yourself that it isn't a factor (it would show up in the FWHM anyways).

The bgd count doesn't reveal much if anything about the focus or seeing, just the convolution of camera QE, filter bandpass and light pollution/natural sky brightness where the scope was pointed.

What is more important is the stellar profile. In Maxim there is a graph window which displays the line profile of star for all three colors simultaneously (for color images), and also a graph of intensity versus radius for each channel (you can do these graphs in excel if you can export the data in tabular txt format, but that's a pain). From this you can quantify the amount of halo in each channel independent of stretch or black point level. Don't have Maxim? send me the raw stacked FITS x3 in floating point or 16bit, cropped around the area of interest.

Now the FWHM calculation is not necessarily sensitive to the low intensity halo, but the aggressive stretching we use in processing to bring out faint nebulosity takes no prisoners. So here are some hypothesis to try:
1) greater reflection/scatter from CCD microlenses in the blue
2) reflection/scatter from telescope surfaces
3) reflection/scatter from filter surfaces
4) limited correction of objective lens in the blue

To confirm/deny these:
1) Use a different camera which has a different chip (e.g. KAF, KAI or DSLR, OSC) the latter two nicely avoid changes in seeing or altitude. You may already have images from this scope with another camera?
2) Not being a refractor guy I am unsure here, but I do know the design and AR coatings need to be right in order to avoid ghosts which can show up as haloes. Do others have the same problem with different cameras?
3) Borrow a blue filter from a mate and see if it goes away?
4) I doubt it is this one since you would think the FWHM would be affected, only pick as right answer if everything else is excluded!

HTH,
EB

[PRejto]

Ray and Eric,

Thanks a lot for your feedback and ideas.

I've done a bit more research and found a spec for the TEC140. It's corrected from 436-1000 nm, but the Baader seems to transmit to 380 nm where the Trius is still at ca 40% QE.

I was all set to try a Hutech IDAS LP filter (cuts at ca 410 nm) to see if this helps. Then the clouds rolled in, of course....

If this works I wonder if there is any way I could just do the UV cut in blue and not the other channels. A shame because these Baaders without any LP filter generate the nicest/easiest colour I've ever had.

Eric, I will test my images in Maxim. I have the program but rarely use it these days. I appreciate the other tips as well. I'm hoping it's just the need for an IR cut. The other issues would be fairly insurmountable!

Peter

[Shiraz]

hi Peter.

figure 149, example 17 in the reference shows the spots diags for your scope design (multi-band is the top spot diagram) - it definitely has an extensive halo at the blue end (about 10x the (black) diffraction spot) and presumably would be even less well corrected in the far violet. http://www.telescope-optics.net/semiapo_and_apo_examples.htm The reference notes that the straight triplet is best for visual and the designer could have chosen to pull in the violet for imaging, by sacrificing performance elsewhere. However, the general characteristic of a significant violet halo will probably still be there. The Trius has much higher blue/violet sensitivity than most Kodak chips, so is better able to detect any such halos.

if it is a violet halo due to residual CA in the APO, it would be in the lum image as well (as Greg has found), so a violet-cut filter would need to be applied there as well - probably best to just put one front of the camera and use it for all channels.

Eric, I think you can discount 1. and possibly 3. - halos do not show up in images taken with reflectors using the same camera (and I think using the same filters).

[PRejto]

Ray,

Many thanks for the spot diagrams link. That is most interesting. Remember that I am imaging with two scopes simultaneously! The TEC180 for luminance with a CLS filter (strong UV cut!) and the TEC140 for RGB/narrow band. I put a similar post at Cloudy Nights and received some very interesting info there re the ICX694 on a TEC140 (not a Trius, a QSI). Using Astrodon blue he is getting decent results and put a link to a blue fits file. It is a big improvement over what I'm getting. The Astrodon seems to cut at 400 nm whereas the Baader cuts at ca 380. Enough of a difference? Perhaps. At least with the QSI it seems to be. Could camera electronics come into the equation, or cover glass material? Also, yes, Greg Bradly shows bloated stars in luminance with his Trius on the TEC180. If you are curious here is the CN post:

http://www.cloudynights.com/topic/476528-bloated-stars-with-my-tec140trius/?p=6219293

Now, if I could just incorporate a CLS filter with just BLUE I might be quite happy. The CLS cuts at 450 nm.

Peter

[JohnH]

So I should have been exposing my B frames for longer than the Rs and Gs...any general guidance on exposure ratios to use or is it too dependent on the filter/optic/ccd combination??

[gregbradley]

The TEC fluorite series are among the most exotic scopes available.
It is most certainly not the scope. I have used several cameras with my TEC180 and only seen this problem with a Trius. I have observed it a few times on extremely bright stars with a KAF8300 camera. Both have small wells. I get perfect stars with a FLI Proline 16803 with over 100,000 electron well depth versus 18,000 with a Trius.

I think the best strategy is to match optics/CCDs and the Trius works better on some scopes than others. Either that or the Trius has has something different from the SX694. Perhaps SX changed something like the coatings or the glass etc.

I put it down to small wells. I see the effect on my CDK17 just not as noticeable.

Why it does appear to be a problem with Mike's scope is strange although he tends to use short subs.

Ray also uses even shorter subs so perhaps that is the best approach - very short subs so the wells don't fill further out from the star's centre.

I can try it on my AP140 and see if there is any difference next time I am using it. I suspect no difference but we'll see.

I am wondering if a FLI Microline 694 would behave the same way. Its only a feeling but I suspect not. This is why the SX is the cheapest, not necessarily the best.

Greg.

[Shiraz]

thanks for the link Peter. One of the posts suggested a Baader UV-IR cut filter, which might be a solution if the problem turns out to be violet CA. If the IDAS makes a significant difference, that could be the next thing to try, since a UV-IR cut filter could stay in place without harming the LRG data.

thinking about it, you guys are really pushing the boundaries by using the scopes outside of their design spectral region, with pixels 1/4 the area of more common ones and with chips that have high sensitivity in the violet region.

[PRejto]

Ray,

You are correct, and personally I don't think it's working out too well for me. I looked again at the link to the photo that was put up over at CN. I now think it is just as poor as what I'm getting. I think I wanted to believe it was better!

Tonight I took a lot of data that I need to think about and perhaps graph. The Hutech IDAS filter certainly helped with a sense of better focus in blue, and reduced the bloat some. It's a step in the right direction, but not enough. I also took 3 sets of images working upward in 30 sec increments up to 5 minutes. There is pretty clear bloating/time in blue. I didn't run the same extensive test in red but there was also some bloat. Compared to blue it wasn't terribly significant. So, it seems there are at least 2 issues going on here. The violet spectrum is out of focus and is causing focus and halo looking issues, and even 5 min exposure of some stars will be far too much.

My luminance image using a CLS filter on the G2-8300 is tack sharp. I took some images with the blue Astronomik filter just to compare. I'd never done that on the TEC180. I noticed a tiny amount of star enlargement along with a tiny bit of soft looking focus. It appears that even this camera will suffer just a tiny bit without a UV block. Previously I had used this camera on my TEC140 but always with the Hutech IDAS filter which produced pretty tight stars in blue. I'm inevitably coming to the conclusion that this camera/scope matchup is not made in heaven. If the filter option fails it might be time for the camera to find a new owner with a reflecting system perhaps.

Peter

[ericwbenson]

Hi Peter,
So while a few more measurements will really confirm this, it looks like it is a combination of 4) + a slightly different 1) and 3).

The camera has to much QE for the exotic refractor hmmm.... I guess I'll ditch the camera and find a worse one... :P

But seriously have you considered a catadioptric? Although if a simple bandpass filter fixes the problem, well that is the best outcome for your wallet.

Best,
EB

[Shiraz]

Peter, perhaps have a look at the Wratten 2E filter - it might possibly do what you want, but I have no idea where to get one. The standard filters are gelatin, but someone may make a glass one - quick search did not find any though, so will keep looking.

Agree with Eric - is probably worth persevering a bit with filters if there is still some possibility of getting a usable system - the idea of using the two scopes is a really good one.

[PRejto]

Eric and Ray,

Ah, I was unclear. I very much want to get the Trius to work. Of course I'm not giving up yet. I'm waiting for a filter I ordered. If it doesn't work I don't really see how or why I should continue to struggle with this issue. Yes, it's a great camera but it reveals something not so great in this particular combination. That's all. I really hope to find a way around all of this!


Peter

[PRejto]

I have continued experimenting with the blue filter and Hutech IDAS. I also was loaned an Astrodon blue which made for an interesting comparison with the Baader. The Baader cuts at 380 nm, and the Astrodon at 400 nm, thus one would expect the Astrodon to be slightly sharper. This was born out in testing though the difference is subtle. The Astrodon appears to concentrate the light a bit more but still showed quite a large halo effect. With the Hutech IDAS in place in front of both filters there is an improvement as the Hutech cuts at 410 nm. To my eyes the effect was better for the Baader; I'm not sure why but I saw it after several iterations.


I am going to be trying out the Baader Semi-Apo filter and depending on the result may purchase a Shott GG435 as recommended by Gerd at Astronomiks. That cut might exactly match the TEC140 lens specification. What remains to be seen is how overall colour balance works with such a filter in place. A look at the graph showing frequency response of various cameras easily shows why the effect I never noticed with my KAF8300 camera is now so obvious with the Trius.



The other issue that clouds all of this discussion is that of star size using the Trius. It's hard to separate the two issues so I ran a bit of an experiment* with the blue filter by gradually increasing exposure times in 30 second increments up to 5 minutes. The stars in question range from about 9 to 11th magnitude. I've plotted the results. Perhaps my test had too many variables; seeing, gradually lowering altitude, tracking (I was guiding), etc. Still, the average result points to a possible enlargement taking off around the 3.5 minute mark. The reason I don't put this on the doorstep of tracking is that the aspects in CCDInspector are still quite good at the 3-5 minute positions. Anyway, I put it out here for what it's worth. Personally I do think that my 5 minute exposures have probably been too long if I want to avoid even bigger and overly saturated stars. But, even 30 seconds gives saturated stars. I wondering if in using this camera one might need to take a whole range of exposures in order to simulate the effect of deeper wells?


Peter

* I focused with FocusMax before each of the three runs. This was with the Baader blue filter.

[Shiraz]

Certainly looks like that is on the right track Peter - the Hutech made a huge difference.

The FWHM result is a bit surprising - I can think of no reason why increasing exposure should change star shape on a linear CCD (it will when the stars reach saturation, but CCD inspector throws them out). Maybe the increasing background level is upsetting the FWHM calculation. The contrast ratio is reducing with longer exposures (I think it should increase) which may suggest that the data has not been dark calibrated - that could explain the change in FWHM with increasing exposure. Perhaps rerun with dark calibration if it was not done - I think that the "Reduce" checkbox lets you do that if it has not already been done elsewhere. If you don't have darks for those exposures, maybe try something like the dynamic PSF in Pixinsight (do you use PI?).

you could use multiple exposures to gain a bit of dynamic range if saturated stars bother you - I think that Rick posted some details on this process (again in PI).

regards ray

[PRejto]

Hi Ray,

Thanks for the input. I did not dark subtract, but I can do that and compare values. I'll post the results!

Peter

[Shiraz]

oops, sorry Peter, I might have erred in my prior post. Thinking about how it probably works, you really need to subtract the total background, including skyglow - just taking off dark may not make much difference. Will PM.

[PRejto]

Ray,

I've done the dark subtraction. The results are confusing to me! The FWHM only changed a little, if at all. But, the aspects became terrible! Jeez, I hope they really are not that bad! I blinked two images - one dark subtracted, one raw. I could not detect anything by eye (other than a little bit of dark noise subtraction). I certainly couldn't see an aspect going from 8 into the 30s.

Check out the photo attached.

Thanks,

Peter

[gregbradley]

Good work on your experiments there. I find the problem in luminance rather than the blue channel (it tends to be in all channels).

So the theory its the colour correction of the scope is likely to be incorrect otherwise it would not bloat in luminance like it does.

Also I see the same issue at times with my CDK17 but that does have a corrector although chromatic aberration is minimal at the eyepiece end of scope as opposed to a refractor objective.

I'd say the problem is more too small wells. The other possibility is IR bloat which may affect all of the filters?

The other practical solution is to take a 10 minute luminance test exposure of a potential target and if you get bloated stars then pick another target. I have used the scope successfully on objects with no bright stars in the frame.

Example Trius and TEC180 images I have taken:

http://www.pbase.com/gregbradley/image/154225916/large Trius 694/TEC180 still some halos but quite minor overall not bad.

http://www.pbase.com/gregbradley/image/154487188 stars in this image seem fine.

http://www.pbase.com/gregbradley/image/154052172 stars are fine.

http://www.pbase.com/gregbradley/image/154241563 luminance and good stars.

http://www.pbase.com/gregbradley/image/156022968 Microline 8300 and TEC180 - perfect stars (same filters as used with Trius)

http://www.pbase.com/gregbradley/image/134312755 Microline 8300 and TEC180 - perfect stars

http://www.pbase.com/gregbradley/image/134561608 Microline 8300 and TEC180 bvery bright central star had a bright halo in red which was toned down in Photoshop but you can still see it somewhat. I put this down to small wells?\

I use Astrodon Gen 11 50mm square filters. They are top notch. Not sure how Astronomiks compare. I don't see blue halos specifically so perhaps they cutoff differently? Also I image with the TEC at a very dark site so that may also give a slightly different result.

One thing is for sure its only the Trius that does this. There are thousands of high end images from other cameras on TEC180/140's with no issue. So I'd say it points the finger at the Trius and perhaps filters if you are only getting the problem in the blue which is different to what I get.

What does work as a strategy on the CDK is to do shorter subs -5 minutes is my standard although sometimes I do 10mins if the target can take it. The CDK will actually burn out a galaxy core on 10 minutes on some subjects with the reducer!

Greg.

[Shiraz]

Hi Greg, I would say that Peter's results and yours show quite clearly that the problem is a violet halo. Peter showed that a filter that removes the bottom 40 nm or so essentially cures the problem. The only 2 channels that can actually see the violet light are blue and luminance - which explains your lum result. This is not a criticism of the scopes - I am sure that they work exceptionally well - they just don't correct well enough in the violet (which is outside of the design range) to be suitable for use with a camera with small pixels and very high violet sensitivity.

Just to be clear, exactly what do guys mean by the term "bloated". If that just means bigger stars, then that is certain to happen - the Trius will produce equivalent stars that are 4x the area of those from the 16803 (or 1.4x the area of those from the 8300), simply because it puts 4x as many pixels under each star. Everything else in the Trius image will also be 4x the area including galaxies etc. So if you want to compare Trius images with 16803 images, you need to downscale the Trius images by 1/2 first. If you mean that the cores are saturated, that is due to to the combination of very high sensitivity and smaller wells of the Trius - saturation can easily be overcome by using short subs (eg a couple of minutes) and the Trius will allow you do that because of the low read noise. But fixing the saturation will not make the stars any smaller - it will just stop the cores of bright stars from losing detail and going white

[Shiraz]

That confirms that I was wrong, as guessed - will have a look at the data with PI, which corrects for background.

[PRejto]

The last thing I want anyone to feel in these posts is that I am in any way criticising the TEC branded refractor. That is far from my intent. They are amazing scopes! But, it's well known that Yuri figures the optics for visual. Now if one puts a camera on the scope that perhaps mimics the frequency response of the human eye the blue channel will have a vanishingly small problem as witnessed by my own photos with the KAF8300 and Greg's many photos. But, put a camera on that has such a highly accentuated response to blue/violet/UV and we have an issue. As Greg has said one must match the camera to the optics. This is something I tried to do when making a purchasing decision about the Trius, but I failed to appreciate this one particular issue in blue. I spent a lot of time agonising over pixel size, etc. And, looking for similar posts such as this one...well, I couldn't find one which I find a bit odd given the popularity of the TEC140 and the Sony chip. I do think that this issue is likely to be raised more and more as the Sony chip takes on ever more popularity. Perhaps opticians will begin to rethink how they figure APO optics in reaction to this issue. Who knows? Time will tell.

I guess some people will take a swipe at the need for a -violet type of filter on an APO. My response is that's pretty much a garbage argument! I try to think of it this way: we don't blame DSLRs for having an IR cut filter do we (unless we are doing astrophotography)? We accept that a filter is necessary to compensate for the CCDs increased sensitivity (compared to the human eye). I think a -violet filter is exactly the same, just at the other end of the spectrum! Now, I suppose the argument isn't particularly helped when one considers that many reflecting type scopes can maintain critical focus into shorter wavelengths and thus manage to avoid blue fringed stars. But, I would argue that this light is no more "real" (by real I mean to the human eye in terms of colour) than IR. Maybe my argument is self serving. I don't know, but I do know that I'm not stressed about using a filter if in the end I can take a great photo with tight stars!

Peter

[PRejto]

Ray, I will crop out the data you requested and put it on Dropbox. I'll send you the link privately. I really appreciate your willingness to take a look at this data and to help me figure it out!

Peter

PS: In your other post tonight your point is well taken about the "magnification" effect of the smaller pixels. Yes, one must compare apples to apples so to speak. Well, this is exactly what started me down this path: I take luminance at .88 arc-sec on the TEC180 with the G2-8300. The colour from the Trius on the TEC140 at .95 arc-sec. It is during the L-RGB combine (where the Trius image is scaled up to match the .88 arc-sec luminance) that I notice that many of the stars coming from the Trius are just larger than from the KAF8300 chip. Of course without a special filter blue was/is huge. But I notice it with other coloured stars as well.

[gregbradley]

A picture is worth a thousand words. Perhaps Peter's and mine setups are slightly different as when I see the problem its in all colour channels and luminance not just blue. When I do colour combines and process I don't necessarily get blue halos but often red sometimes it is blue.

So the conclusion that its the high violet performance of the camera I would question as it does it in every channel. Hence my conclusion its the small wells as I don't see it on a deep well camera.

I also see evidence of hazy bloated stars in others' images sometimes using other brand cameras too so it obviously is something to consider when setting up a system. I don't see it in Mike's images despite using a corrector and I don't see it your images Ray using a Newt with a corrector. I do see it to a much lesser extent on the CDK with corrector but again in all channels not just blue.


Not sure what you were referring to about blue/violet halos in the TEC180/Trius/Astrodon Gen 11 images I posted in my last post as none of them have that except one with rather minor bluish halos.

Below are luminance, red green blue subs usin TEC180 Trius 694 Astrodon Gen 11 filters. As you can see the bright stars are bloated and are much the same in all 4 channels with no one channel much better than the others. I think these were 10 minute subs.

I am interested in seeing the results from different filters like minus violet etc but don't expect it to handle based on my experience. Shorter subs, perhaps a UV/IR block may help. Perhaps its stronger response in UV/IR that is doing the damage?? The bloated stars look a bit like they can go when I use my clear filter with no UV/IR block on it.

I'd also love to see some images from a FLI Microline 694 to see if there is any evidence its really the camera and its electronics - like gain settings, quality of the ADC, quality of the coatings on the CCD window and other supporting electronics that could influence the final image.
Or is it simply the small wells. I see SX quoe 18,000 electron well depth yet FLI quote 20,000+ so there is some difference right there.

Greg

[PRejto]

Greg,

Your photos are superb and if I could get stars like yours I'd be quite happy.

Now, I believe that your Astrodon filter cuts at 400 nm, but the Baader I am using cuts at 380 nm. I use Astronomiks on my G2-8300 and I get good results in blue with that camera. I put that more to the chips QE in blue than to the particular cut off of the blue filter.

A friend loaned me an Astrodon blue filter last night. It produced slightly tighter blue stars than the Baader. But, not good enough to satisfy me. Adding the Hutech IDAS significantly helped as it cuts at 410 nm. Both filters showed improvement with the IDAS. A bit more cutting and I think I will be there.

Peter

[PRejto]

Greg,

I don't mean to beat a dead horse so to speak, but I think it's quite easy to mix up two different problems into one.

I'm quite convinced that in my system, for whatever reason, that I have a real focus issue, and large blue stars with halos, due to excessive violet out of focus light being picked up by the Trius and its high QE in that spectral region. Otherwise, how can I explain that I have so much better focus and smaller haols (in blue) using a filter that cuts at 410 nm? As noted earlier, my Baader filter passes to 380 nm which is 20 nm more than your Astrodons. And as I noted in the previous post the IDAS also improved the Astrodon filter I borrowed last night.

Problem 2 is probably completely unrelated to problem #1. I would also guess that the large stars one sometimes sees in other colours might be due to small well size. It is this second problem that I'm now trying to come to terms with. For me, though, it is much less of an issue than the unusable blue I was generating.

Peter

[gregbradley]

Strange you are only getting bloat in the blue. That would seem to point towards the filter or the scope.I can't say I have ever seen poor performance of my TEC180 in the blue channel though. Blue is often hazier than the other channels due to light scatter but seems no different to any other scope. As Ray points out this chip is also quite small so the magnification factor is quite high. Again not sure why that would be a problem but it may be magnifying whatever that something that is non optimum here.

I am curious though if my AP140 shows any difference. At least that would nail down the possibilities a bit more.

Another possibility for the bloat may be the microlenses on the sensor. I wonder if they don't like F7 and the angles the rays hit them? A bit of a long shot though.

The UV/IR block aspect may be worth an experiment. I have a Baader UV/IR block filter I could tape on the CCD window to see if it changes anything. Another little test next time I have it out.

I was not too worried about it really as my other cameras suit the style of images I want with this scope and my CDK does better than the TEC180 for galaxies although the TEC180 does an excellent job. The CDK at 17 inch aperture simply is better.
Greg.

[PRejto]

Yes, Greg, I agree with you. It's quite important to keep the TEC140 and TEC180 discussion quite separate. I make no "claim" about the TEC180 as I've never tried my Trius on that scope. It may very well have a different colour correction than the TEC140. In fact, I'm unable to find any colour correction data on the TEC site about the TEC180 at all. Do you happen to have anything?

And again, I stress that the blue issue I see on the TEC140 is perhaps due to the way it is corrected AND exaggerated by the super hi QE of the Trius. At least my own tests seem to indicate this. However, the large star issue is probably something quite different once blue is taken care of. It may be something seen by both scopes and probably has much more to do with the camera and the way it is used rather than either scope.

Your eventual test of the AP130 will be most interesting!

Peter

[gregbradley]

Oh I misunderstood Peter. I thought the blue issue was with the 180.
And you are getting no problems in the other channels? Sorry you probably have mentioned all this in previous posts on this thread. You refocus after each filter change? In my experience Baaders are nowhere near parfocal despite their claim. Astrodon's are.

There is a difference in flourite and colour correction and fluorite is chosen because of that. But I would have thought it was more to be able to get the same colour correction with a faster F ratio rather than
better colour correction at the same F ratio over ED glass. There is better scatter which could account for better blue performance (but as you see in my samples not really only if the star is not too bright).

Good luck with your experiments. I also think it may be useful to post on the SX Yahoo Group to see what others are finding. Or use your 8300 camera for colour and the Trius for luminance.

Greg.

[PRejto]

Thanks Greg!

I certainly do focus with each filter change. My Baaders do seem pretty close to parfocal. The whole issue came to light when I just couldn't find a focus in blue that looked anywhere near what the FWHM values indicate. In other words FWHM of say 2 but looks terrible on the actual image.

Yes, perhaps I could switch cameras on the scopes but I would lose the fairly lucky nearly same FOV and similar resolution I get with the cameras set up this way. Also, with the sensitive Trius gathering colour I think it's a win win situation. Once I get the blue tamed I should really be good to go. The resolution of the 8300 chip on the TEC180 is great!

Peter

[gregbradley]

The 8300 is quite a good match for the TEC180 but you still have to watch out for the occasional very bright star and exposure times. That 180 aperture lets through a lot of light fast.

Greg.

[PRejto]

I think this little picture gives this discussion a bit of perspective!

I have 2 filters coming via Gerd Neumann at Astronomiks - the GG420 and the GG435. He is happy for me to keep whichever performs the best in my setup. He also tells me that Astronomiks is developing a set of RGB filters to exactly address this blue issue in refractors. I will resurect the thread once I have a result!

Thanks everyone for the input!

Peter

[PRejto]

I have finally completed some preliminary tests using various UV/blue blocking filters in combination with my TEC140/Trius combination. It appears I need to do a bit more as the data I have so far is not conclusive. Perhaps it will be of interest to others. And, certainly I'm open to suggestions for improvement of these experiments. There are so many variables to consider.

I have 3 filters to consider:
1. Shottt GG420
2. Shott GG435
3. Baader Fringe Killer

I previously tested the IDAS LP filter which gave some improvement in blue resolution and fringing (cuts around 410 nm). The Baader filter has a complex cut (see image below) and the Shotts cut at 420 and 435 nm)

I decided to image NGC1977 (Running Man) with my Trius using the Baader blue filter (which passes down to 380 nm). In front of the SX filter wheel I installed an Orion 4 position 2" wheel containing the 3 filters. I decided on a series of 10 x 2 min exposures for each filter. After collecting that data I had a quick look at the average adu in an area known to be blue. After a bit of trial and error I settled on an exposure of 1.5 min for series of RGB without any UV filter in place. My reasoning (perhaps incorrect) is that a 25% time reduction in blue without a filter would approximately equal the blue transmission with the UV blocking filters in place and lead to a hopefully close to correct colour balance. It appears that I somewhat underestimated the reduction, particularly for the Baader Fringe Killer.

First, a quick comparison of resolution shows that all three filters give a significant visual improvement to raw frames with the Baader doing the best job, though not by much.

I registered all the images at once and then stacked and mean combined for each filter. I then created 4 RGB images, one for each filter and one that used no blocking filter. The results were quite surprising as I fully expected to see a difference in resolution given the bloated appearance of the blue channel in the sub frames without the blocking filter. I cannot detect a difference in the final result in this experiment. I even went back and only registered the blue results that used no blocking filter. Again I saw no difference visually though the FHWM reported by CCDStack was about 10% worse than when I registered all the images at the same time. In any case. with this particular imaging target, it is difficult to conclude that I should be using any blocking filter at all even though with a different target I struggled with quite a lot of blue fringing on medium to bright stars. This time I simply don't see the fringing. But, I do see quite a lot of reflection halos in the sub frams as soon as I use the blocking filters. Again the halo effect goes away in the RGB combine, I guess, because there are no halos in G and R to reinforce. It makes me wonder if I saw the blue fringing in my earlier experiments only because I had not removed the blocking filters shooting R and G and consequently all the channels had refection issues.

I exported the RGB images from CCDStack after slight background adjustments to eliminate blue clipping that was showing with CCDStack background left at default. The images from CCDStack are what comes out of the default settings and no colour adjustment was made to the background. I only adjusted the background to eliminate clipping when exporting to Photoshop. I used Photoshop CS5 to align and balance black points of RGB. By far - to my eyes - the RGB image with no blocking filter looks the best in terms of blue (these were all 1.5 min subframes). As the strength of the blocking filter increases I see progressively more and more green caste which decreases somewhat when the RGB black points are aligned. However, even after alignment the blocking filter images do not look as nice in blue as the image using no filter. I have gone back and measured adu in the images and conclude that perhaps the Baader image should have been 132 seconds rather than 120 seconds. The GG filters exposure times needs perhaps only a bit more time. Here I am assuming a linear CCD response. In any case it looks with the use of these filters needs at least ca 30% increase in exposure time and the Baader perhaps 40% or more. In the next round of experiments I will increase the exposure time in blue to the calculated amount but also do a series with perhaps double the amount of time. It would be interesting to see if any amount of extra blue exposure time would result in as good an image as seen without the blocking filters.

Comments are most welcome!

Peter

[PRejto]

These are the results after adjusting the black points in RGB.

[gregbradley]

Use a different camera. The KAF8300 cameras have no such issue and your off and running. Some setups simply don't play well together and that's part of the trick of astrophotography. Which items work well with what items.

The 694 chipped camera is great for galaxies, narrowband and small objects but its never going to be a good wide field chip. 11002, 16803 are the kings of that arena and will be for some time to come I predict (no new decent CCDs for years now and none I am aware of on the horizon either). The TEC140 is a premium widefield astrograph not so much a galaxy scope anyway.

With Kodak's ex-imaging division being sold around its unlikely they will produce a new hot CCD as it seems its a struggling business. Sony seems to be hot in development of cameras CMOS sensors but not so much in large CCDs where we need them.

There are thousands of excellent TEC140/11002/16803/8300 images.
Using a filter simply degrades the image and is unlikely to ever work well without compromise. You'd better off selling it (its still a great camera and apparently best suited to fast reflector scopes) and get another KAF8300 camera or an 11002/16803 camera.

Greg.

[PRejto]

Well, Greg, I hear you and I have considered that. But, keep in mind what my goal is here. I have both the TEC180 and the TEC140 on the same mount and my desire is to image simultaneously with both scopes: RGB with the TEC140 at .99 arc sec, and Lum with the TEC180 at .88 arc-sec. Remarkably the FOV with the 8300 chip camera and the TEC180 is only slightly larger than the FOV of the TEC140/Trius combination. I think some of my images done this way have come out OK but I've been concerned about the blurry blue sub frames. Other than this one issue it is working quite well. I've not found a different camera that will give me the resolution or ease in combining images. So, I will persevere by either finding the right exposure/filter combination, or just ignore the issue and process out the aberrations. I do appreciate yor input!

Peter

[gregbradley]

Yes that is convenient. You can match images from different focal lengths with Registar as well as an alternative.

One strategy may be to take shorter subs for RGB stars with the 694 and mask them in on the longer exposure images. See if that works.

Greg.