12 replies to this topic

[Gray]

Took this image today and noticed on the bottom half of the disc it appears off band or missing contrast.  I am wondering if this is because of too much tilt on the DS etalon?  Seems my DS unit is far in the red wing and I have turned it counterclockwise a lot.  Wondering if someone (Marty, Bob, Warren) maybe able to tell me what I need to do!  Thanks for looking.  Best 25/250 frames.  Seeing averaging 2 arc seconds per minute.  Coronado SM3 90mm DS, ASI 1600MM Pro.  Processed with AS!3, IMPPG, Pixinsight, & PS CS2.  

 

[BYoesle]

Hi Graham,

 

It looks to me that you have a number of issues, including possibly using too much tilt of the DS etalon where indeed your image appears to be a little off-band. across the disc.

 

You also may be applying too much contrast or other processing effects. With double stacking you should be able to capture disc and prominence detail with a single exposure. Here's a recent screen shot of the exposure parameters I obtained with my DS SM90's:

 

Click for larger.

 

SM90/90/BF30, ED100/900, FLIR Grasshopper 3 IMX253 mono, 15 fps, FireCapture.

[BYoesle]

Here's the minimally processed image after stacking with AutoStakkert, RegiStax6 wavelets, then a litlle unsharp masking and histogram adjustment, and finally compression in PaintShop Pro to meet the file size limitations of the forum:

 

Click for larger.

 

Full resolution image here.

 

So I would take your time in adjusting the tilt to get the ghost image just barely off the main image for best contrast uniformity, and be fastidious in the tuning to be sure you're exactly on-band. Lastly, ease up on some of the contrast and processing - especially important given the MEADE's SMIII etalon contrast uniformity unfortunately appears less than the original (and much more expensive) SM etalons of yesteryear. 

Edited by BYoesle, 19 January 2021 - 01:01 AM.

[Gray]

Thank you Bob. My tilt was almost if not at the limit for one. Two, I thought we weren't to use gamma during capture, but I will try to match your settings. That probably explains your exposure time twice that of mine. I was using hardware accelerate in FC. I wonder if I should uncheck that option? It's been in the 30s here in the mornings when I tried the last few times. I tried using a dew heater wrapped about the blocking filter. Im thinking that was not the problem. It's got to be in the tilting/tuning of the second SM2 etalon. I did play too much in PS with high pass and unsharp. I knew I had bad data, so I over processed. So the goal is to tune without much tilt, just as you and others have described, slightly off to the side for best uniformity. Im wondering if the SM2 etalon is too far in the red wing. When removing the hard stop pin and rotating CCW to move towards the blue wing, I remember it felt as if I was all the way backed off, but I am not certain now. Need to get a good solid day of sunshine and go back to the drawing board so to speak.
Thank you for responding so quickly.
Oh, and I'm sure your etalons are of much higher quality. I'm stuck with what I have and hope they're reasonable enough to keep me happy for the rest of my days above ground.  Clear skies your way!

Edited by Gray, 19 January 2021 - 08:22 AM.

[rigel123]

What Bob said! It’s OK to use Gamma on, I typically do for all of my animations since I want to do as little processing as necessary to all of those frames. Pushing Gain always introduces noise so keeping that low while suffering a little slower frame rate is a decent compromise. Looking at Bob's screen shot it looks like seeing was pretty good at the time.

I think your idea of “Going back to the drawing board” is a good plan and set up as a single stack to begin with, get your best image that way and then after attaching the DS just move the ghost so it is just off of the real image. I barely adjusted the tilt wheel on my Lunt to move that image off.

It would be interesting to see what your image looks like prior to processing too, just out of AS!3.

[rigel123]

I had posted this view through my DS (60mm) showing how far off the disk I typically move the ghost. In practice I would then move the scope to better center the real image.  Bob moves it even less than this and in the FOV with my ASI174 I have plenty of room for the full disk to move it around to get the best even illumination as possible since this is an internal and external etalon set up compared to Bob's external for both.  You could also try Marty's "Cereal Bag" flat method to help with uneven illumination but I think going through the tuning exercise should help as well.

 

[Gray]

What Bob said! It’s OK to use Gamma on, I typically do for all of my animations since I want to do as little processing as necessary to all of those frames. Pushing Gain always introduces noise so keeping that low while suffering a little slower frame rate is a decent compromise. Looking at Bob's screen shot it looks like seeing was pretty good at the time.

I think your idea of “Going back to the drawing board” is a good plan and set up as a single stack to begin with, get your best image that way and then after attaching the DS just move the ghost so it is just off of the real image. I barely adjusted the tilt wheel on my Lunt to move that image off.

It would be interesting to see what your image looks like prior to processing too, just out of AS!3.

Okay.  I'm on the desktop now and I didn't see Bob's zero gain setting which is pretty neat.  Here is the bad data I was talking about.  Straight out of stacking, no other processing.  You can see a bright but non symmetric ring around the limb. 

During tuning, and I am new, I was trying to get the filaments as dark as possible visually.  That seemed to require shifting the DS unit very far to the blue wing, and I had to tilt almost all the way.  I have previously checked each etalon at a time and actually thought the SM2 etalon might put up a better image than the SM3 but I need to try again to be sure.  Wednesday (1/20/21)  might be the day here.  

Oh, I did use a flatcap flat field device from Daystar and applied it here.

 

Thank you Warren for taking your time to help! 

 

 

 

PS: Thanks again  Bob 

He's like a single malt scotch whiskey, even though I don't drink anymore 

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[rigel123]

That looks like the right side is inside the sweet spot and the left is outside.  I guess first remove your DS unit and return the etalon back to the starting position.  Tune your scope in SS for the best view and then attach the DS unit and see where the ghost images are.  If they are just off of the disk you should have your best view.  If not, then adjust until they are just off and see how that looks.  I know the Coronado etalons work a bit differently than the Lunt Tilt ones so maybe Bob or Marty can jump in as well.  Good luck!  I have clear skies but am nursing a bruised knee so I'll have to rely on you guys to get some shots as it looks like there are some areas of interest by looking at Gong images!!

 

Were there any clouds around at all?  I can also get results like this if some of the images in the stack have clouds moving across the disk, but usually a little sharper than this one shows.

Edited by rigel123, 19 January 2021 - 09:18 AM.

[Gray]

No clouds were around. The Sun was low, around 20 degrees by guessing. But, the same results are if it is noon high. So it's got to be the tilting. My fear is I won't be able to get it on band photographically but I could be wrong. The Coronado etalons are encased like the Lunts so I suppose temperature might not affect them. Idk, will try again tomorrow hopefully. Completely overcast here now.

[rigel123]

No clouds were around. The Sun was low, around 20 degrees by guessing. But, the same results are if it is noon high. So it's got to be the tilting. My fear is I won't be able to get it on band photographically but I could be wrong. The Coronado etalons are encased like the Lunts so I suppose temperature might not affect them. Idk, will try again tomorrow hopefully. Completely overcast here now.

Temperature affects the blocking filter more as I can watch the contrast change over time if I don't warm my BF, but with the Lunt BF Heater I don't get that problem in cold temps.  As long as it is above 40F I typically don't have a problem with the BF so I just use the heater on those mornings it is in the 20's and 30's.

[Gray]

I just hope the scope will be able to get on band. I could've bought a Lunt but I chose otherwise. Hopefully I can get it corrected tomorrow morning. 

[rigel123]

I just hope the scope will be able to get on band. I could've bought a Lunt but I chose otherwise. Hopefully I can get it corrected tomorrow morning. 

I remember when I bought my Lunt almost 9 years ago I went back and forth between Lunt and a Coronado and I almost went with the Coronado for a very scientific reason....I loved the gold scope!!  I went with Lunt at the time because the issues with Meade were starting to get questionable, but I'm pretty certain you should be able to get on band with your scope!

[BYoesle]

Here is the bad data I was talking about.  Straight out of stacking, no other processing.  You can see a bright but non symmetric ring around the limb... My fear is I won't be able to get it on band photographically but I could be wrong... I just hope the scope will be able to get on band.

 

Hi Graham,

 

Unfortunately from what I can see in you image, things currently don't look too great. What I have encountered with a small sample of MEADE Coronado etalons is that they are tuned too high above 656.28 nm. This is likely due to the spacers being too thick and the resulting large etalon gap. While being tuned a bit high is ideal for the secondary (DS) etalon so that tilting it removes both the ghost images and brings the filter on-band, MEADE apparently relies on being able to both tilt and compress the etalon gap to correct this - and apparently believes this gives them leeway in not meeting stricter etalon spacer production tolerances.

 

Ideally the CWL of the etalon should be just a bit high so that minimal tilting will bring the etalon on-band. This is a critical feature for tilt-tuned etalons and requires a good deal of time and attention. If the spacers are too thick, the etalon CWL will be too high, and excessive tilt will be needed to get on band (if it can get on-band). Therefore "banding" likely will be present as sections of the etalon will be off-band with respect to the tilt axis portions in which the acceptance angle of the filter is exceeded. This appears to be what is going on the left versus right side of your image.

 

The same thing applies to RichView mechanical pressure tuning. Again ideally the filter CWL is just slightly high, and a small amount of pressure would decrease the etalon gap and bring the filter on-band. Additionally, this pressure would need to be very precisely and evenly applied to both the center and peripheral spacers to maintain a parallel etalon gap. From the two RV etalons I have examined, only the center spacer has pressure applied, and because the etalon is tuned high, a greater amount of pressure is needed to get on-band and significantly deforms the etalon plate. This results in the unevenness of the etalon gap, and therefore differing CWL's across the etalon, and widening of bandpass when the center of the filter is tuned to be on-band:

 

 

Left: excess tilt tuning. Right: excess tilt and RichView tuning. Greatly exaggerated for clarity.

 

The combination of these two issues results in a filter that presents with banding and a broadening of the bandpass, leading to contrast variations across the image and generally having poor overall contrast. See here for additional analysis.

 

Getting back to "square one:" You should evaluate each etalon individually to see which one requires the least amount of tilt to be on-band. Hopefully the tilt required for this etalon will produce little if any banding. I would hope little if any RichView compression tuning will be needed. This etalon should be designated as the primary etalon. The remaining etalon will be your DS etalon, and should only be tilted enough to remove ghost reflections off the primary image. Thereafter, use the RichView tuning to get the best contrast. I believe in this way you'll have the best compromise with minimal banding and bandpass widening. However, flats will likely be required to achieve the best imaging results as far as contrast variation.

Edited by BYoesle, 20 January 2021 - 07:09 PM.