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First light with QHY8L (in Ha)


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Hello ladies and gentlemen,

Last night I put my newly acquired QHY8L to the test. It's also a first light for my flocked tube, I flocked the tube, the inside of the focuser, around the primary mirror and also the primary mirror clips and the secondary mirror edges.

As I don't have a light pollution filter yet (only Ha and OIII) and I could see Vega rising above my roofline, I figured that I should wait an hour and try for the star Sadr and the nebulosity that surrounds it.

I took 5 x 5 minutes using EZCAP. Cooling failed on the third (don't know why?) and as a result the 4th and 5th frames had unacceptably high noise. The first three frames were cooled to -15 and I have subtracted 5 dark frames.

post-18683-133877607437_thumb.jpg

My question is - what is causing the horizontal lines and how can I possibly be seeing an out of focus light source [star?] in the frame?

After Sadr had moved on and I could see Cygnus, I moved straight to the North American Nebula as it was an area of sky that had always interested me.

Since I had to restart the cooling (at -15), I found that it overshot a little (to -40 !!!), so I set the cooling to -35 and it stabilised there for this shot:

post-18683-133877607445_thumb.jpg

Two hours on the North American Nebula. 12 x 5 minutes (with darks) and 6 x 10 minutes (no darks).

No horizontal banding and no out of focus stars.

Now, I notice that there is some strange coma to the edges of the frames. This, I later found was due to an improperly seated camera. It was slightly skewy as I had attached the screws to the center of the coma corrector in order to get focus.

Tonight I have found an extension ring that came with the QHY8L and using that, I'm able to comfortably get focus with the camera nicely seated.

Tonight's images should be better than last nights.

First impressions? This camera does the business. It is more sensitive than a DSLR (I chose Sadr also as I have comparable 1000D frames). I'd say about twice as much. Plus, the noise is remarkably low (to the point where one can use the levels tool to get a quick and dirty image out without careful curves).

Both of these images have had a five minute process. I'm sure that I can do better.

Also note that this is a OSC camera and only 1/4 of the pixels (APS-C sized sensor) have been used. I also note that other than hot pixels, the green and blue channels appear to be silent which is frankly amazing since imaging Ha with a DSLR gave me green and blue noise at least a third of the way up the histogram.

Edited by MikeWilson
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On the first image could the out of focus star be a reflection?....the bright star is near dead centre and the other blob looks like an image of the mirror, could this then be a reflection between the filter and corrector?

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On the first image could the out of focus star be a reflection?....the bright star is near dead centre and the other blob looks like an image of the mirror, could this then be a reflection between the filter and corrector?

I don't think it is (it's not on the other image).

When I flocked the tube I did notice a little bit of damage on the secondary mirror. I didn't notice it before, it's very minor and only on the edge. I wonder if this is the cause?

Mike

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Thanks Martin,

I'm doing that right now and I have to say that I'm a little worried.

All of my images (even the new ones) are riddled with exceptional amounts of coma on each star. If you look very closely, you'll see that each star is accompanied by an out of focus doughnut.

I suspect that this might be caused by the very tiny amount of damage on the secondary mirror (a couple of mm wide there's a spot that is refracting light differently to the rest of the secondary).

I don't know if the damage has always been there, but it's the only lead I've got at the moment unless anyone can suggest something else that it might be? This is the first light after flocking my telescope so it's possible (no matter how careful I was) that something might have come into contact with the secondary mirror and caused this slight damage to the coating?

I have a simple, but slightly destructive test. I have bought some blackboard paint from eBay and I'll blot out the tiny damaged bit and see if this results in the problem going away.

Otherwise in the meantime, the images are ok if I resample them right down to a smaller size :)

Clear skies,

Mike

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The out of focus star is odd - could it be a spacing issue between coma corrector and the camera chip?

Camera looks like it does the business, the NA nebula pic is super.

Try mixing in some bias frames too, my atik tends to band a little down the right hand side & using bias frames sorts this out.

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Thanks for the suggestions John,

Now that you mention the coma corrector and I think about the directions the coma is taking, it looks somewhat like the coma corrector could be struggling.

It doesn't account for the wierd out of focus star effect though. Let me grab a few more pictures.

And I'll get some bias frames in. What sort of exposure times are you using for bias? 0.1s?

The camera itself seems to be behaving, so that's one less thing to worry about :)

Best,

Mike

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Bias frames should be the minimum exposure time you can take on the camera.

The spacing on the Baader MPCC needs to be 56mm from the glass to the chip of your camera. They work out of the box with a T-ring & DSLR but will need a different spacer dependant on your chip depth in the body of the camera.

That could be the cause of the odd reflections.

Edited by johnrt
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I'm testing right now on M57 and here's what I'm getting:

Center (looks spot on):

post-18683-133877607453_thumb.png

Bottom left (a little higher than bottom left. Clear coma and out of focus light):

post-18683-133877607472_thumb.png

Top right:

post-18683-133877607509_thumb.png

Any ideas? I could remove the CCD and test the remainder of the optical train with a DSLR (this would tell me if secondary mirror damage was the cause)? Or I could just throw a bag over it, grab a glass of wine and relax for half an hour before bed. :)

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Bias frames should be the minimum exposure time you can take on the camera.

The spacing on the Baader MPCC needs to be 56mm from the glass to the chip of your camera. They work out of the box with a T-ring & DSLR but will need a different spacer dependant on your chip depth in the body of the camera.

That could be the cause of the odd reflections.

Thanks John,

I didn't know about the 56mm spacing... I suspect therefore that the spacing is certainly not 56mm. I've added a few extra mm since last night's images (posted earlier) and this M57 test.

I'll attempt to get the distance between the sensor and the MPCC identified.

Best,

Mike

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Thanks Martin,

Do you happen to have the dimensions for the Baader MPCC?

I've just been outside setting up an old school DSLR imaging run, but a Ha filter on an unmodded camera, plus clouds rolling in - I think I may be defeated tonight.

However, this 60 second sub of Vega @ ISO 1600 shows little evidence of reflections (focus was quick and dirty).

post-18683-133877607546_thumb.png

My money is on an MPCC spacing fault as John suggested.

Best,

Mike

Edited by MikeWilson
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I think this is the 'tile adjustment ring'?

Thanks for the measurements - I will check out the usual suspects for a suitable t-spacer.

I also have a Brightstar 2" filter wheel coming in the post too. I will check the spacing for that to see which side of the coma corrector it should live (probably closest to the focuser where I have a large 2" extender).

Best,

Mike

post-18683-133877607555_thumb.jpg

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Thanks John,

I owe you a beer (or a coffee!). That's two, now :)

Should I account for the MPCC glass to rear t-thread distance as part of the 56mm figure, or is the 56mm inclusive of the MPCC threads?

I think that it's safe to say that I'm way out at the moment with just 25mm +/- 5mm to the Coma Corrector!

Best,

Mike

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I would put the filter wheel in between then MPCC & chip - you can then just minus the width of the filter wheel from the spacer you need.

I think you need to go from the glass, but +/- 1mm or so is OK.

I think this is the 'tile adjustment ring'?

Thanks for the measurements - I will check out the usual suspects for a suitable t-spacer.

I also have a Brightstar 2" filter wheel coming in the post too. I will check the spacing for that to see which side of the coma corrector it should live (probably closest to the focuser where I have a large 2" extender).

Best,

Mike

Edited by johnrt
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This is the inside of the sensor side of the MPCC. I estimate the distance from glass to tip of the threads to be 9mm.

The outer t-thread part itself is about 3-4mm wide but I suppose this doesn't matter since it won't affect the spacing?

Also: good idea about the filter wheel. It could give me more space to play with on the focuser side, too.

Clear skies,

Mike

post-18683-133877607559_thumb.jpg

Edited by MikeWilson
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I think this is the 'tile adjustment ring'?

Thanks for the measurements - I will check out the usual suspects for a suitable t-spacer.

I also have a Brightstar 2" filter wheel coming in the post too. I will check the spacing for that to see which side of the coma corrector it should live (probably closest to the focuser where I have a large 2" extender).

Best,

Mike

Forget exactly where to measure from - lots of stuff is designed to attach to DSLRs and the distance to the DSLR chip is 45mm plus another 10mm for the T-ring. Hence if the MPCC screws straight onto your cannon, the total distance is 55mm. If you are going to attach anything else instad of a DSLR, that needs to be 55mm too.

Filters go as close to the chip as possible, so attach directly to the camera (or in your case, you can attach to the adjustment ring). I think the Brightstar is 21mm thick, but you need to check for your specific wheel.

Camera CCD distance 20mm

Adjustment ring 3mm

Filter wheel 21mm

Filter -1mm (-1mm effect for every 3mm thickness of glass)

That gives you a total of 43mm with ring, 40 without.

You need 55mm (+/- 1mm), so get spacers of 12mm or 15mm. Fortunately 15mm in T-thread is very easy to come by.

You can see that it would be very difficult to get an off-axis guider or active optics setup in there (it would need to be less than 15mm deep).

Edited by dmahon
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Thanks!!

Good point about not needing to know specs of the MPCC as we just need to replicate the spacing between it's threads and the DSLR chip.

I've got 1 x 15mm, 2 x 10mm and 1 x 6mm and a filter wheel coming. Hopefully I should be able to cobble something together there.

Thanks again,

Mike

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  • 2 weeks later...

excuse me, i need some help with qhy8L

If out temperature is about 19 Celsius what should be a temperature of a camera?

And what gain and offsett do you set in common?

my sky is with a big LP and screen is almost white in a 2 minutes exposure. Any advice please except take filter... ;D can I avoid some LP at my f-6.3?

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Hello Kruska,

"excuse me, i need some help with qhy8L"

Sure :-)

"If out temperature is about 19 Celsius what should be a temperature of a camera?"

The camera has set point cooling, so whatever temperature you'd like up to -35 from ambient. I find the camera reaches -30 (with +10 ambient) easily, although I set it at -15 or -25 depending if I'm running on battery or home respectively.

"And what gain and offsett do you set in common?"

Offset 120

Gain 0

"my sky is with a big LP and screen is almost white in a 2 minutes exposure. Any advice please except take filter... ;D can I avoid some LP at my f-6.3?"

Use a filter :-)

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@Mike. I am using a Skywatcher Field Flattener on my 'frac, which was designed for DSLR. The spacing required is 55mm from the shoulder of the FF to the plane of the imaging chip. On the QHY8L, I sourced 2 x 17mm extension tubes from Bern @ Modern Astronomy. The tubes are made by Brightstar, part number SKU1888.

Be careful using the tile spacer adapter that comes with the camera. It is designed to allow you to tilt the camera in case the sensor is not perfectly square to the cone of light (the Sony chips have a manufacturer's tolerance of 0.25mm). The adapter has a tiny grub screw that you can screw in to tilt the camera body. make sure that it is totally screwed out, otherwise you will tilt the sensor, which might give you coma or elongated stars at the edges of the image.

@Kruska: Here is a link to the QHY forums, where to set the gain offset "First off all start tuning using gain = 0 and offset about 120 and let cool down and stabilize the CCD at the required temperature (for instance -30°). Then activate the "noise analysis" feature in EZCAP and take a short dark exposure with sensor (i.e. an "offset/bias" of 1s or shorter) with a cap on the lens to avoid light exposing the CCD. Then check the RMS value for each analyzed field. It should be between 400 and 1200 but the most important is the min value for each field, it should be higher than 200. (should never be 0, the offset tuning goal is to avoid the null value). -> then if necessary, adjust the offset value to get a RMS between 600 and 1000 for instance (I got 900 to 1000 with the QHY10), the min value should be well above 200.

Then you can adjust a little bit the gain to get the full range: try an 1min exposure (remove the cap of the lens) on a bright light (a bulb for instance): the target is to get 65535 max value and RMS value close or equal 65535 indicating that a full pixel correspond to max value of the 16 bits ADC."

@Mike: I started off with the default 120,0 settings. After using the method above, I ended up with 125,0. It might be worth having a play.

HTH

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Gentlemen,

I think it worked. Thank you all for your advice.

For now I'm using a 35mm of spacers + 20mm CCD depth = 55mm total, plus the filter. It's still not quite right but I think I'm just 1mm out one way or another.

Suffice to say, it worked though - a very quick and dirty process of the Garnet Star Nebula in Ha shows the spacing problem is now resolved :)

post-18683-133877611313_thumb.jpg

@Zakalwe: Thanks for the tip about the adjustment ring. I had no idea what the extra holes for grub screws were for. I don't think I'll need to use it, I'll just keep it in a box for now before I use it and introduce more problems!

Edited by MikeWilson
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