symmetal

Another great image Göran. I've just enabled the LBN catalogue in Stellarium and loads of new targets arrived for RGB imaging. 😊

Might I asked what made you choose 3m exposures. I found that with the RASA 8 and ASI2600MC and no filter, the sky background noise will swamp the read noise by a factor of 5 in Bortle 3 skies in around 30s. I've been using 60s exposures just to avoid too many subs and hopefully not saturate too many stars so the colours can be resolved.

At gain 100 and offset 50 the sky background ADU is 717 to swamp the read noise by 5. 🙂

Alan

No harm in trying an argon purge I imagine. Is that from the aerosol cans used in wine preserving? It's heavier than air so should stay around the sensor while the cover's replaced.

What's it like if you cool to 0C instead of -10C. The difference in dark current between 0 and -10 is fairly small.

Alan

9 minutes ago, Rustang said:

It looks ok,  set to surface, no sliders changed and I believe the remember box was un ticket. The whole sun was cover with the alighment squares. 

What should the average solar file be in size then? 

If you captured the full frame size of your camera then the stacked file should be 1920 x 1200 x 2 bytes = 4.6MB. AS creates a 16 bit stacked image hence the x2

If you used a ROI then it's  ROI width x ROI height x 2 bytes in size.

There will be extra bytes added to the file for the file header but the above figures are close enough. 🙂

Alan

What does the image itself look like. AS will only save files in an uncompressed format.

Did you select 'Surface' and not 'Planet' on the Image Stabilization option. 

Did you change the 'Image Size' sliders at the top left of the preview window from what they were when the file was loaded and is the 'remember' box unticked.

When you placed the AP grid was the whole sun surface covered by the alignment squares.

Alan

I forgot to mention that you may as well use 8-bit capture and enable the 'high speed' option if it's there, in the camera setup panel. This will get the highest framerate. 'High speed' uses less bits for conversion like 10 or 12 bits which you might as well use if you're saving in 8-bit.

Stacking at least 100 8-bit frames will gain you around 4 extra bits of image depth so will be equivalent to a 12-bit image. This is only if noise is present in the individual frames, which there will be. Here's the theory behind this by Craig Stark The Effect of Stacking on Bit Depth, which is an interesting read. 🙂

Alan

The fan coming on isn't an issue. It may turn off by itself later when no cooling is requested.

Camera gain and offset settings should be available in Firecapture itself. There may be options like camera dew heater on/off in the ascom driver which you could turn off but no problem if left on.

Firecapture will detect it's a mono camera so options like R and B colour gain and debayer options will not be available anyway.

Frame rate is set automatically to be as high as possible. Frame rate will be limited by the processing power of the computer running Firecapture and the camera itself, but is initially  determined by the exposure duration. How many exposures will fit in 1 second. It's the reciprocal of the exposure. An exposure of say 10mS or 0.01 seconds gives a maximum frame rate of 1/0.01 fps which is 100 fps. A 5mS exposure gives a maximum of 200fps.

You won't achieve these maximum framerates if the full sensor is used as the camera can't process the full frame data fast enough. A smaller ROI will enable higher framerates to be achieved, up to the exposure determined maximum, though the camera may max out at 250fps or similar where an even smaller ROI doesn't help.

It's only the height of the ROI that determines the frame rate available, so the ROI width can be left at maximum if you wish, for no loss in fps.

Don't enable gamma when capturing the video, leave it unticked or at 50 which is off anyway. Gamma processing each frame takes considerable processing time so your frame rate will suffer. You can easily gamma adjust the final image later in PS or whatever you use. I've found it useful to set the gamma to 0 in Firecapture when previewing the image as it enables focus to be determined more easily as the image has more contrast but turn off the gamma before starting a capture.

Aim for an exposure around 5mS if possible, and adjust the camera gain to get the histogram maximum around 70% of full. Having a very high camera gain is not a problem and will benefit from lower read noise. Although the preview may look very noisy, once several thousand frames are stacked in post processing this noise will disappear.

Check that the histogram peak corresponding to the 'black' sky background is not butted up against the left edge of the histogram and that the left side of the peak is just visible. Set the camera gain first, and then the camera offset to set the 'black' background peak position on the histogram.

Alan

Had an hour of clear skies before the clouds appeared, so could test the replacement RASA 8. Unfortunately, it produces very similar results to the first RASA 8, though the small area of no flaring is more centred on the replacement. The two dark 'spikes' to the right on each image are an artifact of the semi-circular cable router.

I thought that not having an UV-IR cut filter in the path might be an issue as the RASA spec says it focuses wavelengths between 400 and 800nm but doesn't specify the actual pass band width. However the ASI2600MC protect window is an UV-IR cut filter, passing 400 - 690nm so that isn't an issue. The 2" filter holders for the RASA and 17.5mm back focus cameras aren't in stock at FLO at the moment for me to use an external filter yet.

If possible could @ollypenrice, @gorann or @tomato take a similar wideband image of Deneb to have a direct comparison please, as I'd like to see what a 'good' RASA is like. 😊

At the moment, if I have a bright star in the image I'll have to put an aperture mask on and increase the exposure accordingly. I'm impressed with the lower brightness star shapes and sizes over the full APS-C frame on both RASAs. The first one I had to use a 0.5mm metal spacer to get round stars in the corners, though the second one is good without extra spacers. Getting the camera tilt set up on the test jig beforehand was essential though, as both RASAs themselves were tilt free. 🙂

Alan 

6 hours ago, Stuart1971 said:

🤔 Strange you flattener requires it to be the other way round, but the filter could be swapped around in the cell, by removing the locking ring and flipping over, not a difficult job as long as the locking ring is not too tight

The Z73A F/F, which I assume ninjageezer has, has the filter mounted on the rear of the flattener like this so is the 'wrong' way round. It also prevents the adjustable ring from reaching zero if fitted, and adjustment is then from around 5 to 15mm.

The newer, and more expensive Z73R F/F intended for larger sensors does have the filter screwing into the front of the flattener after the camera rotator, so is the 'right' way round and doesn't affect the adjustment range. 🙂

Alan 

Filters where the orientation is important should have an arrow or caret mark on the rim which should point towards the light source. If there is no mark it implies that the orientation doesn't matter.

For filters where it does matter then the most reflective side of the filter, which is the side the caret mark points to, is the side that should be pointing towards the light source. This is to minimize potential reflections between the filter surface and the camera sensor which could have an adverse affect on the image.

The reason the F/F states that the filter should be bi-directional is that the filter has to be mounted the opposite way around in the rear of the F/F, compared to the usual filter mounting method.

Alan

FLO have to date not heard anything back from Celestron, so have kindly offered to exchange my RASA 8 for a new one they had in stock. 🤗 Returned mine last Thursday and new one is due tomorrow. Up until yesterday Clear Outside had three clear moonless nights from tomorrow. Needless to say all three have now gone yellow/red. 😟

Fingers crossed the new one is a good one, and I get a chance to confirm that in the next few days. 🔭

Alan

The anti-dew ring for the camera is to prevent condensation on the front surface of the camera protection glass only and won't have any real effect on dew on the sensor.

If the camera sensor chamber isn't airtight as seems the case on my 071 then it will over a few days or maybe weeks, end up at the average daily RH of the outside air. During the night as the temperature drops some moisture may condense out even if not cooled.

Keeping the camera in an airtight container with large desiccant bag until just before being used means the sensor and tablets are around 25% RH so can then be left on the scope for several days without problems, defore going back in the container. The chance of more than a few days of imaging time in a row being rare this works for me. 🙂

It means the camera doesn't have to be opened to change the tablets, which wasn't successful for me on the icing, and end up getting dust on the sensor as happened when I tried it. The DSLR sensor cleaning kits with the one use pads on a stick work very well though.

Alan

44 minutes ago, Adam J said:

My guess is no simply because frost and consideration normally form dead centre of the chip and that is offset to the side. I think my first move would be to give all optical surfaces a good clean. But to be honest the definitive way to tell is really easy, just take a flat with the cooling turned off. 

Anthony has posted a flat with no cooling and the patch is not present. My ASI071 ices up from one edge of the frame initially and works it way across.

38 minutes ago, Adam J said:

ZWO state that poet on the side is not for customer use. Also you can't dry desiccant out like that, it needs to be removed and heated in an oven. 

The instructions for the original ASI1600 was to put two desiccant tablets in the port adapter to dry out the camera internals. This was actually ineffective as two dry tablets will do little to dry out the four internal saturated ones and they will all equalize to around 75% saturation each at best and also take a long time. This is probably why Zwo don't include the port adapter anymore and have said not to use it.

Silica gel is a moisture buffer. It will absorb moisture from moist air and release moisture to dry air until an equilibrium is reached. I've put saturated small silica gel sachets in a sealed container with a large bag of 'dry' silica gel and a RH meter indicating 20%. After a few days the sachets change colour and indicate recharged and the RH has increased to around 22% or so. 🙂

Putting the camera in the container it would likely take a week or more for the internal tablets to reach equilibrium as the access port area is small.

Heating them just forces the water to be released quicker as the water molecules will have more energy and the RH of the surroundings is very low, as long as there is adequate ventilation to let the released water vapour escape. Microwaves aren't so good at ventilating the water vapour I've found, as the inside is left wet and the silica gel pack very hot and moist, unless it's left in well beyond the recommended 'cooking' time.

Alan

Yes, that looks like dew on the sensor as the spots are in focus. Possibly ice, though I've found the ice generally forms streaky patterns rather than distinct spots. My ASI071 is very prone to ice unless I store it when not in use in an airtight container with a large desiccant bag, and the access port on the side of the camera open, to allow the internal desiccants to dry out.

Alan

Frost or dew on the sensor is pretty much in focus as it's only the sensor cover slip thickness away from the pixels. Flat frames will display it clearly if it's present.

Alan

@ONIKKINEN Thanks for your info on the ToupTek version cameras. I wasn't aware they were that different. I'll do the Sharpcap analysis on the 2600.

Can you turn off the HCG mode on your camera at any gain setting then? This seems contrary to what I read about the sensor which I admit were more reviews rather than actual facts. With HCG off, gain below 100 on the Zwo) the read noise is over 3 electrons. The low noise option you have seems to be taking a dark frame for subtraction after each light, like some DSLRs can do, which as you say isn't what you want if you're calibrating anyway.

Even at 0.9 electrons read noise I wouldn't be able to swamp it unless I exposed for over an hour at Ha. What sky background ADU values do you get for your usual narrowband exposures?

Alan

Your graphs are a bit confusing pipnina. There are no units on the vertical scale, and the gain in e-/ADU has no actual values. Here's the graphs for the ASI2600 (same sensor) below.

Your read noise is quoted below 1.0 for all your gain ranges and could be assumed to be in electrons (e-) but this seems a rather low figure. The ASI2600 read noise varies from around 1 to 1.5 electrons once the HCG mode is enabled. This implies that HCG mode is on all the time for your camera and so implies your gain is 10dB (100 in ZWO 0.1dB gain units) when your gain is set to 100 linear units (top row) which is really no gain (ie. a linear gain of 1) which has been called 0.0 dB (bottom row), incorrectly I believe. The IMX571 data sheet quotes gain can be set between 0 and 37dB and the HCG mode comes on at gain 10dB and above.

Your full well of around 11,000 at quoted 0.0dB gain coresponds to the full well of the 2600 at actual gain 10dB, which again implies the HCG mode is on all the time on your camera.

Your e-/ADU gain graph has no value stated, and at your 0.0dB gain setting (really 10.0dB) is actually around 0.25 e-/ADU on the 2600.

Using the ASI2600 figures for your minimum gain setting of 100 or 0.0dB which I assume you are using.

Sky background ADU = (read_noise * swamp_factor) ^ 2 / gain + camera bias ADU

= (1.45 * 5) ^ 2 / 0.245 + bias ADU

= 214 ADU + bias ADU.

The bias ADU is required as that's added to every frame you take and more importantly it includes the camera offset value used. You'll find for images at low camera gain the offset contributes most of the ADU value in your image. Just take the mean ADU value of a bias frame or your master bias if you have one as your bias ADU value. My ASI2600 offset is set at 50 and it adds 10ADU per offset unit. My bias ADU is actually 503 ADU which is 500 ADU from the offset and 3 ADU from the camera electronics, so my Sky background ADU value is 214 + 503 = 717 ADU.

It's easier to have the bias added on as that's the value displayed on any capture software when you hover your mouse over the image, and you can quickly read the Sky background value.

@ONIKKINEN has quoted the value before the offset (bias) is added on and has used different values interpreted from your graphs which may be more correct. 😉 Use whichever you want as a guide. 😊

Hope this helps pipnina.

Alan

I expose to the point where the sky background noise swamps the camera read noise by a specified factor, and so making the read noise contribution of the total noise in the image insignificant. At this point I start another exposure as exposing beyond this duration has no advantage in S/N and just overexposes the stars more. I used to swamp the read noise by a factor of 3 but will use a factor of 5 in future to reduce the number of subs taken. Advice from vlaiv taken here. 🙂 At factor 3 the read noise contribution to the image is fairly insignificant while at factor 5 it's very insignificant and can be ignored.

It depends on your sky darkness but for read noise swamping by a factor 3, I expose 1 min for L and 3min for RG and B with the ASI6200 and ASI2600 with an f6.3 scope. However for narrowband imaging I would need very long exposures to significantly swamp the read noise. I'd need around 90 mins at Ha and over 3 hours for OIII and SII. This is in bortle 3 skies. If you have more light pollution that these optimum exposure values will be lower.

So, unless you have high light pollution, for narrowband, it's still beneficial to expose for as long as you are able, before mount or guiding issues, or the chance of a plane etc. flying through ruining the image becomes too great. I therefore use 900s exposures for narrowband. Well below the optimum duration for adequate read noise swamping, so read noise is still an issue for narrowband imaging, even with the latest HCG low read noise cameras.

I don't know what sensor your camera has pipnina, as RisingCam's AliExpress page doesn't seem to give model numbers, just specs. If you have graphs of Read Noise and gain in e-/ADU as ZWO and others provide I can give you a sky background ADU value to aim for, for the above Read Noise swamping factors. If not, just specifying the sensor used, I can provide values for Zwo cameras with the same sensor which should be close. 😀

Alan

I've used Baader fluid with microfibre cloth on mine. It made no difference on my flaring issue though. 😉 When adjusting the camera orientation, I've sometimes brushed the surface of the corrector plate with a knuckle, leaving a smear requiring the baader fluid treatment. 🙂

Alan

@ollypenrice @gorann @tomato Thank's for your help and comments.

I sent all my previous test images to FLO earlier in the week and they replied they will share them with Celestron and work out a plan of action. FLO said I'm the only one who has reported an issue with the RASA 8 to them.

I find it strange that on the CN thread, Celestron are still saying no replacement mirrors are available to properly repair those sent back to them, but RASA 8 scopes are still available for sale. I assume they know which mirror manufacturer or batch caused the problem scopes and that they aren't still selling them without checking them for this issue first.

Putting an aperture mask just in front of the mirror and losing a couple of mm or so, I thought would be a better fix by Celestron on the problem mirrors, and would save having to use a larger mask further away like I've had to do.

For those willing to accept 180mm aperture or so, Celestron could offer to refund a portion of the cost of the scope.

I'll send the latest tests to FLO as well and await their reply. Hopefully some time next week.

Alan

Here you go @ollypenrice

Again full Moon and I used Deneb as the subject for 10s exposures and screen grabs of a standard stretch in SGP for the images. I think this rules out the camera as having an effect on the flares. The dark cutouts of the flares seem predominantly on the right for the first composite and the bottom for the second composite are effects likely effects due to the cable routing. 

Here's the best spot in the image for minimum flaring. Moving slightly from this spot will give the asymmetric flares.

Alan

7 hours ago, ollypenrice said:

It might be worth finding a flaring star, imaging it, rotating the camera and imaging it again,

just in case the flare is produced outside the telescope. Does the flare follow the rotation or not?

Olly

I'll give that a go tonight Olly. I've tried it with and without the dewshield I made and there was no change in the flaring. I was actually surprised that with the full Moon yesterday around 70 degrees away there was really no significant difference in the look of the image at all.

Alan

4 hours ago, gorann said:

When you ask for it I realize, like @ollypenrice that I have not aimed my RASAs at any object s with very bright stars, but in any case, like Olly, I have not seen anything like the star anomalies you have so something must be very wrong with the scope, and apparently Celestron admits it.

I hope you get it sorted in one way or another because a well-functioning RASA8 is just a worm hole into deep imaging!

Thanks Göran. Actually medium brightness stars do show the same flaring effect though at a much lower level of course. I took some images of Sadr today with a U shaped cable router I printed to compare with the image @tomato posted and the asymmetric flaring is still present on mine. It was a full Moon though so the darker parts of the flare were washed out and hidden. Mine was 10s RGB while tomato's was 180s narrowband so isn't really a fair comparison. I gave both a similar quick stretch and set the background level to be the same. Here's the result. The dimmer stars in the lower image are totally washed out by the Moon in the top one

My RASA 8, 10s RGB at full Moon, cropped

tomato's RASA 8, 180s NB, cropped

Sadr was in a similar position in the frame for both images. Actually once you place the bright star outside the good area with no flare, which at full aperture I found is a small area about 400 pixels across, just below the centre of the frame, the flare suddenly appears and doesn't change a lot as you place the star closer to the edge of the frame.

Alan

8 hours ago, ollypenrice said:

We do get rather hard-edged but circular disk-like halos on some stars but what you have are highly asymmetrical flares and those we don't get.

To my mind, BTW, StarXterminator is a game-changer for the RASA, which goes gloriously deep but gives indifferent stars. (Certainly not refractor standard.) However, my present method, still under experimentation, is to give the original image a partial stretch, save it, de-star it and give the de-starred image a little more stretching and cosmetic fixing of artifacts around where the larger stars were. I then put the still starry version as a layer on top in blend mode screen or blend mode lighten, combine them and go for the final stretch. In a nutshell the stars get a lower stretch than the rest and can be made to look pretty decent.

Without StarX I'd be considerably less enthusiastic about the RASA. With it, I'm a happy bunny.

Olly

Thanks for the starless image tips Olly. I've tinkered with Starnet++ but when I add the stars back in it looks like an image which has had the stars added back in. 😁 I must try harder.

Alan

9 minutes ago, tomato said:

Attached is an image of Sadr (180 sec exposure)taken with my RASA8/QHY268c/NBZ combination, the scope was purchased from FLO in August 2020, the cables were in a circular arrangement.

Thanks tomato. I'd be pretty happy with that. 🙂 I'll take some images of Sadr tonight without a mask for some comparisons. My test images used Vega and Deneb which are a bit extreme I admit.

I think you'll find that the darker notches at top and bottom of Sadr are effects of the cable routing arrangement but they're relatively minor.

Alan

Thanks @ollypenrice. I've experimented with different shape 3D printed cable guides and while they remove the typical spikes they seem to cut little notches out of the stars and/or add extra little spiky flares. This might be due to the edge flare issue I currently have affecting it, and have found that the straight cross cable guide gives the roundest bright stars at the moment so used that. There are some halos visible when fully stretched and processed, but they are at least even around the stars so don't stick out like the flares. 🙂

Got several clear days forecast at the moment, though full moon of course, so can experiment more. I have noticed that the moon will also cause extra small spikey flares even with a dew shield and pointing over 90 degrees away. I found that out when testing my latest aperture mask until the moon set when it came good. The large edge flare effects are there though, with or without the moon.

Alan