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I think that, in this analysis, you are looking at raw data and not at data as is it will be, or can be, processed for a final image. My capture procedure is based on what I will do with the data in processing, not on its relationship with raw signal coming from the sky. I do not want equal SNR in all channels. If my data were to remain unprocessed then, yes, I might want that - but my data will be processed. If I expose for faint tidal tails, rarely seen, in luminance then my stars in luminance will be over exposed. This is not a problem because I will not use luminance on my stars, I will use RGB only for stars. If galaxy cores are, in the same way, over-exposed in luminance, I won't use them, I will use the RGB-only cores. One of my fundamental principles in imaging is to ask myself, What am I going to do with this layer?  The answer to this question determines how I will shoot the layer. Olly

An adjustable Lego frame. 

I've been adjusting focus and colour balance, through I'm not using the camera in a typical location but through a lean-2 roof with very limited sky line. The camera picks up IR and I'm not using a block so that does effect the resulting images. I'll see if today's focus tweak has helped tonight.

It seems simple but is actually a difficult question, not because of the math involved but because of the accuracy of the parameters that have to be used in the math. In order to get an accurate exposure time for a given SNR, the most important factor is the sky brightness. Many professional observatories have tables relating the sky counts to moon illumination in different filters. To further complicate things, sky brightness changes during the night and the proximity of the object being imaged to the moon is also important. I started making tables relating the sky rate to moon illumination, and in makes a substantial difference than just relying on a ballpark sky mag chart. From what I found if you want to have equal SNR in all channels, you will have to expose more for RGB than L, contrary to what people are used too. A ratio for 1:3:3:3 is a good starting point (L:R:G:B)   If someone is interested on how to get the sky counts from your images during a night session, the basics steps are simple: Calibrate the subs using darks and bias (flat is optional) Ideally mask the sources in your image (stars, galaxies) Compute the median value of the background  Optionally you can get sky magnitude from counts, but not really needed for anything since we already have the sky count rate Do this for every frame acquired during the night, then take a median value (you will notice the sky rate changes during the night) I suppose the steps can be done in Pixinsight, maybe involving pixel math, but I work in python Here are the main steps in python: def calibrate_image(raw_frame, master_bias, master_dark): """ Preprocess a raw image frame by applying bias and dark corrections. Parameters: ----------- raw_frame : array-like The raw image data. master_bias : array-like The master bias frame. master_dark : array-like The master dark frame. Returns: -------- corrected_frame : array-like The preprocessed image data. """ # Bias correction bias_corrected = raw_frame - master_bias # Dark correction corrected_frame = bias_corrected - master_dark return corrected_frame def sky_brightness(sky_rate_electrons, gain, exposure_time, zero_point=27.095, image_scale=0.92): sky_rate_adu = sky_rate_electrons / gain total_sky_counts_adu = sky_rate_adu * exposure_time mag = -2.5 * np.log10(total_sky_counts_adu) + zero_point pixel_area_arcsec2 = image_scale ** 2 surface_brightness = mag + 2.5 * np.log10(pixel_area_arcsec2) return surface_brightness def calculate_sky_electron_rate(corrected_frame, exposure_time, gain): bkg_estimator = MedianBackground() bkg = Background2D(corrected_frame, (50, 50), filter_size=(3, 3), bkg_estimator=bkg_estimator) median_sky_level = bkg.background_median sky_electron_level = median_sky_level * gain sky_electron_rate = sky_electron_level / exposure_time return sky_electron_rate    

There will be six lengths of studding equally spaced through the outer holes. The centre and three of the inner set, again equally spaced, will bolt to the mount. The extras allow the whole plate to be indexed round one position (30 degrees) and offset the mount by plus or minus one degree. This will make polar aligning the studding in the concrete less critical. There's method in my madness - allegedly.