Pompey Monkey

"Turn that bloody light off!"

43 minutes ago, ollypenrice said:

I experimented with this in a previous discussion with Alacant over the histogram and evidence of black clipping. I showed that a screen grab of his image (not this one) with its Levels histogram in Ps showed black clipping. But did this arise from the screen grab itself? I then took a screen grab of an image of my own, posted on here, opened it in Ps and looked at its histogram. There was no difference between the histogram of the screen grab and that of the original image. (I posted the evidence.) So for the purposes of looking at the histo related to black point I think the screen grab is roughly OK. It certainly won't work for all tests though.

Olly

Useful to know

One advantage of the HEQ5 is the ability to transport it all over. I took mine, with all the kit, to La Palma a few years back.

There were two of us travelling and, because the weather is nice out there, it only took one item of excess baggage between us. Although I do confess to removing approximately 30 cm off of the tripod legs to make it fit in the suitcase.... lol

On 23/02/2020 at 15:33, ollypenrice said:

🤣 I knew it! I knew I shouldn't have posted that screen grab with you on the case!!!  I somehow managed to muck up my Ps profile settings when I opened it but rather than search for a solution then and there I pressed on with my reply to Alacant's post.  The evidence regarding the black point remained valid so I left it. Ignore the colour profile.

(Out of interest I don't know what I altered but a net search told me to close Ps then open it again while waiting a fraction of a second before hitting Shift/Control/Alt.  Well that might be easy for somoeone with the fingers of a concert pianist but it took me a good few attempts. Now fixed.

Olly

Personally, I would never trust screen captures. Who knows what dithering/approximations/normalising etc. that your operating system does in a capture.

Cant help you on the saddle front other than i found the adm to be excellent on my heq5

However, if you only have a quarter turn of the bolts holding the dovetail, DO NOT USE IT! 

The usual advice on bolts is that you need a minimum of three turns with the threads engaged.

I have an "identical" no branded guide scope. It works fine, BUT...

I needed to add two extra locking screws to the sliding part of the focuser as it was flexing like mad!

I believe that the holes are already drilled and tapped in the tube - M4 or M5 I think. Once focused, I "carefully" nipped up the three screws and now it is solid.

I'd be interested to hear if a similar problem exists with the branded versions...

Bias:   Yes

Flats:  Yes

For a non-cooled DSLR, darks and dark flats are pretty much the proverbial "urinating into the prevailing airflow" for reducing noise. and can actually make things worse.

If you are guiding, then implement an aggressive dithering plan (min 12 pixels between subs) and use sigma pixel rejection in the stacking process.

If you are not dithering, then you really should. This is the most effective noise reduction method for DSLR imaging.

Yes you did 😃

I agree that subtracting the uncalibrated darks also removes the bias from the lights.

I just prefer to remove the  bias from all my files so I know that I have "a level playing field" from the start. 😃

1 hour ago, vlaiv said:

That really depends. I most cases you in fact don't need bias to be subtracted and if you follow "standard" workflow you can actually use any file as bias - even Picasso painting digitized to exact size as your subs

Let me explain and show why is that:

If we observe "regular" calibration procedure being (same thing happens to flats, so we will skip flat calibration for now, and just mention it at the end):

- Master bias is stack of bias subs (for now, we will later substitute in Picasso painting instead)

- Master dark is made by stacking "calibrated" dark subs.

- Calibrated dark sub is dark sub minus master bias

- Calibrated light =  (light - master bias - master dark) / master flat

Let's do a bit of substitution

Calibrated light = (light - master bias - average(dark - master bias) ) / master flat

Now average is regular average sum and division, and if we have "constant" term we can pull it in front of the brackets so let's do that

Calibrated light = (light - master bias - (average(dark) - master bias) ) / master flat.

Let's rearrange that a bit:

Calibrated light = (light - master bias + master bias - average(dark) ) / master flat

Now you will probably notice that we have -master bias and +master bias and fact is that two numbers with same absolute value - one negative and one positive added together will give 0, and we can have any old "number" there it won't change a thing so it is safe to also write this:

Calibrated light = (light - Picasso image + Picasso image - average(dark)) / master flat

and that is equal to

Calibrated light = (light - average(dark)) / master flat

You don't need bias to do proper calibration, and in fact if you use above "standard" calibration flow - you can use any, and I mean literally any sub as master bias - it will make no difference at all.

You only need bias in very special cases - like mentioned above by @Merlin66 - scaling darks - either in form of different exposure length or when trying to optimize dark calibration (darks at different temperature).

Wow! that's a lot of explanation. Thanks.

The way my, rather limited, interpretation is if you subtract the bias (and dark if necessary on long exposures), then:

• Flats (Vignetting/dust bunnies) are a multiplicative correction factor,
• Gradients are subtractive.

Yes/no?

It's the little things that count...

1 hour ago, dph1nm said:

Both CMOS & CCD manufacturers tend to add a constant value to the image before it is read out, in order to avoid negative values. This needs to be subtracted (both from flats and lights) before flats will work correctly.

 

They do indeed add a constant value to the image to avoid negative numbers. However, this is a constant and is called the pedestal. It is not the bias signal.

The bias is electronic noise generated by the sensor and camera electronics as the image is read off the chip. Fortunately, this noise is fairly consistent for every image read and, by averaging many bias files, a model can be built ("master bias") that can be subtracted from every image to dramatically reduce the read noise from the camera/sensor.

20 hours ago, Merlin66 said:

Why take bias frames?????

There are only of use when scaling darks for different temperatures????

All images include bias.

Flats and Flat darks should be all that is ever required.

Agree/ disagree??

 

Bias should be subtracted from every image read from the camera i.e. lights, flats and darks. Except bias frames, of course

On 16/01/2020 at 21:21, Jonk said:

I have just bought an FTDI cable but not had a chance to test it yet. Tomorrow night's looking good and dare I say it, so is Saturday!

The ftdi chipsets assign themselves a com number on first plugin. After that, it matters not which usb socket you connect to, it will retain the port number (unless you want to change it).

The improved reliability over the prolific chips is like night and day!

Ad-blocker is your friend

Although I may have a go at making a Pi-hole soon: https://pi-hole.net/

You could always lend it to another local astronomer who would benefit from it.

Your scope is a reflector = no chromatic aberration (CA).

The (cheap) barlow has at least one refracting lens = possibility of CA.

99.5% it's the barlow.

On 03/01/2020 at 15:58, alacant said:

Hi

Keep it simple? One of these and one of these will just work.

HTH

 

Yep. This^^

On 01/01/2020 at 13:57, Victor Boesen said:

A couple of days after Christmas I was out with my star adventurer to capture the pleiades and the mount didn't have any serious problems with 90sec subs with my skywatcher evostar 72, however when I was going to capture the orion nebula the next day the tracking was so slobby it couldn't even manage 60sec exposures and for a minute I questioned the batteries(that I had just changed a month before and only used a couple of times) so I tried changing them with no succes. Then I began to think if it's motor backlash but I really don't know and that's why I post this question. Keep in mind that my polar alignment wasn't the issue cause it was somewhat spot on. I do also have a little imbalance in the declination because of my scopes dovetail.

This image is a 2min sub just to exaggerate the error:

I hope someone can help me here because I really don't know what the problem is. Could it be because of the orion nebula being further away from polaris than M45 and I simply should expect more from the mount?

Victor

East-West is almost Up-Down in you image. As the (very wiggly) star trails in your image are neither vertical or horizontal, I'd say it was a balance/weight/gear slipping/bearing issue. So it is not the RA tracking.

The movement is largely in the East-West direction, so my guess is that the DEC axis was continually slipping in little increments, and then these movements were enough to also upset the RA tracking somewhat. This also fits in with the Pleiades tracking OK, as the more vertical the scope gets, the lower the turning moment it produces on the DEC axis.

Balance again and make sure that the clutch is done up a wee bit tighter next time, and see what happens.

Great idea, I will borrow it. 

Of course the total eclipses will take priority!

I'd say go home and have a nice hot chocolate.

25 minutes ago, BabyPepper said:

Okay so i focused on the "random noise", and what do you know.  Its not random noise after all ;  its like a little tiny river network of lightning streaks.   Sit and stare!

 

I do believe that you have imaged the Solarians mass transit system!

Seriously, that is absolutely brilliant!

If it doesn't cost you anything/much to try - why not have a go?

I think your limitation might be with your mount - what one is it?

Also, I think that @Stub Mandrel has probably done what you are suggesting - and he will know everywhere it could go wrong!

Super!

What's all that black tape doing?

I hope it's just a low voltage cable!! 😱😱

1 minute ago, teoria_del_big_bang said:

I have had so much help from this forum I am more than happy to give a little back if I can 🙂 

Steve

Agreed!

There is so much negativity and hate going on online.

This is a smashing haven for the sane and well-meaning.

Are you using the correct bahtinov mask for the new focal ratio?

It can make a difference.

I have an Esprit ed80 too.

I'm afraid  that there is no way to get the oag and the fw behind the flattener without exceeing the back-focus distance of 55 mm.

As for tuning the spacing, some imagers luck out with the exact recommended 55 mm, but i didn't! Two years on and i stil haven't  got it perfect,  but it is "good enough" 😁