QHYCCD

QHY294c or QHY294PROC - FLATS - it seems I cannot do them - please help!

Hello
This thread is from other one: https://www.qhyccd.com/bbs/index.php?topic=8408.90, as I was crowding it with problems that have nothing to do with other thread - sorry - I let myself go!

Can anyone help in taking FLATS using SharpCap and QHY294PROC ?

I have not certain idea and my situation is, I have mostly bad weather and time limited and every time I can match 'free time' / 'decent weather' / 'nobody bothering me!' I get to solve problems rather than do any decent shots!

Very frustrating and I also am sited within a small gap of sky in a back yard!

Any help would do.

I am guessing  - based on what I have read, you generally have to set SharpCap to same Gain [maybe]/ focus [obviously] as the Lights, then change Exposure [usually 2-5ms if I am not wrong] and keep Histogram at around 50% white wave peak?
I cannot do it and it seems extraction of flat while on DSS or similar program does not actually do much!

i.e. SharpCap states:
An ideal flat frame histogram would have a peak at about the 50-60% level and have all the histogram between the 20% and 80% levels. This is shown below for monochrome and colour cameras

I will try again tonight, but if anybody can give us some useful practical tips ASAP, I will try tonight and also do them after focusing and before taking lights.

Thank you in advance to any one.

Mauro

PS in a few years I will retire and i am slowly trying to built a few setups and then when all works, sell whatever I do not need. Trying to do things as simple as possible ans since here in North East England UK is generally cold and have very few good nights and one need to get to use as many as possible.
I have slowly managed to setup Computer/controls form inside - not far from tripod and used old EQ6 motors to adapt them to 2 telescopes with SW old control - I do gross focusing by hand - unless telescope is already set and then attach belt and fine tune focusing indoor.
« Last Edit: April 07, 2021, 07:09:30 AM by QHYceced »

Hi Mauro,

I'm taking may flats with SharpCap and the 294c (non pro version) and a dimmable Lacerta flat field panel.
I guess your flat field panel is too bright if your exposure time is only 5ms.

I'm setting my camera to about 500 ms (i.e. 0.5 sec) and then use the dimmer to tweak the histogram a bit, so that the white peak is roughfly in the middle. It's quite normal that the green peak in this case is a lot further to the right side at about 75%. This is due to the bayer pattern which has two green pixels so there is more green signal.
You have to pay attention however that the green peak doesn't move too far towrads 100% lest it saturates.

It is also important that you don't change the focus point and filter before taking the flats.
Best would be to not touch the scope at all after imaging which of course isn't practicable.
I usually take my flats the other morning so I tear down my setup but I don't change anything in the optical path!
That's very important for succesful flats, the vignetting and size of dust donuts is depending on the focus position so don't change it for the flats!

Another vital point is to use darkflats instead of bias frames for calibrating the flats, thus you can succesfully deal with the significant amp glow of the 294c.

best regards
Bernd

Hi Bernd
Thanks for the info.

I agree with you - last night I did put 2 layers of white plastic wrapping [did not have shirt at hand that time] and that is also slightly shiny !!!
Yes, I thought exactly the same about the brightness, I need to check if it is dimmable - I hope so or I will have to modify it somewhat.

This morning I focused my SW72ED towards a neighbour house and slightly forward focused [I now it is almots same as focusing a star - I tried it already and a bit off focus does not change much as Sharpcap's Robin says.

This time I got a very good flat when applied with greyish screen and all peaks overlapping on a nice sinusoid - I guess, as one would expect and was VERY clean.

I will have to get an old white tee-shirt to make image well diffused with shine and reduce light somewhat.

If I can do it electronically, I will .

Late will check if it is dimmable, I have not used it in 2 years - I forgot things + I am getting older!

yes, I have seen people stating something like 0.5s to 5s - depending on light and mine YES it is HUGE - in fact I told my wife last night, it is stronger than our outdoor light!

On a similar matter I re-did Sensor Analysis and this time I got it right, so the Brain function in Sharpcap should work better now, if not perfect - judging by the data I am starting to understand.

Thanks
Mauro

😂🤣





UPDATE

Yes, my LED A3 panel is dimmable.

At lowest and without shirt - just using it own white cover I get a mainly RED wave and can do Flat at ~ 400-500ms, but I guess I might have to raise brightness to get more colours out and use shirt [I already gathered - I got larger!] to diffuse it.

I might have solved all my problems at once, this time.

May the Gods of Astronomy help me next time to finally find my 'way'.

I also guess I will have to set my 294PROC gain to less than 1601 I had, as telescope + flattener stops at 5.2 now and I have read QHYCCD page saying to reduce - they are correct, in fact I get Panda eyes - i.e. main stars a bit bloated and with violet/blue halo. Although that might be 'also' due to my CLS-CCD, I might have to add another filter to stop UV or IR? Unless I can afford to get a L-enhance /Tri-band or similar - not a quad-band, it is for clearer areas. I am Bortle 5, maybe 4.5 now with slightly reduced street lights glow.

But this is another matter, not for this thread.

Regards
Mauro

Hi Mauro,

glad to hear that your flats improved with decreased brightness.

Although that might be 'also' due to my CLS-CCD, I might have to add another filter to stop UV or IR?

You definetly have to use at least an UV/IR cut filter because the 294c only has AR coating on the sensor window.
It's written in the specs:

Quote
For color camera user need to add a UV/IR filter in the light path
 

As far as I know this might be adding to your problem of bloated stars.

best regards
Bernd


Hi Mauro,

glad to hear that your flats improved with decreased brightness.

Although that might be 'also' due to my CLS-CCD, I might have to add another filter to stop UV or IR?

You definetly have to use at least an UV/IR cut filter because the 294c only has AR coating on the sensor window.
It's written in the specs:

Quote
For color camera user need to add a UV/IR filter in the light path
 

As far as I know this might be adding to your problem of bloated stars.

best regards
Bernd


Correct - thanks for reminding me, I will check 'now' and I believe I do have an extra 2" UV/IR to add although we know the more glass in front the more problem might come out - it might be over my Canon 100D !

I do also keep a huge black blanket to screen me from neighbour's light, which I have now raised it on one side to avoid any shine from my garage side fascia ... painted brilliant white!!

Got some grey undercoat + black matt paint, I might paint it when it is hotter = end of shine!

Regards
Mauro

Hi Mauro,

regarding the stray light a dews shield might also help a lot.
I've started using a dewshield last autumn to prevent dew on my SCT corrector plate, but I suppose it also blocks a huge amount of stray light falling from the sides on the scope's aperture.

best regards
Bernd