Drizzle causing errors in PCC

Andrea Girones

Hello

I have noticed that PCC is not working for my images when I use drizzle data. I found this explanation on the Pixinsight forum.

But I do not know what scale I am using nor how to find it when I generate drizzle data during "Star Alignment"

Can anyone help? thanks Andrea

If you are using drizzle, please remember to change the focal length or the pixel size according to the Scale parameter value used in DrizzleIntegration. For instance, if using a Scale value of 2, you should then multiply the focal length by 2 or divide the pixel size by 2.

BlockHead

Yes, I demonstrate in my drizzle tutorial as well. 

When you use PCC (without drizzle)- what numbers do you usually use?

(Did you review my PCC section?)

To calculate your platescale use the formula 206265/(Focal length of your telescope in millimeters). 

This will give the platescale in arcseconds per millimeter. Then you multiple this number by the size of a pixel of your sensor (in millimeters). So if you are using a sensor with 9 micron pixels you multiply by 0.009. This will give you a final platescale in arcseconds per pixel which is the "SCALE" value you are looking for. When you drizzle, you are changing this scale by the factor you used.

Write back with your telescope aperture and f-number or focal length and the size of your sensor's pixels..and we can determine this. 

-the Blockhead

Andrea Girones

Hi So i have gone through the PCC section  ( watched it again this morning) and usually get the numbers from the FIts header. PCC worked for a while and then-stopped working. 

I have NOT gone though the drizzle section ( Adam give me time there are a MILLION videos in Fundamentals lol) I also just figured out it was the Drizzle causing the issue as I don’t always use it. I thought maybe the fact that my Hyperstar images are mirror inverted was the problem 

I am using my C11 SCT with Hyperstar so 540mm focal length  f1.9 and Asi533 camera with 3.76 pixels 

And then i will go look at the drizzle videos because I am not even sure I need to drizzle with this setup! Thank you, you are the reason I even know about how PCC works!

BlockHead

Thanks.

But still... write back with your telescope focal length and ccd chip pixel size... lets compute it real quick and make certain the Universe is in order!

-the Blockhead

Andrea Girones

I though I did?

540mm focal length

Pixel size 3.76

Thank you

BlockHead

ooh... yes you did.

So... 206265/540 =  381.97 arcsecond per millimeter

But... we want arcseconds... so we take 381.97 x .00376 =  1.43 "  per pixel

If you are performing PCC on an image that was drizzle upsampled by 2... then you need to make it 0.718 (0.72) arcsecond per pixel .... OR you could leave it the same and double the focal length to 1080 mm. I think manipulating the plate scale number is best. 

So... is this the value you used when you did PCC on your drizzled image?

-the Blockhead

Andrea Girones

Honestly i just checked off “create drizzle data” when i did star alignment.   When I tried PCC it worked when I did not generate drizzle data and did NOT work when I used drizzle data.

 

Of interest I just tried Pcc again today with a different camera and NO drizzle and it still did not plate solve.  So I may need to go though the videos again to trouble shoot. 

I will  do a more careful and scientific look into this and get back to you!

Thanks!

Andrea

BlockHead

Oh... checking "create drizzle data" does not do anything if you do not utilize those special files. 

You only need to create them when you plan on drizzling. Drizzling data is actually a high bar of quality to reach... most people who do it don't actually have the data quality to do it well (or get better results). 

This applies to me...and I am not taking bad data. 

-the BLockhead