Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

What happened to these flats?

  • Please log in to reply
16 replies to this topic

#1 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 15 June 2019 - 02:12 PM

I have a new 8" Celestron RASA that I have been able to use three nights so far. I have been using Newtonian reflectors for AP for years, but this is my first experience with a catadiatropic scope for AP.  I am very impressed with the RASA, but it presents some challenges I have not encountered before.  One such challenge is flats - until I get a dew shield, I won't be able to use my flat box, so I have been taking twilight sky flats, which I have done many times with my newt.

 

When using the Celestron LP filter made for the 8" RASA, strange artifacts appear in the flats.  They do not appear in flats made with the AR filter that comes with the RASA, and they seem to only affect the twilight sky flats made with the LP filter - light subs made with the LP filter when the sky was dark do not show the artifacts.

 

Below are three images showing the issue.  They were produced in PixInsight from individual flat frames, debayered, converted to grayscale and stretched:

 

filter_problem.png

 

 

The first (top) sub was made with the AR filter installed in the RASA.  It appears normal and the master flat made from this sub combined with 19 others worked well.  The other two were made on two consecutive nights with the AR filter removed and the LP filter installed.  These flats made with the LP filter were unusable.  Fortunately, the RASA has such a naturally flat field that if your sensor and filter are clean, and you can do without the corners of the images, you barely need flats.

 

The camera sensor was cooled to 0 degrees C for the AR filter subs and the first LP filter sub, and it was cooled to -5 degrees C for the second LP filter sub.  At first I thought the artifacts were caused by frost on the camera sensor, even though I had the anti-dew heater on, but I removed the camera and looked at the sensor.  It was clean and dry.  The LP filter and the Schmidt corrector plate were also clean and dry.

 

Has anyone seen artifacts like this in twilight sky subs, or any other kind of subs?  I am inclined to believe there is a problem with the LP filter, though visulally it appears normal.  I have not been able to repeat taking flats with the AR filter yet, perhaps will be able to do that tonight, in order to rule out the possibility that something happened to my camera or to the telescope after that first night.

 

Any ideas?

 

Cheers,

 

Don


Edited by DonR, 15 June 2019 - 02:58 PM.


#2 zakry3323

zakry3323

    Viking 1

  • *****
  • Posts: 967
  • Joined: 11 Apr 2016
  • Loc: Pittsburgh

Posted 15 June 2019 - 02:24 PM

Wow, that's some intense weirdness. I'm going to follow your thread because I've never seen anything like this, and I want to know what it is. 



#3 Lead_Weight

Lead_Weight

    Viking 1

  • *****
  • Posts: 921
  • Joined: 21 Jul 2016
  • Loc: Houston

Posted 15 June 2019 - 02:26 PM

If this was from the 071, then it's ice from the cooling. I see this on mine. And now that it's summer, it's really rearing it's ugly head. It's not the filter. I find I have to cool the camera slowly so that it doesn't ice over. Eventually the ice will cover the whole image area. I cool it slowly by lowering it in 5° increments, rather than going from 30° to -15° immediately. There is also a heater for the window that needs to be turned on if you haven't done that. It should be in the camera settings. Additionally, I find that the heater only partially works, and I have to be imaging with the sensor on to provide enough heat, so I'll just have the camera snapping images of whatever while I cool it down. Hope that helps.


  • Dynan likes this

#4 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 15 June 2019 - 03:19 PM

Hi Andrew,

 

Thanks for the response.  Ice was my first thought, though I have never experienced it with this camera.  The anti-dew heater was on, and I always cool the camera slowly, though maybe not as slowly as you do (about 25 C to 0 C in 10 minutes), and I cool the camera to the target temperature about 30 minutes before beginning the flats.  

 

When I saw the weird artifacts on the first night with the LP filter, I removed the camera without disconnecting it or warming up the sensor and examined the window and the sensor.  I did not see any frost.  On the second night after noticing the same artifacts, I waited an additional 10 minutes, taking a test flat about every minute.  It appeared as if the artifact was decreasing in size over the span of a few minutes.  Of course this could be explained by frost being slowly dissipated by the anti-dew heater.  I saw no trace of artifacts about an hour later when I began shooting lights.

 

Tonight if the clouds clear I will try shooting the twilight sky flats at ambient temperature.

 

Thanks,

 

Don


Edited by DonR, 15 June 2019 - 03:22 PM.


#5 Lead_Weight

Lead_Weight

    Viking 1

  • *****
  • Posts: 921
  • Joined: 21 Jul 2016
  • Loc: Houston

Posted 15 June 2019 - 03:40 PM

Hi Andrew,

 

Thanks for the response.  Ice was my first thought, though I have never experienced it with this camera.  The anti-dew heater was on, and I always cool the camera slowly, though maybe not as slowly as you do (about 25 C to 0 C in 10 minutes), and I cool the camera to the target temperature about 30 minutes before beginning the flats.  

 

When I saw the weird artifacts on the first night with the LP filter, I removed the camera without disconnecting it or warming up the sensor and examined the window and the sensor.  I did not see any frost.  On the second night after noticing the same artifacts, I waited an additional 10 minutes, taking a test flat about every minute.  It appeared as if the artifact was decreasing in size over the span of a few minutes.  Of course this could be explained by frost being slowly dissipated by the anti-dew heater.  I saw no trace of artifacts about an hour later when I began shooting lights.

 

Tonight if the clouds clear I will try shooting the twilight sky flats at ambient temperature.

 

Thanks,

 

Don

That's exactly how I've experienced it. It seems the act of using the camera (to heat the chip up) makes the anti dew heater work. So you have to be using the camera to get the frost to disappear. Also, once it's cooled to temperature, (and there's no frost) it doesn't seem to re-frost after that. So I typically end up shooting my flats after a session when it's still cooled down.



#6 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 16 June 2019 - 07:35 AM

Hi Andrew,

 

You were right - I shot the flats last night at ambient temperature, then started cooling the camera slowly, in five degree steps, one degree per minute.  The flats were fine, and I made test exposures every few minutes during the cool-down.  The "artifacts" appeared just after the camera reached 0 degrees C.  I warmed it up to 5 degrees C and they disappeared quickly.

 

So it seems the dew heater on this camera doesn't have much effect.  My ASI071MC Pro is about a year and a half old.  Looks like I need to start managing the desiccant tablets.  This is a little disappointing.

 

Thanks for your help.

 

Cheers,

 

Don


  • jdupton and zakry3323 like this

#7 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 16 June 2019 - 10:46 AM

On the bright side, I do like the 8" RASA.  I only got four one minute light frames last night, before giving up due to the almost full moon and intermittent clouds.  Calibrated with the ambient temperature flats and a master bias (no darks), the four minutes of total integration time yielded a respectable image of M5.  It's under-sampled, with too few subs to drizzle, but I like it for a four minute image.

Attached Thumbnails

  • m5_done.jpg

  • kathyastro and starbuckin like this

#8 Lead_Weight

Lead_Weight

    Viking 1

  • *****
  • Posts: 921
  • Joined: 21 Jul 2016
  • Loc: Houston

Posted 16 June 2019 - 10:53 AM

Cool, wish I could get a decent image that fast :)

#9 kathyastro

kathyastro

    Viking 1

  • *****
  • Posts: 749
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 16 June 2019 - 11:15 AM

My Atik 383L+ usually frosts up.  My standard practice now is to fire up the camera and start the cooling (10 minutes to reach operating temperature) at least an hour before I start to grab images.  Running for an hour at operating temperature gives the frost time to sublimate off the glass.

 

I have tried changing the dessicant pill, including rejuvenating the dessicant in the oven, but it makes no difference.  What it needs is time.



#10 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 16 June 2019 - 12:57 PM

Thanks Kathy.  I will try that next time out.



#11 555aaa

555aaa

    Vendor (Xerxes Scientific)

  • *****
  • Vendors
  • Posts: 1432
  • Joined: 09 Aug 2016
  • Loc: Lynnwood, WA, USA

Posted 17 June 2019 - 09:05 AM

I shoot my sky flats at +5 C and then I let the camera cool and go through this phase during twilight. My camera is an old STL-11000m, but I always shoot darks with those flats at the same temperature.

Edited by 555aaa, 17 June 2019 - 09:06 AM.


#12 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 17 June 2019 - 01:22 PM

That is a good idea, I will try that.  I am also going to recharge the drying tablets and put some silicone grease on the camera's  o-ring seal.  From what I have read this may or may not help.

 

Thanks,

 

Don



#13 Alfredo Beltran

Alfredo Beltran

    Viking 1

  • *****
  • Posts: 888
  • Joined: 01 Aug 2010
  • Loc: Bogota, Colombia

Posted 19 June 2019 - 08:10 PM

That is a good idea, I will try that.  I am also going to recharge the drying tablets and put some silicone grease on the camera's  o-ring seal.  From what I have read this may or may not help.

 

Thanks,

 

Don

I saw frosting also on my 071 Pro. How is the seal working?



#14 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 20 June 2019 - 08:09 AM

Hi Alfredo,

 

I have not performed the maintenance yet - I will be doing it today or tomorrow, and will report back here with the results.

 

Cheers,

 

Don



#15 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 20 June 2019 - 11:17 AM

I just finished drying the desiccant tablets and applying silicone grease to the o-ring in my ASI071MC Pro.  The procedure was quick and easy, following the directions here:

 

https://astronomy-im...e_desiccant.pdf

 

I did not take any photos, I was working quickly to minimize the exposure of the camera's internals to the humid Georgia air.  But here are my observations.

 

Removing the nine socket head cap screws necessary to open up the ASI071MC Pro requires a 2mm hex key (Allen wrench).  The screws in my camera were not overly tight, and were easy to remove.

 

After removing the three cap screws that hold the "tilter" onto the camera body and removing the tilter, I found a black foam rubber ring below the tilter, sitting on but not attached to the camera's cover.  This is not shown in the PDF file - I just lifted it off and replaced it in the same position during re-assembly.

 

After removing the camera cover, I found the four desiccant tablets seated in cups on the under side of the cover.  The cups held them in place so that they did not fall out, but the tablets were easily pried out of the cups with a finger nail.  I set them on a clean saucer and microwaved them on high for two minutes, as specified in the PDF file.

 

The tablets were hot when they come out of the microwave.  I used tweezers to pick them up, place them in the cups, and gently press them into place.

 

It appeared that the desiccant tablets contain an indicator dye, but the color change of mine (slightly yellow before microwaving, slightly pink after microwaving) was very subtle.

 

The o-ring seal is red, and sits in a groove around the edge of the camera body.  The o-ring appeared to have a very light coat of grease on it.  I carefully removed the o-ring (you would not want to touch the top of the sensor with it), gently ran it through my fingers to make sure it was clean, and applied a light coat of fresh silicone grease (from the plumbing department at Ace Hardware).  Again, be careful when re-seating the o-ring that you do not let it touch the top of the sensor.

 

When replacing the camera cover, take note that there are two shiny metal electrical contacts on the under side of the cover (on cameras with the anti-dew heater only) that must be aligned with the two posts sticking up from the camera's circuit board in order for the window heater to work.

 

The whole procedure took about 10 minutes.  Time will tell if it had any effect.



#16 Alfredo Beltran

Alfredo Beltran

    Viking 1

  • *****
  • Posts: 888
  • Joined: 01 Aug 2010
  • Loc: Bogota, Colombia

Posted 20 June 2019 - 12:31 PM

Thank you very much for the detailed report Don.

 

I did exactly that on Saturday (except applying the silicone grease) and the camera has been since then in a hermetic jar with silica gel. Let’s see if recharging the tablets is enough to keep it from frost.

 

Regards,

 

Alfredo


Edited by Alfredo Beltran, 20 June 2019 - 12:32 PM.


#17 DonR

DonR

    Apollo

  • -----
  • topic starter
  • Posts: 1351
  • Joined: 15 Nov 2006
  • Loc: Georgia, USA

Posted 21 June 2019 - 08:58 AM

Good news - this morning I ran "ceiling flats" indoors, on the day after drying the camera's desiccant tablets and lubricating the o-ring.  It is about 24 degrees C outside and 75% relative humidity (dew point 18 degrees C).  The air conditioning is on, so it's at least that humid indoors.  I acquired the flats at -10 degrees C immediately after the camera reached the temperature setpoint.  There is no sign of frost in the flats.  It looks like drying the tablets worked, and hopefully the light silicone grease on the o-ring will help keep the camera dry inside for a while.

 

Cheers,

 

Don


  • hoxca likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics