Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Flats issue


Andyk93

Recommended Posts

Good! Thanks for the images - I think we're getting somewhere. Attached is a greyscale version of your single flat image. I'm not sure if it corresponds with the masterflat at the start of this post ( later image is possibly darker at the bottom). Possibly taken in a slightly different way but perhaps shows that illumination hasn't always been the same. A single flat should look much like a master. The only reason for taking multiple flats is to reduce the noise in the image.

Still, the flat shows the circular artefacts.

Possibly more importantly is the single raw image. The Orion nebula is a very bright target, though your image is very dark and lacking in detail. It needs a much longer exposure. This will provide the dynamic range that will be corrected by a masterflat. If a very dark image is stretched too much it will show limitations in the underlying process - this looks like what has happened with your image. Go for a much longer exposure if you can - the whole of the nebula (including the 'head') should be visible in a single captured image. 30 seconds is often a good starting point for the Orion nebula.

Any questions, fire away...

s_flat_processed.jpg

Link to comment
Share on other sites

  • Replies 35
  • Created
  • Last Reply

Thanks very much for your help. So when I take a 30 second image, on my camera it looks roughly the same as the stretched one, when put into PI it looks like the attached one. I assume the camera stretches the image slightly. This stack of subs was to capture the Triangulum ready to blend. Then I would use 5-10 min exposures to capture the rest. I can guide for 30mins but it tends to go too far at 20+. This was taken with an hours worth of data but with a Nikon d3300, same telescope. 

orion_embed_DBE softer.jpg

Link to comment
Share on other sites

That was with a different non modified camera. The camera I use now is a modified 1000d and is where I get the problems. I know it's a dust issue but they don't calibrate out with flats. It's obviously me that isn't taking accurate enough flats so that's why I'm struggling. The only reason I did those 30second images is to blend into the core of m42 so it didn't look blown out. Hope that helps

Link to comment
Share on other sites

OK - I understand more now. Perhaps the mod introduced some dust. I recently modified my 1000D and (luckily) didn't seem to introduce dust except for a tiny bit of plastic sticking out in one corner. Also, not long ago, I put together a composite of the Orion nebula, though not as good as yours. I used 45 sec for the body and 10 sec for the core. 

So if you are just imaging for the core and not the body, only the core matters. Flats aren't important for such a small part of the image, though perhaps purists would disagree. The round artefacts are far away from the core too.

Somehow I think the quality of your work is far ahead of mine. Great to see it!

Link to comment
Share on other sites

It doesn't seem to matter how long the exposures are. It still shows the dust, I have loads of data on different targets but it's all ruined by the dust. If I can't find a way to correct it, I will be taking it apart and finding out where the dust is. Thanks for the kind words! Once this problem is sorted I will be much happier. At the minute it feels like I'm wasting my time and it's more and more tempting to buy a ccd

Link to comment
Share on other sites

Hi. One last try? Remove DSS' .txt files. Stack one of each light, flat and bias. One very last chance: remove the camera from the telescope. No lens. Set to Av. Take a flat pointed at an evenly lit sky. For me, it's not dust. Otherwise I'm out of ideas. HTH and good luck.

Link to comment
Share on other sites

MasterFlat_ISO800.tifI havn't had time to do as suggested but I have just used my light panel to get 2 second exposures for my flats. I put the dslr straight onto the light panel with coma corrector and lp filter in place. Before I started I also used a rocket blower to clean the sensor. I THINK this has got rid of the dust spots? is it at the correct focus to show the spots?

MasterFlat_ISO800.jpg

Link to comment
Share on other sites

The blower may have done the trick, but I don't think the light from the panel will show this - I've previously tried taking flats with just the camera and found the unfocussed light doesn't show the dust as well. Light from the telescope will show dust as the light is focussed and will show up anything in its path. Light from the panel is not focussed. Following our previous exchange of messages, I've had a closer look at flats taken after modification of my camera. There is dust! See a stretched part of a flat below. The effect of the dust is visible on a raw image but is not evident on a stacked image using flats, showing the flats are doing their job. I think that is what you need to work towards - getting flats to work. That means obtaining even illumination, although dust can (and will) move around, so there may still be a bit of fixing to do manually.Dust.jpg

Link to comment
Share on other sites

Hi. I don't think it's going to work. The flat has to be taken with exactly the same focus, orientation, optical path and dust in which the light frames were taken. Worth a try though; it will be interesting to see if they calibrate the light frames any better. HTH.

Link to comment
Share on other sites

38 minutes ago, bobro said:

Dust.jpg

This is exactly what I wanted to see! Atleast I know it's actually dust, I'm going to have a look at the sensor and see if I can see anything then wait till it's clear again to get proper focus 

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.