Noob with Rasa 11 and center of images are super bright! Help!

Say hello to Mr. Vignetting.
Have you taken flat frames to correct your light frames?
If so how  does your flat frame look?

Very curious.    What do your raw stretched subs (no filter) and flats look like?  How long are your exposures, and what Bortle are you imaging at?   Also, are you using a dew shield?

From the specs (see link), you should probably be using a gain of 100 (at the step).     There's definitely no significant advantage at 60 and likely just more trouble.  Might be wise to start your diagnosis by imaging broadband (MUCH shorter exposures).   Broadband exposure duration for this setup in moonless conditions should be on the order of 30 secs.   Once you see those raw frames, you'll have a good idea of what's going on.

edit:  deleted link.   check post below for correct diagram.

Here's a spreadsheet you might want to play with....it's a 2600 exposure calculator from cloudy nights.

https://drive.google.com/drive/folders/1CnKISk-4X0pTo6emReMIO099JlgPHIT6

Cheers,  Doug

A couple more thoughts.   First, are you using an LP filter in place of the optical window?   In my case, I replaced the RASA's clear optical window with a Hoya UV-IR filter (full time).   You might want to revisit the RASA white paper (page 6) and look at the spot matrix diagram to convince yourself that you don't want to have any UV below 436nm or IR above 707nm reaching the camera.

Second thought is a sanity check on the light fall-off.  From the white paper (pg 11), for APS-C, you should only expect a light fall-off of on the order of 5%.   It looks like you're getting much more than that.   So, something's not adding up yet.....

https://celestron-site-support-files.s3.amazonaws.com/support_files/RASA_White_Paper_2020_Web.pdf

Looks like a dew problem to me…

I have the RASA11 and I shoot with a ASI071MC at times so a few questions.  In general, what your seeing is normal and i'm assuming that your image is stretched and the intensities have been adjusted to clearly and boldly show the rings / vignetting..... correct? A few things that caught my eye:

1) You mentioned that the rings show in your darks. If so, you did your darks incorrectly.  You need to detach your camera, from the RASA, put the cover on the camera threads and do your darks that way.  What you stated indicates that you did your darks with the camera attached to the RASA which is a big no no. So that's issue #1.

2) You need to ensure that your using the M48 plate when you use a APS-C sensor with the RASA. Although the rings shown and the  vignetting will still be present under a hard stretch, the M48 plate will minimize this more than using the M42 plate.

3) When you've completed your imaging session, do NOT move the RASA until you've completed your flats. It's ok to slew to the Zenith but other than that, do not move the system until your flats have been completed.  Even the slightest movement will prevent this system from producing quality flats, and with the RASA's low tolerance, it's very easy to do.

Hope this helps. Good luck.

Thank you very much for the feedback!
I was originally taught to always take my darks with the camera on ( with my refractors that is ) I never heard of taking the camera off to do them but will now!

I am using the M48 plate as I wanted the most light possible with such a large sensor, so good there.

As for the flats, in the past I have always used the white tshirt technique to take them. I dont think I have ever taken dark flats. How should i go about taking them?

I have always had refractors so the Rasa is completely new to me but really want the learn.

Thanks again for all your help.

Another way of doing flats is to do it at night when you have full cloud coverage. just take pictures as you would do for normal images. i found this method to be best for avoiding pollution of the flat from reflections inside the scope and the flats naturally become 1 to 2 minute exposures, so you can just use your normal darks for calibration.
Added plus is, you have something to do on those annoying cloudy nights.

I see two big problems in the image that you posted.

1) As others have pointed out, you have very significant vignetting.  The RASA 11 is designed to cover a 42 mm image circle and you are using a camera with a APS-C sensor with a significantly smaller sensor so I’m not sure why your image shows so much vignetting.  The RASA may have some vignetting over that sensor, but not like what you are showing.  How do you have the camera attached to the telescope?  It sure looks to me like your adapter may be too small in diameter.  The beam from the RASA is very fast and it sure looks to me like you are seriously clipping the marginal rays at the edge of the field. Remember that to eliminate vignetting, you have to pass the sum of the marginal and chief ray heights throughout the system.

2) I see significant stray reflections that appear to be coming from the inside of the camera adapter.  Stray reflections like this will completely screw up image calibration.  Remember that vignetting modulates (by multiplication) the image irradiance distribution.  Therefore, to remove the effects of vignetting, the calibration process divides out the flat data to calibrate the image.  Any additive signals coming from stray reflections will not be removed during the calibration process and worse, they may disrupt the normalization process needed to properly scale the calibration process.  The best way to look for stray light is to remove the camera and to look backwards through the system.  That’s hard to do with a RASA system so you might just start by pulling things apart and either painting everything on the inside of your adapters with ultra black paint or covering everything with ultra black flock paper.  Strays coming from the inside of an adapters is always aggravated when the adapter diameter is too small, which fits with my feeling that you might be using an adapter that is too small.  I didn't look at how far recessed the sensor is inside of your camera but you should also consider the size of the opening in the camera housing as a potential source of vignetting and/or strays.

I want to add that in general, the problems that I see in your data have nothing to do with the camera settings.

John

Hi Nathan,

I also have the RASA 11 with the 6200MM camera and I don't get this kind of effect.

is the image you posted calibrated or not?

The reason why I am asking: plain vignetting (which is to be expected with the 48mm connection) would explain a bright center and getting darker towards the edges
Your image seems to have a ring where the center is bright, the ring is even brighter and then it quickly falls off towards the edges.
So it cannot be just vignetting, probably something is wrong with your calibration routine or your calibration frames.

Do you also see the ring (with the slightly darker center) in the single light frames?

All the best,

Ray

Here is a picture i took with my phone of a single image. The second ring is not there.  The dark spot at the top left was dust on the sensor it self! That was fun clearing off.....

I have the ZWO filter drawer directly on the camera then 48mm all the way to the plate that came with the rasa.

I just ordered everything I need for the Baader UFC setup and will be here Friday!

I have all 2in mounted filters. Do I need different filters?

I apologize for the dumb questions and I know I probably bit off more then I can chew with the rasa so soon, I just really love this hobby!!!

Thank you all again for the great advice, I am learning so much!

I re-did my darks and got this.....
Huge difference!

With the new baader ufc setup, better darks and flats, I think I've got it figured out now.

Thank you all for your help.

Hope to be posting some new images soon!

–deleted as I hadn't noticed the problem was solved–