Rasa 8, horrible flares on bright stars

[gorann]

Hi Alan

I did what you asked for last night. I went for Arcturus, the brightest star in the northern hemisphere. Here are three single 1 second frames at gain 100 with an ASI2600MC. So, left edge, center and right edge of the field.

As you see there is none of the flares you see, so it is most likely a problem with the current RASA batch.

Cheers, Göran

[symmetal]

10 hours ago, gorann said:

Hi Alan

I did what you asked for last night. I went for Arcturus, the brightest star in the northern hemisphere. Here are three single 1 second frames at gain 100 with an ASI2600MC. So, left edge, center and right edge of the field.

As you see there is none of the flares you see, so it is most likely a problem with the current RASA batch.

Cheers, Göran

Thanks very much Göran, that does confirm that some newer RASAs have a problem. Can you share the raw fits files for these three images, so I can send them to FLO who can then use them in their discussion with Celestron about the current RASAs.

Had two clear nights the last couple of days and did an hour or two on various targets with and without an aperture mask. Here's a comparison with M33 given a quick process in Startools with no star manipulation, sharpening decon. or noise reduction. The image without a mask doesn't have a correponding flat frame I'm afraid so is a bit uneven brightness. These show that even modest brightness stars show the flares quite noticeably with no mask. The one with an 180mm mask is much better with just the corners and extreme left and right edges showing some flaring.

RASA 8, ASI2600MC, 180mm aperture mask, 142 x 60s subs 🙂

RASA 8, ASI2600MC, no aperture mask, 110 x 60s subs 😟

Alan

Edited August 30, 2022 by symmetal

[gorann]

The one with mask looks rather good so I suggest you just make the best out of the RASA you have until you get an replacement. Here are the raw fit files.

Cheers, Göran

2022-08-29-2022_1-CapObj_0001.FIT 2022-08-29-2023_9-CapObj_0001.FIT 2022-08-29-2024_9-CapObj_0001.FIT

[symmetal]

1 hour ago, gorann said:

The one with mask looks rather good so I suggest you just make the best out of the RASA you have until you get an replacement. Here are the raw fit files.

Cheers, Göran

2022-08-29-2022_1-CapObj_0001.FIT 49.77 MB · 0 downloads 2022-08-29-2023_9-CapObj_0001.FIT 49.77 MB · 0 downloads 2022-08-29-2024_9-CapObj_0001.FIT 49.77 MB · 0 downloads

Thanks for the fits files. Yes, I'm happy with the masked results until the issue is resolved. 🙂

Alan

[tomato]

Apologies Alan, for not responding to your request.  I’ve been dealing with a serious tilt problem on my RASA8 which I think I have now resolved. 
So if it’s clear tonight I’ll point it at Deneb and take some subs before I start my imaging run.

Steve

[symmetal]

3 hours ago, tomato said:

Apologies Alan, for not responding to your request.  I’ve been dealing with a serious tilt problem on my RASA8 which I think I have now resolved. 
So if it’s clear tonight I’ll point it at Deneb and take some subs before I start my imaging run.

Steve

That's OK Steve. If your Deneb images are similar to Göran's Arcturus then it should provide solid evidence that a batch of current RASAs have a problem which must be addressed by Celestron who seem to be dragging their feet with the issue. 

I had to set the camera tilt very accurately on the test jig before putting it on the RASA. The first RASA I had to add a 0.5mm spacer to avoid elongated edge stars  while the second was good without it. Edge stars pointing towards the centre means the spacing is too great, the opposite of what refractors tend to indicate. Getting the same tightness on the locking ring is critical to maintaining the spacing distance too.

Also I found that over an imaging run one side of the image may start developing elongated stars and a refocus fixes it though the change in focus isn't enough to make the central stars appear out of focus. I tell SGP to refocus every hour which helps. 🙂

Alan 

[tomato]

Just got these before a cloud bank came over, RASA8/QHY268c, 10 sec exp, Gain 30, offset 30, cables arranged in a circle. There is a light house effect on the star but not sure if this is cable diffraction. Still some tilt on my sensor I'm afraid.

2022-08-31_21-33-02__-10.00_10.00s_0000.fits2022-08-31_21-37-44__-10.00_10.00s_0000.fits2022-08-31_21-41-13__-10.00_10.00s_0000.fits

[symmetal]

55 minutes ago, tomato said:

Just got these before a cloud bank came over, RASA8/QHY268c, 10 sec exp, Gain 30, offset 30, cables arranged in a circle. There is a light house effect on the star but not sure if this is cable diffraction. Still some tilt on my sensor I'm afraid.

Thanks for those images Steve. All three show no asymmetrical flaring which is good and backs up Göran's images. I'll add them to the evidence given to FLO.

The 'dark' spikes seem to be an effect of the cable routing. I've been doing tests on different arrangements and so far the semi circular arrangement in front of the corrector plate seems to be the best with the least artifacts. I also tried fitting the semi circular loop level with the base of the camera to avoid running the cables down the side of the camera but any arrangement fitted further away from the corrector plate seems to add many small spikey artefacts all around the bright stars. Not sure why this happens.

Your tilt is quite noticeable unfortunately. Did you build one of the wooden tilt test jigs? I think they're likely a requirement with a RASA which has such a small depth of focus. Your stars on the right pointing towards the centre indicates the camera right side is too far away. I believe you have the Artesky filter holder which is a single piece so can't make it shorter. The Starizona one I have on order from FLO incorporates a removeable 5mm spacer which I thought would be useful if the distance needs to be reduced. 

Comparing the size and shape of your elongated stars compared to the first RASA I had which needed a 0.5mm spacer yours look to be greater than 0.5mm out on the right compared to the left. Hope you can get it sorted.

Alan

[tomato]

Thanks for the feedback. I have a custom made adapter which allows a 2” filter to be screwed in, and employs the tapered adapter for the QHY camera. I’m currently experimenting with  home made semi circular shims and do all my adjustments on the scope,  but I will look at making a test jig.