Rasa 8, horrible flares on bright stars

[symmetal]

Can I ask if anybody with a Rasa 8 has excessive uneven flares on bright stars. This lengthy topic on CN shows similar issues, with Celestron acknowledging the problem but haven't yet fixed the problem on scopes returned, due to a lack of replacement mirrors. It seems to be an issue with the mirror edge finishing on some mirrors.

I did some tests using Lyra in various positions in the frame and I need an aperture mask of 180mm too fix the flares over most of the frame though the corners and extreme left and right still show it to some extent. I think an aperture mask of around 170mm or smaller is needed to fix them over the whole frame of an ASI2600.

This shows the problem using no mask and a 194mm mask. With 194mm only a small area near the centre is OK and this area increases with a smaller mask aperture. I used a simple cross shape cable routing to just give standard diffraction spikes to avoid a more curved routing possibly confusing the results.

I'd have thought that the best option is to fit a mask just in front of the mirror on those mirrors with the problem as only a few mm will then be lost which won't be a significant light loss.

 

On a side note I've made a chart showing the actual aperture as if it were a lens with the same exposed frontal area and the actual light loss with a frontal mask. I've wondered why the central obstruction is ignored when stating scope apertures. The 200mm Rasa  8 has a 94mm diameter central obstruction. If it had say a 190mm central obstruction would it still be classed an f2 scope as far as focal ratio is concerned even though it is of course not actually f2 aperture. 🤔

If it were a lens the Rasa 8 would not be f2 but around f2.26 or AV 2⅓. With the 180mm mask I'm currently using it's now f2.59 or AV 2¾. The focal ratio would be 2.2 though.

To have an AV of 2 or f2 the Rasa 8 needs a front plate diameter of 221mm with a 94mm obstruction.

A mask of 137mm would reduce the actual aperture by 2 stops to f4 though the focal ratio would be stated as 2.91 

Alan

 

 

 

[ollypenrice]

I think the simplest calculation may be to find the area of clear aperture which, in the case of the RASA 8, is 27,572 square mm.

It's a shame about your stars. So far we haven't seen this with the one I'm involved with. I think Celestron just need to fix it for you. 

Olly

[gorann]

Yes it is a shame. Must be a bad batch of mirrors, I have not seen this on my two RASA 8, but they are a 2-3 years old. Maybe it is getting too popular and QC is slipping.

[symmetal]

Thanks for your replies Olly and Goran. Looking at previous images you've posted with the RASA it didn't look like you had this issue but I wanted to check before i pursue it further. it looks to be RASAs purchased in the past year or so having this problem, and it looks like Celestron are still having difficulty sourcing new mirrors. One person on CN had their RASA returned with the appearance that Celestron had gone over the mirror edge with a course file. It actually performed better like this though. 😲

I'll message FLO about it. I bought the last RASA they had in stock at the time a few weeks ago, though they have another one in stock now. It may well have the same issue though.

For RGB imaging it's not too big an issue stopping it down. At 200mm I was swamping the read noise by the sky background noise by a factor of 5 at 45s exposure at SQM 21.5, and masked to 180mm it takes around 1 min for the same figure. With a narrowband filter it will be more noticeable. With my FLT98 I have to expose RGB for 4 mins to swamp the read noise by a factor of 3.

Alan

[symmetal]

@ollypenrice and @gorann If possible, could you post any RASA 8 image you have with a bright star in the frame, like Sadr or Alnitak, to give a comparison. A raw fits file would probably be best, with if possible, the star in different positions in the frame. Thanks. 😊

Some flaring is expected but it being so uneven on my images being angled towards the closest edge is what's rather annoying. I've sent my test images to FLO for them to assess.

Alan

[ollypenrice]

29 minutes ago, symmetal said:

@ollypenrice and @gorann If possible, could you post any RASA 8 image you have with a bright star in the frame, like Sadr or Alnitak, to give a comparison. A raw fits file would probably be best, with if possible, the star in different positions in the frame. Thanks. 😊

Some flaring is expected but it being so uneven on my images being angled towards the closest edge is what's rather annoying. I've sent my test images to FLO for them to assess.

Alan

We don't have any bright stars in images since we changed to a U-shaped cable guide but we''ll try to shoot some for you next time out. I really don't think we'll see anything like yours, though. Our previous O shaped cable guide gave elongated halos on M45 but we had no flares at all.

Olly

[symmetal]

Thanks @ollypenrice. I've experimented with different shape 3D printed cable guides and while they remove the typical spikes they seem to cut little notches out of the stars and/or add extra little spiky flares. This might be due to the edge flare issue I currently have affecting it, and have found that the straight cross cable guide gives the roundest bright stars at the moment so used that. There are some halos visible when fully stretched and processed, but they are at least even around the stars so don't stick out like the flares. 🙂

Got several clear days forecast at the moment, though full moon of course, so can experiment more. I have noticed that the moon will also cause extra small spikey flares even with a dew shield and pointing over 90 degrees away. I found that out when testing my latest aperture mask until the moon set when it came good. The large edge flare effects are there though, with or without the moon.

Alan

[ollypenrice]

1 hour ago, symmetal said:

Thanks @ollypenrice. I've experimented with different shape 3D printed cable guides and while they remove the typical spikes they seem to cut little notches out of the stars and/or add extra little spiky flares. This might be due to the edge flare issue I currently have affecting it, and have found that the straight cross cable guide gives the roundest bright stars at the moment so used that. There are some halos visible when fully stretched and processed, but they are at least even around the stars so don't stick out like the flares. 🙂

Got several clear days forecast at the moment, though full moon of course, so can experiment more. I have noticed that the moon will also cause extra small spikey flares even with a dew shield and pointing over 90 degrees away. I found that out when testing my latest aperture mask until the moon set when it came good. The large edge flare effects are there though, with or without the moon.

Alan

We do get rather hard-edged but circular disk-like halos on some stars but what you have are highly asymmetrical flares and those we don't get.

To my mind, BTW, StarXterminator is a game-changer for the RASA, which goes gloriously deep but gives indifferent stars. (Certainly not refractor standard.) However, my present method, still under experimentation, is to give the original image a partial stretch, save it, de-star it and give the de-starred image a little more stretching and cosmetic fixing of artifacts around where the larger stars were. I then put the still starry version as a layer on top in blend mode screen or blend mode lighten, combine them and go for the final stretch. In a nutshell the stars get a lower stretch than the rest and can be made to look pretty decent.

Without StarX I'd be considerably less enthusiastic about the RASA. With it, I'm a happy bunny.

Olly

Edited August 10, 2022 by ollypenrice

[tomato]

Hi Alan,

Attached is an image of Sadr (180 sec exposure)taken with my RASA8/QHY268c/NBZ combination, the scope was purchased from FLO in August 2020, the cables were in a circular arrangement.

2021-11-22_18-54-09_-20.00_180.00s_0009-crop.fits

Crop

[symmetal]

9 minutes ago, tomato said:

Attached is an image of Sadr (180 sec exposure)taken with my RASA8/QHY268c/NBZ combination, the scope was purchased from FLO in August 2020, the cables were in a circular arrangement.

Thanks tomato. I'd be pretty happy with that. 🙂 I'll take some images of Sadr tonight without a mask for some comparisons. My test images used Vega and Deneb which are a bit extreme I admit.

I think you'll find that the darker notches at top and bottom of Sadr are effects of the cable routing arrangement but they're relatively minor.

Alan

[symmetal]

8 hours ago, ollypenrice said:

We do get rather hard-edged but circular disk-like halos on some stars but what you have are highly asymmetrical flares and those we don't get.

To my mind, BTW, StarXterminator is a game-changer for the RASA, which goes gloriously deep but gives indifferent stars. (Certainly not refractor standard.) However, my present method, still under experimentation, is to give the original image a partial stretch, save it, de-star it and give the de-starred image a little more stretching and cosmetic fixing of artifacts around where the larger stars were. I then put the still starry version as a layer on top in blend mode screen or blend mode lighten, combine them and go for the final stretch. In a nutshell the stars get a lower stretch than the rest and can be made to look pretty decent.

Without StarX I'd be considerably less enthusiastic about the RASA. With it, I'm a happy bunny.

Olly

Thanks for the starless image tips Olly. I've tinkered with Starnet++ but when I add the stars back in it looks like an image which has had the stars added back in. 😁 I must try harder.

Alan

[tomato]

I find StarXTerminator to give better results than Starnet most of the time, particularly when adding the stars back in. The latest version has an enhancement that works well with PixelMath, ~((~starless)*(~stars)).

[gorann]

When you ask for it I realize, like @ollypenrice that I have not aimed my RASAs at any object s with very bright stars, but in any case, like Olly, I have not seen anything like the star anomalies you have so something must be very wrong with the scope, and apparently Celestron admits it.

I hope you get it sorted in one way or another because a well-functioning RASA8 is just a worm hole into deep imaging!

Edited August 11, 2022 by gorann

[symmetal]

4 hours ago, gorann said:

When you ask for it I realize, like @ollypenrice that I have not aimed my RASAs at any object s with very bright stars, but in any case, like Olly, I have not seen anything like the star anomalies you have so something must be very wrong with the scope, and apparently Celestron admits it.

I hope you get it sorted in one way or another because a well-functioning RASA8 is just a worm hole into deep imaging!

Thanks Göran. Actually medium brightness stars do show the same flaring effect though at a much lower level of course. I took some images of Sadr today with a U shaped cable router I printed to compare with the image @tomato posted and the asymmetric flaring is still present on mine. It was a full Moon though so the darker parts of the flare were washed out and hidden. Mine was 10s RGB while tomato's was 180s narrowband so isn't really a fair comparison. I gave both a similar quick stretch and set the background level to be the same. Here's the result. The dimmer stars in the lower image are totally washed out by the Moon in the top one

My RASA 8, 10s RGB at full Moon, cropped

tomato's RASA 8, 180s NB, cropped

Sadr was in a similar position in the frame for both images. Actually once you place the bright star outside the good area with no flare, which at full aperture I found is a small area about 400 pixels across, just below the centre of the frame, the flare suddenly appears and doesn't change a lot as you place the star closer to the edge of the frame.

Alan

[ollypenrice]

5 hours ago, symmetal said:

Thanks Göran. Actually medium brightness stars do show the same flaring effect though at a much lower level of course. I took some images of Sadr today with a U shaped cable router I printed to compare with the image @tomato posted and the asymmetric flaring is still present on mine. It was a full Moon though so the darker parts of the flare were washed out and hidden. Mine was 10s RGB while tomato's was 180s narrowband so isn't really a fair comparison. I gave both a similar quick stretch and set the background level to be the same. Here's the result. The dimmer stars in the lower image are totally washed out by the Moon in the top one

My RASA 8, 10s RGB at full Moon, cropped

tomato's RASA 8, 180s NB, cropped

Sadr was in a similar position in the frame for both images. Actually once you place the bright star outside the good area with no flare, which at full aperture I found is a small area about 400 pixels across, just below the centre of the frame, the flare suddenly appears and doesn't change a lot as you place the star closer to the edge of the frame.

Alan

It might be worth finding a flaring star, imaging it, rotating the camera and imaging it again,

just in case the flare is produced outside the telescope. Does the flare follow the rotation or not?

Olly

[symmetal]

7 hours ago, ollypenrice said:

It might be worth finding a flaring star, imaging it, rotating the camera and imaging it again,

just in case the flare is produced outside the telescope. Does the flare follow the rotation or not?

Olly

I'll give that a go tonight Olly. I've tried it with and without the dewshield I made and there was no change in the flaring. I was actually surprised that with the full Moon yesterday around 70 degrees away there was really no significant difference in the look of the image at all.

Alan

[symmetal]

Here you go @ollypenrice

Again full Moon and I used Deneb as the subject for 10s exposures and screen grabs of a standard stretch in SGP for the images. I think this rules out the camera as having an effect on the flares. The dark cutouts of the flares seem predominantly on the right for the first composite and the bottom for the second composite are effects likely effects due to the cable routing. 

Here's the best spot in the image for minimum flaring. Moving slightly from this spot will give the asymmetric flares.

Alan

[ollypenrice]

8 hours ago, symmetal said:

Here you go @ollypenrice

Again full Moon and I used Deneb as the subject for 10s exposures and screen grabs of a standard stretch in SGP for the images. I think this rules out the camera as having an effect on the flares. The dark cutouts of the flares seem predominantly on the right for the first composite and the bottom for the second composite are effects likely effects due to the cable routing. 

Here's the best spot in the image for minimum flaring. Moving slightly from this spot will give the asymmetric flares.

Alan

Right, so you can pin the issues on the scope, I'd say. It might be worth giving this info to Celestron and/or the dealer.

Olly

[gorann]

I do not think you need to put any more effort into diagnosing the illness. The conclusion is that Celestron / your dealer has to replace your scope. I hope this new disease will not strike many more before it is cured!

[tomato]

Well, at least you now have more information on the issue than before you started the thread. 
Have you decided on a course of action? As you say, a replacement RASA could possibly have the same defect, but I suppose FLO could easily test it before shipping.

The stop down mask is a quick and relatively easy fix but that solution would rankle with me on a scope costing in excess of £2K.

A third option might be to obtain a refund and wait for one of the right vintage to come up on the classifieds, they do appear from time to time.

The filed down mirror photo on CN does look rather grisly, but it appears to have reduced the flaring. It does look to me like poor tooling set up at the factory.

Steve

[symmetal]

@ollypenrice @gorann @tomato Thank's for your help and comments.

I sent all my previous test images to FLO earlier in the week and they replied they will share them with Celestron and work out a plan of action. FLO said I'm the only one who has reported an issue with the RASA 8 to them.

I find it strange that on the CN thread, Celestron are still saying no replacement mirrors are available to properly repair those sent back to them, but RASA 8 scopes are still available for sale. I assume they know which mirror manufacturer or batch caused the problem scopes and that they aren't still selling them without checking them for this issue first.

Putting an aperture mask just in front of the mirror and losing a couple of mm or so, I thought would be a better fix by Celestron on the problem mirrors, and would save having to use a larger mask further away like I've had to do.

For those willing to accept 180mm aperture or so, Celestron could offer to refund a portion of the cost of the scope.

I'll send the latest tests to FLO as well and await their reply. Hopefully some time next week.

Alan

[symmetal]

FLO have to date not heard anything back from Celestron, so have kindly offered to exchange my RASA 8 for a new one they had in stock. 🤗 Returned mine last Thursday and new one is due tomorrow. Up until yesterday Clear Outside had three clear moonless nights from tomorrow. Needless to say all three have now gone yellow/red. 😟

Fingers crossed the new one is a good one, and I get a chance to confirm that in the next few days. 🔭

Alan

[symmetal]

Had an hour of clear skies before the clouds appeared, so could test the replacement RASA 8. Unfortunately, it produces very similar results to the first RASA 8, though the small area of no flaring is more centred on the replacement. The two dark 'spikes' to the right on each image are an artifact of the semi-circular cable router.

 

I thought that not having an UV-IR cut filter in the path might be an issue as the RASA spec says it focuses wavelengths between 400 and 800nm but doesn't specify the actual pass band width. However the ASI2600MC protect window is an UV-IR cut filter, passing 400 - 690nm so that isn't an issue. The 2" filter holders for the RASA and 17.5mm back focus cameras aren't in stock at FLO at the moment for me to use an external filter yet.

If possible could @ollypenrice, @gorann or @tomato take a similar wideband image of Deneb to have a direct comparison please, as I'd like to see what a 'good' RASA is like. 😊

At the moment, if I have a bright star in the image I'll have to put an aperture mask on and increase the exposure accordingly. I'm impressed with the lower brightness star shapes and sizes over the full APS-C frame on both RASAs. The first one I had to use a 0.5mm metal spacer to get round stars in the corners, though the second one is good without extra spacers. Getting the camera tilt set up on the test jig beforehand was essential though, as both RASAs themselves were tilt free. 🙂

Alan 

[gorann]

A bit too late - I had a clear sky last night and was shooting away with my dual RASA-rig, but now clouds have moved in. I will try to remember your request next time it clears😉

[Fedele]

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