Bad stars

[spectre]

Hi All,
I have been having some problems with deformed stars as well as color gradients across the stars. This is only 12 X 60 seconds of NGC 7789 using my ES ED102 CF APO with the ES field flattener and my unmodded DSLR. Can you tell me what would cause the color gradients across the stars?
The deformed stars in the corners are radial to the center of the image and I am not sure if this is due to too much spacing between the FF and the DSLR sensor or not enough. I know I saw this discussed in another post but I have not been able to find it again.

This image was fully processed in PI. I didn't spend a lot of time on it since I am only trying to diagnose my problems.


Thanks in advance

John

[STEVE333]

Hi John - I have an ES ED102 with a Stellarvue FF/FR and I have the same problem. My color gradients are actually much worse than yours.

What image processing software are you using? The reason I ask is that there is a simple solution using PixInsight thar corrects the problem.

Steve

[Stuart]

It doesn't look too bad to me, actually.

[spectre]

Thanks for your quick replies.
Steve, I'm using OK for all my processing. I would appreciate it if you could let me know what that simple solution is. I confess that I'm just scratching the surface with PI.

Stuart, The color gradient gets lost when I downsize my pictures for posting. The color gradient is quite obvious in the full rez image, blue on one side and red on the other.I

Thanks for any help you can provide.
John

[Baskevo]

Spectre, I just posted similarly about this, as I am having the same issue. I have been researching all night and found a few things.

I read this on the reviews for the field flattener on Explore scientific's website:
My understanding is that this flattener is intended for use with only CCD cameras, not DSLRs and requires a distance to the sensor of 45mm.

And then this one also...
I have been successfully using this Field Flattener with A DSLR. I just went to a CCD and also had to scour the internet to find the back focus which I have read is 45mm. I have asked Explore Scientific what the back focus is without a response.

I have also read on cloudy nights that ES states the back focus is 45mm (which I'm not sure exactly what that means... I think 45mm from the focuser? or maybe from the sensor?), while others say it's actually 55mm... So we would need additional focus extenders, maybe..? I extended to about the 47/50 marker on the focuser (which I don't think is a marker for the back focus, but again I'm not sure), with both extension tubes attached, and if I went any further, I would lose focus (using a bahtinov mask).

I found this on cloudy nights:

I have that field flattener and it is not intended for use with dslr's only dedicated astro cameras. It has only 44mm of backfocus. I have a ES ED102CF and I use this one:

https://www.astronom...refractors.html

It works great with my Canon/ED102 combo and should work with any f6 to f8 refractor. It has a 57mm (+or- 4mm) of backfocus.

I use it with this Canon t-ring (which happens to be a t-ring OAG combo)

https://www.teleskop...the-T-ring.html

but should work with any t-ring (depending on camera and type of t-ring you're using you may need a small spacer). As long as the distance from the sensor on your camera (marked on the top of the camera) to the end of the threads of the field flattener is between 53 and 61mm you should be good.


So apparently it doesn't work with DSLR's? I don't know if anyone here with the ES has any experience with this... Others recommended the Orion field flattener for short refractors.

[Baskevo]

Grr... I then just found this on Highpointscientific.com in the description of the product:

[/I]This version of the FFEDAPO-01 has 55 mm of back focus (plus or minus 2 mm) and is designed for cameras with APS-C or smaller sensors. It will not produce a flat field when used with a full-frame camera.[/I]

And when I had talked to them on the phone about my order for this being delayed, the guy was super helpful, giving me recommendations on my guiding... I think he would've told me, if he knew, that the flattener would not work for the DSLR... So I'm still lost.

And this was in one one of the reviews on highpointscientific
[/I]I use this with my Canon DSLR and the Explore Scientific ED80. I need to use two extenders to get focus. My stars are slightly elongated towards the edges but that might be because I’m not autoguiding yet.[/I]

[Baskevo]

Got an update for you Spectre, and I hope this isn't considered spam. ES customer support got back to me and said that the back focus for the 2" 55mm Field Flattener (if it doesn't have 55mm engraved on the flattener, than it is a 45mm back focus) will either be 55" or 57" back focus..? So I think we need more space between our DSLR's and the Field Flattener. You have a different camera than me, so it might be different. You'll have to find the flange to sensor distance (mine was marked with a phi symbol on the top of the camera, and I measured it with a caliper).

I hope this helps...

[JayTee]

Got an update for you Spectre, and I hope this isn't considered spam. ES customer support got back to me and said that the back focus for the 2" 55mm Field Flattener (if it doesn't have 55mm engraved on the flattener, than it is a 45mm back focus) will either be 55" or 57" back focus..? So I think we need more space between our DSLR's and the Field Flattener. You have a different camera than me, so it might be different. You'll have to find the flange to sensor distance (mine was marked with a phi symbol on the top of the camera, and I measured it with a caliper).

Please keep in mind that the distance from the Canon camera flange to the sensor is ≈44mm then when you add the 10.5 mm for the T-mount, there is your almost 55mm. So you need to screw it directly onto the T-mount.

Cheers,
JT

[Baskevo]

Got an update for you Spectre, and I hope this isn't considered spam. ES customer support got back to me and said that the back focus for the 2" 55mm Field Flattener (if it doesn't have 55mm engraved on the flattener, than it is a 45mm back focus) will either be 55" or 57" back focus..? So I think we need more space between our DSLR's and the Field Flattener. You have a different camera than me, so it might be different. You'll have to find the flange to sensor distance (mine was marked with a phi symbol on the top of the camera, and I measured it with a caliper).

Please keep in mind that the distance from the Canon camera flange to the sensor is ≈44mm then when you add the 10.5 mm for the T-mount, there is your almost 55mm. So you need to screw it directly onto the T-mount.

Cheers,
JT

Awesome I'm glad you said that... 44mm is what I found what one person said online, but I didn't know if it was right... I also measured it myself and that's about what I got, along with around 10mm is what I measured for the T-ring.

So those pictures that I shared, with stars looking similarly to spectre's, was with the FF screwed directly into the T-ring and attached to the camera. I'm guessing then that I have a FF that is a big longer, at 57mm back focus? I bought some spacers online... I'm going try those out.

[spectre]

Thanks everyone. I measured my camera and t-ring and it looks like I need more space between the sensor and the FF. I have some delrin spacers and I'll try it all again if, I mean, when, the clouds go away on a weekend.
John

[Baskevo]

I ordered some spacers myself, will be here in about a week. I'll let you know if it works. I have a feeling it's not going to make much of a difference though...

[Baskevo]

I put on the spacers tonight, and they didn't make a difference for the stars... I don't know what to do now

[Baskevo]

So it looks like, with the spacers, the field flattened out better, but my stars still look really weird... Here's a screenshot of an auto-stretched, stacked image (looks like this when it's not stacked, too):

I tested it out and the stars only look like this when I have the field flattener on. If I go out of focus a little bit, the stars look normal.I can't add anymore spacers... Should I ask for them to send me a new field flattener?

[Baskevo]

New Field Flattener came, but of course it is ridiculously windy But I can already tell the new Field Flattener is NOT the same as the one I had... Weird. It just feels like better material, the serial number is printed on it, and the t-threads on the female side is taller. It's almost like the one I had originally was a knock-off version!!

[spectre]

Well, I've finally had a chance to try this again with more spacers. It seems that the more space I have between the sensor and the flattener, the worse the stars become in the corners. Seems like I need less space than I am able to get with the T-ring I am using now. Does anyone know if there is a 'thin' T-ring for Canon EOS cameras? Or if there is another way to reduce the spacing?
Maybe I need to get a different flattener even though this is the one that came with the scope.

Thanks in advance
John

[pakarinen]

It doesn't look too bad to me, actually.

Looks pretty good to me, but I don't do AP, so...

[Baskevo]

Well, I've finally had a chance to try this again with more spacers. It seems that the more space I have between the sensor and the flattener, the worse the stars become in the corners. Seems like I need less space than I am able to get with the T-ring I am using now. Does anyone know if there is a 'thin' T-ring for Canon EOS cameras? Or if there is another way to reduce the spacing?
Maybe I need to get a different flattener even though this is the one that came with the scope.

Thanks in advance
John

(I'm a total pro at this now ;D lol)

Does your Field Flattener have "55mm" engraved on it spectre? If not, you have the 45mm Field Flattener which is the older version. They do sell a thin t-ring specifically for that version. I hadn't seen in your original post that your stars are going TOWARD the center. That means that you need less spacing.

But honestly looking at your original image, it doesn't look that bad... I would download CCD inspector which helped me a lot, because it can be hard to tell what looks better than before... I finally got mine to "good enough," with around 85mm of total spacing in between the Field Flattener and the camera sensor... So that goes to show you that "it's never what it says it is" which is something I heard a lot trying to figure this out

[KathyNS]

Please keep in mind that the distance from the Canon camera flange to the sensor is ≈44mm then when you add the 10.5 mm for the T-mount, there is your almost 55mm. So you need to screw it directly onto the T-mount.

Cheers,
JT

Any DSLR with its T-ring has a flange-to-sensor distance of exactly 55mm. This is a defined standard that all T-ring makers follow. So if your T-ring is 10.5mm deep, you can say with some certainty that your camera body is 44.5mm deep from flange to sensor.

I point this out because it is a useful number to know: all DSLRs are 55mm deep when the T-ring is installed. It is no accident that so many devices like field flatteners and coma correctors are designed for a back-focus distance of 55mm.

[Baskevo]

https://www.highpointscientific.com/exp ... ing-510369 That's the t-ring I'm talking about, that is made specifically for the older 45mm back focus explore scientific Field Flattener.. If your explore scientific Field Flattener does not have "55mm" engraved on it, then you need this t-ring

[spectre]

Thanks James, that looks like exactly what I need. The trouble with AP is that, when something is a little off, it really gets to be a bother when you know it could be better. That's where I'm at right now. It really doesn't look that bad, but I know it could be better and it'll just keep bugging me.
I'm off to Highpoint to see about getting that t-ring. With any luck this will fix my problem.
John