Edge HD 8” M57 & NGC6888

[JonHigh]

Managed to get my rig out the other night and captured the Crescent Nebula (30x 120sec exp, 1hr) and M57 again (300x6Sec exp, 30 mins) through awful seeing conditions and hard to focus tack sharp. Going to do some tests next time out to try and establish where in the image train I’m getting the elongated stars from. I have a feeling this might be camera tilt as my collimation seems spot on? Or is my back focus a bit too close? Although I’m currently using the matched reducer I might check without it next time. Also, considering the price of the reducer my copy seems to have more CA than I would like. I don’t think I’m alone on that matter! 

Edited August 8 by JonHigh

[assouptro]

Hi Jon 

great images for the short  exposures and integration

Regarding the elongated stars I have owned and regularly image with a traditional non “edge” SCT for many years.

I have spent hard earned money and more importantly precious imaging time trying different focal reducers/field correctors, tilt adjusters trying to combat it including hours of experimenting with distances and agonising over the bad corners or sides of my subframes using software that is useful such as astap and Nina that give you numbers on a screen in real time allowing adjustments but sometimes distracting me from the beauty and wonder of what is achievable from a suburban back garden with consumer grade equipment 

I would love to have perfect optics and repeatable results but I’m not sure I will ever achieve this without buying a high end well corrected triplet refactor and at the image scale I enjoy this would be financially unachievable without a lottery win! (Note to self, start buying lottery tickets!) 

Having never owned a flat field SCT with matching reducer I can only imagine your concern with the less than perfect results and if it was new I would feel a bit “cheated” by the ads and claims of these manufacturers, questioning the quality of the examples I had been sold 

It could potentially be some sag in your imaging train, a potential tracking issue (although your central stars look good so this is unlikely), slightly misaligned optics, camera tilt, or a faulty reducer? There looks to be some astigmatism in one corner that I think can only be a result of the reducer? (I’m in no way an optical specialist and could be wrong?) your proposal of imaging without it will help with this question but the smaller fov could reduce exposing the possibility of sensor tilt? 

One option, if you bought second hand and have no means of recourse is to use blurXTerminator software, it really does take the pain out of struggling to achieve optical perfection, although I’m sure we would all prefer the former but  it just works! 
 

Sorry for the long ramble only I feel your pain trying to solve issues like this that can cause stressful pixel peeping and can be a “joy killer” in what should be a rewarding hobby allowing us to revel in the majesty of the otherwise invisible heavens! 
 

I wish you well and hope you get to the bottom of the issue 

Thanks for sharing 

Bryan 😊

[Roy Foreman]

Nice captures Jon. I have both 8 and 11 Edge models and have never had elongated star problems. Unlike a regular SCT the back focus has to be just right. At prime focus you can be a few millimeters out, but with the reducer it has to be exact. Using celestron's T adaptor gives you the correct spacing with the usual 55mm to the sensor. A pair of digital calipers is a great aid in measuring from the nut to the camera faceplate. Good luck.

[JonHigh]

Hi and thank you for your replies. I am pleased with the results especially from such short acquisition times. I must admit I do feel like I’m starting to go down a rabbit hole so to speak to try to figure out what is going on as this scope is an awesome bit of kit nonetheless! So after much tinkering, did you nail down your issues and if so, what did it come down to?

Buying really high-end equipment is so far out of reach it’s not even funny however although my equipment is not high-end, it still wasn’t cheap so there is an expectation there for the common man spending what I would consider a lot of money on a hobby (Mount, 4” triplet, EdgeHD, 26c camera, ASI585 and all the accessories don’t half add up!). I guess it’s all relative! I bet observatories spend a lot of time figuring these things out too! 😂 I am surprised about the amount of CA produced by the reducer but not sure if that's just to do with the telescope design or the QC isn't that great at the Celestron factory? Maybe they forgot the retail price of these things! 
With regards to the elongated stars, I haven’t used digital callipers so that’s something I will get out later just in case, so thanks for the suggestion Roy. I use the Celestron EdgeHD T-adapter so I know the backspacing should be correct up to 55mm which I’ve made up from extension rings, filter holder and then the camera. So I assume the back focus is correct but again I will double check that. 
I have used ASTAP and Siril to show the tilt on a sub frame but each come out somewhat differently so not sure which to believe! So going to do some tests tonight if it’s clear! I will be using different cameras with/without the reducer and filterless. Must also remember to lock off the focus! Just hoping that it isn't an issue with the 26C sensor! Yes, I do agreed that taking off the reducer may not show the issues as the FOV will be narrower but need to check. OCD!!! 

Shame BlurXTerminator is only available for PI and not intergrated into Siril like Starnet++ otherwise I would buy it based on the excellent results from NoiseXTerminator in Affinity Photo! 

Edited August 9 by JonHigh

[Roy Foreman]

One more thing to add. I don't use the mirror locks on my Edge HD's as they move the mirror slightly and throw collimation off a bit. That could cause star shapes to distort a bit too. I have noticed a bit of CA with the reducer, so it must be a design flaw.

[JonHigh]

Oh!  That's interesting.  Thanks for that. I didn't think it would alter collimation but thinking about it the shift would knock it out. The forecast predicts its going to be clear tonight so thats good advise when I set up later!.

Do you use the reducer on your EdgeHd's? 

[Roy Foreman]

1 hour ago, JonHigh said:

Oh!  That's interesting.  Thanks for that. I didn't think it would alter collimation but thinking about it the shift would knock it out. The forecast predicts its going to be clear tonight so thats good advise when I set up later!.

Do you use the reducer on your EdgeHd's? 

Yes, but only on the 11. Good luck !

[JonHigh]

Took the shots the other night, posted one with and one without the reducer and has been sent off to a Celestron engineer. So hopefully I will get a response soon! 

Edited August 13 by JonHigh