Jump to content
Sign in to follow this  
timthelder

Of Anomolies and Gradients

Recommended Posts

Revisiting my last short image session. I am wondering why I am getting this annoying gradient in my final stacked images. It appeared actually a few objects ago, I attributed it to exposure time or ISO setting, etc. Maybe time for a new camera? My 450Da is nearing 5000 clicks...

Another thing, (which I think might be those pesky lens spacers on 100ED doublets,) is the weird spikes I've started getting.

A few examples...A 300 second sub.

IC396-002_example_zps509595bc.jpg

Master Dark

MasterDark_ISO800_300s_example_zps2f09e252.jpg

Master Flat

MasterFlat_ISO800_example_zps9bc60223.jpg

Master Dark Flat

MasterDarkFlat_ISO800_0s_example_zps8d81845b.jpg

The resulting stacked images...Gradient very noticeable across the image

IC396-4example_zpsfd638e84.jpg

And a reprocess of the Pleiades

IC396web_zpsce72372d.jpg

Any ideas?

Tim.

Share this post


Link to post
Share on other sites

Tim I had something very similar with my Orion 120mm EON ED. It was that during cold weather imaging that the lens cell that is made of aluminum would contract faster than the optics and cause it to pinch the optics. I sent a copy of this image to Orion and they had me return the scope for another one. The new one I received has not had that issue. I submit this image and you can see it has 5 distinct spikes. Your spikes are to a lesser degree but you may not have tried to image when it is very cold. I took this image when it was 10F and the optics pinched very badly. I think it is the design of the lens cell not allowing enough space for contraction. It would also contribute to the gradient as it is warping the optics.

This image was taken with an Orion 120mm EON ED Refractor and should have no diffraction spikes at ALL!

11112218425_c88bbc0e7d_c.jpg

This refractor was returned to the vendor (Orion) for replacement. The replacement has not exhibited this anomaly and that was well over 5 years ago.

Eric

Share this post


Link to post
Share on other sites

Thanks Eric. It wasn't that cold when I captured the above images, only about 50F or so. When I first got my 100ED, my first images the stars looked more like Maltese crosses. I found this fix for the problem...

http://www.andysshotglass.com/100EDfix.html

I wasn't aware of another one, that you mentioned, (although I remember a past member sending their 102ed back to Orion.) Per my posted fix link, I have had my doublet apart before to correct my first experience with the felt spacers that protrude into the FOV by about a mm. On one other occassion I had a small 'bug corpse' lying at the bottom of the lens retainer ring which created similiar results.

Putting the two lenses back together again will be a bit of a bugger, but that's what I'm thinking needs to be done ATM to see if it provides a fix.

I DID find out several years ago, to NOT turn on dew heaters in the middle of a session as it will change the image focus. If I use heaters I turn them on an hour or so before focusing and/or imaging to allow things to be stable as possible.

The gradient I'm getting is an entirely different animal. 1.) It didn't used to be there. 2.) I haven't changed my capture and/or stacking routine 3.) Darks, Flats, etc. doesn't show the gradient.

Also in the last couple of images the camera orientation was different, (north-up on one, north to the left on another, also a different scope too I think.) That's why I am also leaning to a potential camera problem.

Nothing like multiple problems...

ATM, I'm thinking it's time for some planetary imaging...Different camera, different scope, different programs.

Afterthought...maybe I tightened the lens retainer ring down too much to allow for some expansion/contraction.

Gotta luv it!

Tim.

Share this post


Link to post
Share on other sites

Tim. It indeed could be the retaining ring is to tight. Another thing is that the objective lenses are indexed. There are subtle markings on the lens edges that indicate how the 2 are aligned to each other. I know this applies to the 120mm ED optics so I don't know why they would not also do it with the 100mm optics. Look for markings like in the pictures I list below. These markings are subtle and only seen when light hits them just right. They are made with a grease pencil.

Link to image 1: Index marks on 120mm ED

Link to image 2: Index Marks on upper and lower elements

Link to image 3: Negitive image showing more detail

Also look at this site on page 5 in the middle of the page there are some more shots of the lens disassembled. :

Photo bucket Page 5

Eric

Share this post


Link to post
Share on other sites

Grease pencil...really?! When the opportunity arises I will definitely give that a try. As I said, had it apart before but those markings must be extremely subtle.

Thanks, Tim.

Share this post


Link to post
Share on other sites

Real quick...I previously, (a few years ago,) looked for marks on the side of the lenses and coud discern none with 100% certainty. BUT I wil re-check.

Share this post


Link to post
Share on other sites

Can't really offer anything here that hasn't already been suggested.

It was that during cold weather imaging that the lens cell that is made of aluminum would contract faster than the optics and cause it to pinch the optics. I sent a copy of this image to Orion and they had me return the scope for another one. The new one I received has not had that issue.

Eric how long did that to work out? Some serious detective work.

Ray

Share this post


Link to post
Share on other sites
Can't really offer anything here that hasn't already been suggested.

Eric how long did that to work out? Some serious detective work.

Ray

Ray It only took them about a day to send me that RMA and shipping stickers and about another week to evaluate the unit. I had the new scope with in 2 weeks.

Eric

Share this post


Link to post
Share on other sites

Hi Eric,

I meant how long did it take you to realise you were dealing with structural contraction / expansion? That would be a last resort for most.

Ray

Share this post


Link to post
Share on other sites
Hi Eric,

I meant how long did it take you to realise you were dealing with structural contraction / expansion? That would be a last resort for most.

Ray

I miss-understood your question. I got the scope in late summer early fall and it performed quite well, then there was a month or so of bad weather where I did not use it at all. Then finally in mid December, and it was quite cold, I set up to image Orion. That is when I noticed the spiking and had seen this before on a friends refractor. At first I thought it was that the objective retaining ring was to tight from the factory. I loosened it and retried it the next night, still the same problem. I tried a third night and realized that when at room temperature the anomaly wasn't there but as it cooled rapidly to out side air temp the anomaly became more and more pronounced. I tried running my heater wide open and that did not seem to help much. It was obvious to me that if the anomaly wasn't present in the warmer months and showed up in the cold ones that the cell was pinching the optics. Also the images taken while it cooled indicated the cell was contracting at a rapid rate. I also thought that the optics needed collimation but the cell is CNC machined with no way to collimate it. Tolerances are very tight on it. That being said, the answer to your question would be about 4 months and it was a last resort for me also.

Eric

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×