Refractor - spikes in image center. What am I looking at? - Forums

Topic starter

Jan 20, 2022

Hi ,

so I recently got an Esprit 100 (optimized) and a QHY268m camera. Tonight, third night of usable skies since the purchase, I noticed something disturbing when imaging bright stars.

Whenever a star is moved into the center of the image, it starts displaying severe spikes, like I was imaging through a Newtonian. The pattern mostly dissapears towards the edges of the image.

I have zero idea what this could be about; is it the lenes being out of alignment, something wrong with the lense holder? Or is something wrong with the camera?

I tried various read out modes and filters. But the pattern remains. I am a bit lost - maybe someone here has seen this and knows what could be wrong?

Star in image center (crop):

Same star, at the edge of the image (crop)

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Marc

Topic starter

Jan 20, 2022

Well, apparently the most likely culprit are cobwebs…. difficult to say in the dark, but seems some tiny spider may have gotten into the tube (unclear how, camera is permanently connected to the tube…).

Unless someone has an alternative explanation.

Björn Arnold

Jan 20, 2022

Hi Marc,

You can certainly check for undesired inhabitants within the tube.

A first check to see if the effect is caused from the telescope is to rotate the camera w.r.t. the scope. If the pattern rotates, it’s from the scope and not the camera part.

My guess is diffraction on lens spacers. I don’t know the exact design of the Esprit but some lenses have three or six spacers (I‘d guess it’s six in your scope based on the angle between the spikes).

CS!
Björn

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Michael

Jan 20, 2022

I'm not sure as my problems look a bit different… (I didn't have as bright of star in frame) but my Esprit 120 is possibly showing some pinching in colder temps (-14c during the session in question). Haven't confirmed but I'm pretty sure it's what I was looking at. Stars look almost hexagonal shape in the image (stacked).

I haven't addressed my issue yet as I'm just hoping for warmer weather for now. Was this a cold session for you?

andrea tasselli

Frequent contributor

Jan 20, 2022

If it were either cobwebs or spacers the pattern will be seen across the field, which isn't. I had it sometimes with a number of lenses and my explanation is that it is due to small condensation droplets at the centre of the lens (or a serious defect on one of the lens).

Stjepan Prugovečki

Jan 20, 2022

Michael:
I'm not sure as my problems look a bit different... (I didn't have as bright of star in frame) but my Esprit 120 is possibly showing some pinching in colder temps (-14c during the session in question). Haven't confirmed but I'm pretty sure it's what I was looking at. Stars look almost hexagonal shape in the image (stacked).

I haven't addressed my issue yet as I'm just hoping for warmer weather for now. Was this a cold session for you?

My Esprit 120 shows pinched optics already at -6C and stars are hexagonal across the field , getting worse at corners. However, Marc's case is not like that . Marc has something in the light path . Fingerprint or something similar somewhere ?

Leonardo Landi

Jan 21, 2022 · Edited: Jan 21, 2022

I've had a very similar problem with an astroprofessional 80 ed. There was a fingerprint between the doublet leaved by the manifacturer. Your esprit is a better quality instrument indeed, but the pattern seems quite similar.

Marc

Topic starter

Jan 21, 2022 · Edited: Jan 21, 2022

Yeah so the spikes do not move when I turn the camera/filter wheel - so it's probably nothing wrong with the tube. Need to dissamble the whole camera setup. I am starting to think it might be some spec of dust on the sensor, or close to it - at least that would perhaps explain why the problem only occurs in a very narrow part of the imaging field.

Marc

Topic starter

Jan 28, 2022

I have now dissambled the entire setup, looked at everything under a magnifying glass (not kidding…) and found..well, nothing. A few specs of dust on the various surfaces, but nothing substantial that would obviously explain my problem here.

From the rotation (or lack thereof), it does seem to be between the rotator and the camera.

I am wondering now if this could indeed be moisture-/condensation related. The camera has a heated window, so probably not the source of the problem. The corrector is perhaps a different story… but nothing I can easily check in a dry run. Gotta wait for another clear night.

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Carles Rabassa guixé

Jan 28, 2022

I had that some time ago, and it was due to an scratch on the IR filter coating… but since you say you don't see anything then no idea. :/

Marc

Topic starter

Feb 14, 2022

After disassembling the entire setup and wiping all the surfaces, the spikes appear to be gone. Never really figured out what caused them, but I am not complaining .

rhedden

Feb 14, 2022 · Edited: Feb 15, 2022

Glad your spikes are gone! I also have the Esprit 100ED and the QHY268M, and they work fantastically well together. Not only is the field of view enormous compared to my past rigs, but the whole sensor is well-illuminated such that flat frames really are almost flat. Once I minimized tilt, the star shapes were good and the color fidelity was excellent across the sensor. I am hoping to do some widefield galaxy imaging with this rig in the coming month or two, since two or three targets can often be captured in one field of view, with just enough aperture and focal length to provide satisfying results at 1.4" per pixel (and 2x drizzle for good measure).

My Esprit 100ED shows signs of pinched optics in cold weather as well. I sometimes get hexagonal stars with six or more spikes. Here is an example from last night's imaging session; ambient temperature was -12.7 C or 9 F.

When I have obtained ugly/asymmetric spikes like you are showing (with any telescope), it has always been something with the dew shield. If there is a protrusion of some sort in front of the objective, ugly spikes can result. A piece of debris sitting on the objective (e.g. fallen leaf, patch of water/ice, or insect) can also produce spikes of this sort. Double-check the inside of you dew shield to make sure it's completely free of foreign material next time you set up. It sucks to have a long night of perfect imaging weather, only to find out that your stars have an ugly protrusion when you stack the data later on.

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