Internal reflection? trouble shooting suggestions

[obrads]

Hey everyone :hi:, i have had two imaging nights so far with my ASI1600mm, the first night i had some focuser tilt issues so i fixed that by tightening things up and then last night when i went shooting i noticed i had these strang ghost stars in my images. any idea what it might be and what steps i should take to try eliminate the issue? :question:
im using a saxon ed 100, 0.85x ed 100 focal reducer and flatterer, ASI1600mm with filter wheel and the zwo off axis guider with a QHY5L 11. thanks guys

[obrads]

here the the images btw

[JA]

The edges of the OAG will (may) cause some noticeable diffraction and reflection. Who knows how much? It would depend on alignment & other conditions....

There are 8 spikes (diffraction spikes) and fuzz around some of the brighter stars? You are using a refractor so I wouldn't have expected any diffraction spikes in the image, so look for some sharp edge/s in the optical path or near the edge of the path. Can you try to image WITH the OAG in place and WITHOUT the OAG in place? That might be something to rule out. Any obstruction (including an aperture stop in a lens) can cause such things, although the number of spikes and the look of the diffraction will vary.
Best
JA

[lazjen]

The images do not appear to be in focus to me - I think you need to tighten this up as well.

Perhaps use a very short time, high gain exposure to get the focus itself better? Are you using any automation to help with focusing?

[PKay]

I have ASI1600MC and field flattener.
The required distance between camera sensor and flattener is 55mm.
Without this, focus issues.
See diagram. Note the 6.5mm back focus to the sensor (included in the 55mm).

[obrads]

Hey everyone, thanks for the replies, yes i know diffraction spikes are not normal on a refractor, i dont mind them though since they are even. i do have a motorized focuser but these where just single test images and i wasnt to stressed about the focus. there are a few sharp edges with the spacers for my camera and OAG, is there some special sort of paint i can use to make sure they are not reflective?

[obrads]

someone told me that light could be reflecting off the sensor or the filters? is this true? if that can happen i dont see how you can fix it

[bojan]

Well, you can't fix it... maybe in final, retouch phase.
Or, try with combining couple of images taken at slightly different directions...
I had those too (Canon FD 400mm F/2.8), and knowing what they are, I am simply ignoring them.

[obrads]

hmmm interesting, ill be really disappointed if it is something that can not be fixed, how do you go about dealing with them? i get one of those glowy patches for each bright star

[Merlin66]

May be worth trying a circular polarising filter in the train....these effective suppress reflections between glass elements - works very well with "problem" set-ups for solar imaging where reflections can be a real PITA!!

[billdan]

Hi, I just had a look of your 2 images.

The Halo around Alnitak (in the horsehead image), is a reflection off the camera window. Alnitak is so bright it bounces off the chips' cover glass onto the camera window glass and then back to the chip out of focus thus it becomes a halo.
As Bojan explained you can rub it out in post processing, but its a pain in the neck to do. Alternatively you can remove the glass window on the CCD and re-install it slightly tilted. Like windscreens on cars are tilted to stop headlights reflecting back into oncoming traffic.

Above Alnitak is what looks like a football and you see it in the Pleiades image, I thinks this is a dust bunny. A speck of dust on the camera windows, or on a filter.

As for the spikes, no idea where they are coming from.

There you go, my 2c worth.

Cheers
Bill

[obrads]

merlin ill have a google see what i can find out about those filters :)
billdan, ah ok now i know what the hales are thank you! with the dust bunnies however, they move around the imaging depending on where the bright star is? would dust do that? also when combined with LRGB one glow dot has red blue and green at slightly different positions

[obrads]

i found this on a site and it looks very similar to the glow spot i have, but the site doesnt explain what it is or how to fix it
http://www.darkerview.com/CCDProblems/internalreflection.php

[billdan]

Dust bunnies don't normally move around. If you were to take a flat frame this would confirm whether or not dust bunnies are present.

[bojan]

Here is mine (greenish blob).. taken with Canon FD 400mm f/2/8 (@f/4) and Canon 60D
I believe it was due to flat front glass, which I removed.
Not sure if it's still there as I haven't done bright stars recently.

[obrads]

ah yes not dust bunnies then, i cleaned my scope before use and took a flat to test. it was perfectly clean

[obrads]

ah very nice bojan, i can see the difference in the haloes for sure, but you still have that green dot on the other side of the horse head which is what i assume is the same as mine

[billdan]

Alex, if you want to get technical this is the maths for working out where the offending dust bunny or reflection is coming from.

d = (p x FR x diam) / 1000

d = Distance from CCD (mm)
p = pixel size (microns)
FR = Focal Ratio
diam = diameter of reflection or dust bunny in pixels

[obrads]

awesome, i gave this a try,
pixal = 3.8mirconmeter
FR=765
Diam of bunny=140pixals? using photoshops ruler tool? its oval so i dont know which way to measure.
and i get that the bunny is being made 400mm from the sensor. hmmmmm

[billdan]

Your FR is 7.65 not 765.

Then 3.8 x 7.65 x 140 / 1000 = 4.06 mm

But that doesn't sound right either, as the CCD window on the ASI1600 is 6.5mm from the chip

You may have to rotate the image slightly to get a better reading on the oval shape, but its pretty close to 6.5mm.

[obrads]

ah correct, hmmmm im stumped. ill look down the scope and look for anything shiny as well as adjust my OAG