is this slight egginess in my edge stars down to inaccurate backfocus from flattener to camera?

[irtuk]

First light on my new Esprit 80 ED yesterday in a brief cloud break. I have the dedicated field flattener for it as well.

It's Quite a large image, and this is right in the corner. is this par for the course or can I improve my back focus from camera to flattener, or anything else? some slight egg-shaping on stars going on

 

Ed.

 

[symmetal]

Your posted image is rather small so it's hard to be definate, but the other three corners look reasonable which implies more a possible tilt problem with the sensor. You can try altering the spacing distance to improve it in that corner, though it's likely the other corners will then start showing poor star shapes.

Sensor tilt jigs have been widely talked about on the forum, and are fairly cheap to make, so it's worth checking the tilt is correct, ie no tilt, before making any other adjustments.

Alan

[Ouroboros]

@irtuk I take it this is just the bottom right of a larger image. Is it? If so it’s a bit like the egginess I have observed in this thread I started 

 

Note the second post which suggests I might need to increase the distance to the sensor. You might have the same problem. 

What size camera are you using? 

[tooth_dr]

Is that a stack or single sub?

[irtuk]

its a snippet of the corner of the stack. the stars are lovely and round in the centre and quite a way out to the edge. its just at the extremities I get the eggs.

[tooth_dr]

6 minutes ago, irtuk said:

its a snippet of the corner of the stack. the stars are lovely and round in the centre and quite a way out to the edge. its just at the extremities I get the eggs.

Could be a stacking artefact, what’s the subs like?

[rickwayne]

Definitely want to look at a single sub. Also this example of incorrect-backfocus artifacts may be helpful:

https://cdn.shopify.com/s/files/1/1594/4815/files/Back_Focus_Spacing_Guide_1024x1024.jpg?v=1606528627

[rickwayne]

I can't look at that thing for more than a few seconds without getting dizzy!

[irtuk]

Here's a single HA sub, 600 sec, guided

IC1396_LIGHT_2021-10-17_22-40-13_HA_-10.00_600.00s_139_0006.fits

it really is just in the corners so I am not that bothered but if its an easy thing to diagnose and fix then why not, I certainly have bigger problems to address

 

[Spongey]

20 minutes ago, irtuk said:

Here's a single HA sub, 600 sec, guided

IC1396_LIGHT_2021-10-17_22-40-13_HA_-10.00_600.00s_139_0006.fits 7.82 MB · 0 downloads

it really is just in the corners so I am not that bothered but if its an easy thing to diagnose and fix then why not, I certainly have bigger problems to address

 

I would guess that, based on this sub, your backfocus is incorrect.

The esprit 80 flattener has a backfocus of 55mm from the M48 thread. What does your imaging train look like? You may need to add a spacer or two to get the sensor at 55mm.

[irtuk]

ahha. this has come up before some time ago. I used the 61mm from the manual

here's my imaging train, bit tricky to photograph with one hand but should give you an idea:

I recall when I set it up it was 61mm from the front of the flattener to the little black nubbin on the camera which I am assuming indicates where the sensor is within the case, that's on the underside of the camera in this shot

 

[irtuk]

I just took another measurement and its 51mm from the front of the thread to the same position. is this the measurement we think should be 55mm?

 

[Spongey]

12 minutes ago, irtuk said:

I just took another measurement and its 51mm from the front of the thread to the same position. is this the measurement we think should be 55mm?

 

That's correct, the M62 to M48 adapter that comes with the flattener should be 6mm thick. 

It might be better to try to work out your backfocus using the widths of each component, rather than assuming where the sensor is in the camera. IIRC the ASI1600mm has 6.5mm backfocus, so add whatever spacers you are using and 1/3rd your filter thickness. You want that value to be 55mm the to M48 thread of the flattener adapter. 

Edited October 22, 2021 by Spongey

[irtuk]

Righto. thanks. I will see what I can do with the baffling array of rings and gizmos I have in the box

I don't understand the statement about spacers as 1/3 the filter thickness, the filters are these ZWO 1.25" Ha SII OIII 7nm Narrowband Filter Set | First Light Optics and it says the glass in the filters is 2mm +/- a very small amount so 1/3 that would be 0.66

Is that what these mysterious fellas are for? they are about half a mil thick each

Ed.

 

 

[tooth_dr]

2 minutes ago, irtuk said:

Righto. thanks. I will see what I can do with the baffling array of rings and gizmos I have in the box

I don't understand the statement about spacers as 1/3 the filter thickness, the filters are these ZWO 1.25" Ha SII OIII 7nm Narrowband Filter Set | First Light Optics and it says the glass in the filters is 2mm +/- a very small amount so 1/3 that would be 0.66

Is that what these mysterious fellas are for? they are about half a mil thick each

Ed.

 

 

They are for holding frameless/unmounted filters and not for fine tuning the backfocus.  

You are correct - you need to add 0.7mm backfocus to account for the glass, so if eg you need 55mm backfocus, then if you have filters in the light path, this needs to be adjusted to 55.7mm.

I agreed with Rob above, measure to hard edges and add the 6.5mm on

[irtuk]

Great thanks. solid advice there. I did wonder what those discs were for.

I think I should also check to see if a snail has taken up residence in my scope from that picture.

 

Ed.

 

[The Lazy Astronomer]

If you find you need to add a very small amount to the optical train, your camera should have come included with a set of shims of varying thickness (my ZWO camera did, anyway).

[irtuk]

So lets see if I have got this right.!

By my calculations I need 55mm of back focus PLUS 0.7mm of filter allowance for a total of 55.7mm

I then subtract the 6.5mm of back focus that's provided by the camera itself between the edge and the sensor

and I come out with 49.2mm

so the distance X in the picture from Tooth_Dr should be 49.2mm?

 

[rickwayne]

Assuming  that the line to the left of the knurled wheel is the back of the flattener, yes.

[irtuk]

Yep. we should be good to go. 

At this point my wife walked in from work and said she had managed to get tickets to a show in Canterbury this evening.

Both "Yay" and "Doh"

Ed.

 

[teoria_del_big_bang]

I can't quite see from that image but I am assuming the point you are measuring from on the flattener is the front of the M66 to M48 adapter on the rear of the flattener as ringed in red below.

If so then the distance between the point marked on the left to the CCD sensor for the Esprit ED80 is 66mm.

+ 1/3 of the filter = approx 0.7 mm 
- distance from front of CCD to Image plane  6.5 mm

So that distance in white should be 66 + 0. 7 - 6.5 = 60.2 mm

Might be easier to measure from back of the spacer so that distance is 60.2 - spacer width (not sure what this is I think was 8mm on my Esprit 100ED)  = 60.2 - 8 = 52.2 mm.

 

EDIT - MAYBE IGNORE THIS LAST BIT I SEE LOADS OF CONFLICTING INFO ABOUT THAT REAR SPACER I ASSUMED WAS 8MM LIKE MY 100ED. IT MAY BE IT IS 11 MM. Iin that case then the distance from the back of the M66 to M48 adapter will be 49.2 mm as stated in earlier posts so sorry for that confusion, I just downloaded the ED80 manual from SW but since then have read loads of threads about this.

Also sometimes if you really look close even when you think you have the exact right spacers you can see a little distortion of stars in very corners and sometimes just needs an an extra 0.3 or 0.5 mm spacer, or more annoyingly one less 0.3 or 0.5 mm spacer.

Steve

 

Edited October 22, 2021 by teoria_del_big_bang

[irtuk]

Cheers Steve. Thanks for the input though, I went out and measured the optical train and it was already pretty close to 49.2mm so it would have been when I took the sub. I am going to experiment with some very small spacers, see if I can fine tune it, but it really is just the corners.

[inFINNity Deck]

On 22/10/2021 at 13:43, irtuk said:

Is that what these mysterious fellas are for? they are about half a mil thick each

Ed.

 

 

These are filter masks to avoid light leakage around the edge of unmounted ZWO filters. I explain that in an article on a Dutch forum (opening in Chrome should translate properly):

https://www.starry-night.nl/forums/topic/maskers-voor-zwo-efw/

As can be seen they do make a huge difference in flats quality.

Nicolàs

[VincenzoRoma]

Hi,

This can be useful to you