elongated stars in the corners

[red dwalf]

hi all, I have a William optics gt81 with a 0.8 x reducer flattener on, also William optics, connected to a filter wheel and an Atik 460, I have acquired a set of calipers from a friend and set about trying to get the correct distance from reducer to chip, I see from FLO`s website that the distance should be something around 64.38mm although it states above that it should be Lens-to-chip suggested distance: 55-57mm, minus the 6.68 from the back of the reducer to the lens does make it 57.7 mm if I have understood it correctly, then minus the 13.5 from the front of the camera to the chip leaves 44.2 mm, I have got the distance as close to this as I could i.e. 46mm but I am getting elongated stars around the edges of the images attached below, how can I tell if this is a result of the slightly incorrect distance from lens to chip or have I made a calculation error somewhere, and would losing another 2mm make the odd stars look normal ?

 

[gnomus]

Take a look at this:

https://stargazerslounge.com/topic/294708-stars-dont-look-right-help/?do=findComment&comment=3226353

[red dwalf]

interesting, thank you, I thought I was already 2 mm more than I should be but looks like I need to increase it yet further.

[ronin]

I would presume that the distance quoted is the classic "in free air", as the camera will have a piece of glass in front of the sensor, for protection if nothing else but likely an IR cut filter, then this has to be taken into account by you. I say by you as if one manufacturer puts a 2mm think piece in and another a 3mm piece and yet another a 4mm piece then the position will alter for each make. So the figure given will be for air only and the user has to compensate.

The difference is about 1/3 the glass thickness as 3mm seems about the usual (filter thicknesses) then I would guess that you need to incorporate a factor of about 1mm in. Now whether this is 1mm more of 1mm less I cannot be sure. I half picture arguements either way.

[MarsG76]

Hmm... I must have been luck with the Hirsch f6.3 FR. I just slapped it on, with and without filter wheel, on that a OAG and never seem to have major issues... Any slight star oblonginess and there is enough resolution just to crop away that 50 or so pixels in each direction.. maybe I'm just not picky...

 

[ollypenrice]

2 hours ago, MarsG76 said:

Hmm... I must have been luck with the Hirsch f6.3 FR. I just slapped it on, with and without filter wheel, on that a OAG and never seem to have major issues... Any slight star oblonginess and there is enough resolution just to crop away that 50 or so pixels in each direction.. maybe I'm just not picky...

 

SCT reducers are not fussy about chip distance. Faster optics are far more demanding. Chip size also makes a huge difference. The bigger the chip the more precision is needed.

 

10 hours ago, red dwalf said:

interesting, thank you, I thought I was already 2 mm more than I should be but looks like I need to increase it yet further.

I'd agree with you on the evidence. However, the diagram is theoretical. It will be interesting to see if you really do need more chip distance. Keep us posted.

Olly

[MarsG76]

14 minutes ago, ollypenrice said:

SCT reducers are not fussy about chip distance. Faster optics are far more demanding. Chip size also makes a huge difference. The bigger the chip the more precision is needed.

Canon 40D.. 24mm chip

 

[AlistairW]

I am interested in how this turns out. I thought the 64.38mm was the correct distance. The 55mm is (i thought) the distance from chip to shoulder of flattener ( but crucially including the GT81 adaptor that is supplied). I would therefore imagine the adaptor is 64.38-55mm or 9.38mm ( which, as I remember, it is ). Now, I dont use a DSLR (or adaptor) so use the 64.38mm distance for my CCD.

[red dwalf]

I have thought about it last night and think your suggestion of 64.38 mm including the 9.38 mm adapter that screws onto the reducer, flattner is correct, I was going to try it last night, just as I went out it tipped it down, so hopefully I can try it out tonight.

[AlistairW]

Let us know how it turns out as i am doing something similar too

[red dwalf]

With luck I'll try it tonight

[red dwalf]

clear ish tonight so managed a 5 minute exposure in ha and it look like the problem is solved, the distance is indeed 64.38 mm or near as can get, this does not include the 9.5mm adapter that comes with the reducer, flattener.

glad that is sorted.

 

[AlistairW]

Glad you have that sorted. Thanks for the update?

[Tim]

Looks better

Getting spacing right has to be one of the most fiddly aspects of imaging.

There is something amiss with the central stars to my eye, although it is late, I have a stinking cold, and can only find my rubbish "ready reader" glasses! 

If you took a short, say 10 second luminance exposure of some centrally placed bright stars, do they have a slight three sided appearance? Or is it just my silly old eyes?

Cheers,

Tim

[ollypenrice]

7 hours ago, Tim said:

Looks better

Getting spacing right has to be one of the most fiddly aspects of imaging.

There is something amiss with the central stars to my eye, although it is late, I have a stinking cold, and can only find my rubbish "ready reader" glasses! 

If you took a short, say 10 second luminance exposure of some centrally placed bright stars, do they have a slight three sided appearance? Or is it just my silly old eyes?

Cheers,

Tim

I picked up on the three sided main stars as well, before coming to your post. The corners look great.

Slight pinching of the optics? If so it might be worth just running a dewheater round the outside of the objective whether it was needed against the dew or not. Was it cold for your shoot? It has been unusually cool here in France the last few nights.

Olly

[red dwalf]

looking at the middle again i see what you mean, i had the dew heaters on, always leave them running and last night was cool but not that cold, i`ll try a few shots with a luminance filter and see what that shows.

[Tim]

Don't forget you only need the heaters on the bare minimum to keep the dew away Rob.

Trial and error will reveal whether warming the lens cell improves or increases the apparent pinching. 

I'm off to Kelling soon  but would like to keep up to date with how you get on, if possible could you @Tim any posts that follow so I remember to pick them up in  my half dazed state. After all it is bound to be perfectly clear every  night and I'm taking imaging and visual gear this year, so sleep won't be an option....

Thanks

[red dwalf]

I'll also be at kelling, Thursday, Friday and Saturday, maybe Sunday too, I'm in the blue field b238, your more than welcome to pop round anytime for a natter, beer, tea or coffee, you can learn so much from others at these events.

[Tim]

Perfect. I have pitches in the top left corner of red, near the entrance to the dog walk, likewise you are most welcome. I'll be doing some imaging with any luck, and also have an 18" dob if you fancy a bit of visual. Look for a dirty brown Navara with a huge dent in it

[red dwalf]

long time since i`ve looked through a 18" dob, will take you up on that with luck, what pitch are you on

[Tim]

I'm not sure, but we are right in the corner over two gravel pitches. You wont miss us, our camp has been described as a refugee site