Guding and Star refraction

[Rezn8]

Hi All , please help with some info.

1. I have started guiding my DSLR camera and lens ( sitting in my guidescope brackets ) with my 8" SCT. In PHD the graph is quite eratic, and the corrections are quite sharp, however, if I am using like 400-700mm focal length lenses, how much of an effect would this have on the final image ?

2. I am getting strange star difraction spikes when using a Canon 300F4 ( with and without 2 x converter ), and was wondering if this is common on all lenses, or maybe just this particular one I am using. ( see attached 3 x 5 min stack ) ?

[Zakalwe]

2. I am getting strange star difraction spikes when using a Canon 300F4 ( with and without 2 x converter ), and was wondering if this is common on all lenses, or maybe just this particular one I am using. ( see attached 3 x 5 min stack )

The spikes are caused by the aperture blades in the lens. Perfectly normal AFAIK

[Rezn8]

Thanks Zakalwe. I forgot to check what F-stop the lens was set to. Would changing the F-stop, change the nature/ pattern of the spikes ? Would using a faster lens F2.8 make a differance ?

Last question, what is AFAIK

[ndastgir]

there are two small black circles near the lower left corner of the image. do you know what are those?

[Rezn8]

I think those are grease spots on my CCD sensor, or dust sitting somewhere inbetween . Normally when I stack ( with dithering ), it removes them.

[Zakalwe]

Thanks Zakalwe. I forgot to check what F-stop the lens was set to. Would changing the F-stop, change the nature/ pattern of the spikes ?

Possibly, as the f-stop is controlled by the aperture that the lens is set to. If it was wide open, then the blades are probably not in the light path.

Would using a faster lens F2.8 make a differance ?

Yes. It would allow you to collect more data in the same time. Fast = good in astro-photography.

However, f2.8 lenses are expensive. Very expensive. Plus, unless they are very good you might find that they are a bit "soft" wide open.

Last question, what is AFAIK

As Far As I Know.

[Rezn8]

Have you ever seen lenses produce 4 point spikes ? I know F2.8 would gather more light,which is helpfull but a tradeof against the wight of the lens, but would it produce the same difraction spikes , sorta funny looking in my opinion... I checked, the lens was wide open, at F5.6 with 2x converter , maybe if I stopped it down it would change blade position and spikes ?

[Rezn8]

I would try it myself, except I havnt had a clear night in two months, and am going away for some dark skies next week to a gamereserve, and want to take the right lens with to take advantage of my limited time there...

[Gina]

Cheaper lenses often have 6 blades and produce 6 spikes. A bit dearer and you get 8 and really expensive lenses often have more and produce much smaller diffraction spikes. I'm told you can make your own aperture mask with a nice round hole and put it in front of the lens. However, I'm sure I remember that the aperture vanes are placed at a particular place in the light path, otherwise there's some image degradation, but it's too many decades ago that I learnt about optics and I can't really remember now

[ollypenrice]

Forget the internal aperture control for AP and run the lens wide open with a front aperture mask which you can make for the price of a cuppa. This will lose your diffraction artefacts.

Olly

[Rezn8]

Thanks Olly. If I understand this all correctly, the blades of the lens open is not perfectly round , even wide open, and the shape it has, makes the spikes. Therefore by using the above, we are making the aperature round, instead of like hexagonal ? Do any lenses open up fully round ?

[Gina]

Do any lenses open up fully round ?

Yes, I have several that do.

[Rezn8]

Hi Gina...please would you list the longer focal length ones that open up fully , so I can try them ?

[Gina]

I'll check out my lenses and get back on that.

[Rezn8]

Thanks Gina, I dont suppose you have used ( or heard about for AP ) a Nikon 200-400 F4 ?

[Gina]

'Fraid not. But prime lenses are better than zoom.

[Gina]

I think all my lenses open right up (pure round aperture at max f) but here's a short list which I'll add to later :-

Asahi SMC Takumar 200mm f4

Vivitar 200mm f3.5

Vivitar 300mm f5

Kaligar 135mm f2.8

[Lars]

Regarding your question 1: Since you are guiding with 2000 mm focal length, the guiding graph will be erratic unless you have a very heavy-duty mount and excellent seeing. Mirror shift in an SCT can add to guidng errors, but I suspect those problems will not be an issue if your imaging set-up is at 400-700 mm. I have used my C9.25 for guiding when imaging with a 480 mm refractor and that worked well.

/Lars

[Rezn8]

Thanks Gina. Lars, that is great news, thanks. I hoped it would work like that.