diffraction spikes and focal length

[Moonshane]

True Andrew although sometimes the seeing is so poor that full aperture is pointless. You can stop a scope down but not up.

It also tightens up star images a lot making tight doubles easier.

[nightfisher]

I

Really would like to get the Altair astro 150 f6 newt as on paper this is going to work well on Luna/ planets and still okay with a lot of dso objects, but I don't want to be viewing Jupiter with spikes, this to me is like false colour to other people

[Moonshane]

I think it will have spikes Jules sadly.

[andrew s]

True Andrew although sometimes the seeing is so poor that full aperture is pointless. You can stop a scope down but not up.

It also tightens up star images a lot making tight doubles easier.

Fully agree with your first point not so sure about the second as the diffraction disc gets smaller with increasing aperture.

Regards Andrew

[Moonshane]

I can only say what I see. Have you ever tried it?

[faulksy]

The single curved vane is probably the optimum design I think. David Lukehurst uses them and I've a lot of respect for his knowledge and expertise

I think these scopes are capable of very high planetary performance - most of the time I think it's the seeing conditions that limits mine on lunar and planetary detail. Get the occasional really steady moment and it rather blows your sock off   Wish I had more of those

I'm glad your Hotech is working well for you. I tried one for a couple of weeks but could not get on with it. I'm currently trying a set of the replica Tectron tools that Astroboot are selling. They seem to work quite well but I'm not "getting" the autocollimator so far

i am struggling with the autocolimator as well john, i bought the cateye kit. i have spoke to jason d and starman and they have sent me some brilliant links on how to use it. if you want i can pm you the details. also gerry uses the same kit and said it makes a massive difference compared to just using a cheshire

[andrew s]

I can only say what I see. Have you ever tried it?[/quote

]

Yes, I was into high resolution imaging at one time. I tried many ideas out, masks, zerodur mirror, high quality mirror finish, microscope objectives etc. I did this both visually and with video and conventional ccd imaging.

As a rule I only comment on practical threads if I have both practical knowledge and some (albeit limited) theoretical understanding of the point under discussion.

I hope that satisfies your concerns.

Regards Andrew

[Moonshane]

No offence intended Andrew although I seem to have caused a little. I see imaging and visual as quite different and agree that for imaging e.g planets aperture matters a lot in terms of resolution. Of course it does for visual but in my experience not in poor seeing conditions for apertures over maybe 6". I am solely a visual observer and as I said can only report what I see. Theory while interesting has no impact on what I see unless I am keen to know why.

[andrew s]

No offence intended Andrew although I seem to have caused a little. I see imaging and visual as quite different and agree that for imaging e.g planets aperture matters a lot in terms of resolution. Of course it does for visual but in my experience not in poor seeing conditions for apertures over maybe 6". I am solely a visual observer and as I said can only report what I see. Theory while interesting has no impact on what I see unless I am keen to kynow why.

No offense taken, I did a lot of visual in my youth but got tired of my eye freezing to the eye piece. One slight problem with visual is that one can be fooled by ones expectations as illustrated by the the canal's on Mars. I am however sure you report what you see honestly and if you find a reduced aperture helps you see more then I would not contest your experience.

Regards Andrew

[HD-AP]

Hi

Focal length will have little impact on the diffraction spikes ,as others have said in other replies it's the size of the central obstruction and thickness of the spider vanes that have the most impact so minimising these will help.

My 10 inch f8 planetary newtonian for visual use has a small secondary and an optical window to eliminate diffraction spikes ,and all mirors and the optical window are of extremely high accuracy .It gives great views but is an unweildy beast on an eq mount .

And optical windows using suitable glass figured to high accuracy are very expensive but it does work but for the same sort of money you could have a much larger newtonian .

[jetstream]

Oddly I don't seem to get diff spikes on the 20 inch on planets. We had it trained on Jupiter last night and there were none. We do see them on bright stars. I've never worked out why it's so variable.

Olly

Olly, I don't see them either in my 10" on planets, just on very bright stars.

[jetstream]

i am struggling with the autocolimator as well john, i bought the cateye kit. i have spoke to jason d and starman and they have sent me some brilliant links on how to use it. if you want i can pm you the details. also gerry uses the same kit and said it makes a massive difference compared to just using a cheshire

I use my autocollimator for high power lunar as this object allows very high detail to be seen and as such my scope is never very far off for the rest of the time, using the Cheshire.

Mike, I'm happy you can now collimate your telescope after getting the sight tube and Cheshire

[Moonshane]

No offense taken, I did a lot of visual in my youth but got tired of my eye freezing to the eye piece. One slight problem with visual is that one can be fooled by ones expectations as illustrated by the the canal's on Mars. I am however sure you report what you see honestly and if you find a reduced aperture helps you see more then I would not contest your experience.

Regards Andrew

Good points Andrew. I do concede that averted imagination can take effect but I am convinced that visual astronomy is a lifelong interest. Thankfully with six Scopes from 80mm to 400mm I have lots of options :0)

[faulksy]

I use my autocollimator for high power lunar as this object allows very high detail to be seen and as such my scope is never very far off for the rest of the time, using the Cheshire.

Mike, I'm happy you can now collimate your telescope after getting the sight tube and Cheshire

still havnt got to grips with the auto sorry for spamming to the original poster (jules)

[Stu]

Fully agree with your first point not so sure about the second as the diffraction disc gets smaller with increasing aperture.

Regards Andrew

Andrew, I totally agree with this point in theory, but I do not see it borne out on star images in practice. Of course resolution for planetary and lunar observing improves with aperture, but any large scope I have looked through tends to show more bloated star images which does not match the theory. I cannot explain this, perhaps someone else can?

I am sure Shane is referring to image quality under poorer conditions when he says the aperture mask gives better images on planets. Ultimately under good conditions, aperture should win.

[andrew s]

Andrew, I totally agree with this point in theory, but I do not see it borne out on star images in practice. Of course resolution for planetary and lunar observing improves with aperture, but any large scope I have looked through tends to show more bloated star images which does not match the theory. I cannot explain this, perhaps someone else can?

I am sure Shane is referring to image quality under poorer conditions when he says the aperture mask gives better images on planets. Ultimately under good conditions, aperture should win.

There could be two reasons. Firstly, larger aperture scopes tend to have a longer focal length and hence image scale and secondly, the impact of atmospheric turbulence changes as the aperture goes from under to over the typical size of the atmospheric cells.

Again I am not challenging anyone's experience.

Regards Andrew

[scarp15]

When initially used my 14" OOUK dobsonian, I would see diffraction spikes quite clearly on each of Jupiter's moons but not on Jupiter. I considered this to be a distraction after the clean crisp images of satellite discs that I was used to formed by my former SCT. More recently I have started to use a paracorr and I no longer see the spikes on the moons, they are now perfect discs. I am not confusing this with coma, but cannot provide an explanation why this is so, except that perhaps the additional glass in the optical path has something to do with it. I also use the TV planetary filter and as I think it was Stu has remarked, it appears to remove any trace of spikes.

[nightfisher]

Guys, thanks for all the replies, i dont think i will go for this 150 f6 as i dont think i will be happy with it due to this issue