First Light 80 mm astrograph

[DaveS]

I gave my 80mm six-element f/4.4 astrograph from TS first light last night. Nothing special, just 1 hour of H-alpha on the Rosette in 10 min subs.

Already I can see issues in the corners, I suspect I have to re-check my sums over the spacing as I can see comatic stars in the corners, and I may have squaring-on problems too, as the coma isn't symmetrical around the frame.

 

Possibly guiding / PA problems too. Ah, well nobody said this was an easy game!

[wimvb]

A very nice image, despite the problems with coma.

Thanks for sharing

[Davey-T]

Nice first image Dave, no sense of achievement if it's too easy

Dave

[laudropb]

Very nice, especially for a first attempt.

[DaveS]

Well, after checking my sums I realise that I had originally allowed another 10mm for a tilt adaptor which I missed from my order to FLO, so instead of having 68mm back focus I only had 58. That would be plenty enough error to give comatic stars.

So either co-opt the 10mm spacer that I have on my 130 PDS or stump up for the tilt-adaptor that I had originally planned. I might need it to deal with the squaring error.

[DaveS]

I have just now pushed the button on a 48mm tilt adaptor and a set of Baader delrin spacers to fine-tune the spacing and get rid of any tilt.

[ollypenrice]

I hope you get this to play nicely because the numbers are excellent - 80mm at F4.4. The distortions aside, this looks good.

Olly

[DaveS]

Thanks Olly

I have hopes for this 'scope, I think my first real imaging run will be a LRGB on Markarian's Chain when it gets nicely placed, but the weather forecast looks dire for the week ahead.

When I was focusing I found that the focus for red and blue was as near coincident as I could tell with a Bahtinov mask. It's an interesting design, essentially a 80mm f/6 triplet apo (Their photoline 'scope) with a built in matched triplet reducer.

[laser_jock99]

Looks good to me.

[Chris]

Looks like this scope has really good potential

[Pompey Monkey]

On 15/03/2016 at 08:45, DaveS said:

Well, after checking my sums I realise that I had originally allowed another 10mm for a tilt adaptor which I missed from my order to FLO, so instead of having 68mm back focus I only had 58. That would be plenty enough error to give comatic stars.

So either co-opt the 10mm spacer that I have on my 130 PDS or stump up for the tilt-adaptor that I had originally planned. I might need it to deal with the squaring error.

Is there not a tilt adjuster fitted as standard? 

[gnomus]

I thought that you didn't need to worry about spacing with these flatfield scopes with built in correction?  

[DaveS]

I thought it had a built in tilt adjuster too, but I guess they must've changed the design. Well, the one from FLO came yesterday and I have it on the 'scope now.

With some of the flat field 'scopes you don't have to worry about spacing as they are designed as a unit, but this one, together with the 100mm f/5.1 and 130mm f/5.0 are actually triplet apos with built in 3 element reducer-flatteners, so the same principles apply.

[gnomus]

3 minutes ago, DaveS said:

I thought it had a built in tilt adjuster too, but I guess they must've changed the design. Well, the one from FLO came yesterday and I have it on the 'scope now.

With some of the flat field 'scopes you don't have to worry about spacing as they are designed as a unit, but this one, together with the 100mm f/5.1 and 130mm f/5.0 are actually triplet apos with built in 3 element reducer-flatteners, so the same principles apply.

OK - I hadn't realised that.  Apologies for any confusion.  In that case it should be possible to eliminate the coma by doing as you suggest: https://stargazerslounge.com/topic/225831-flattener-spacing/?do=findComment&comment=2439819

It is an interesting scope for sure as it has very nearly the same F/L as my Star 71.  I am looking for a second unit for a dual rig.  The Mk II Star 71 is due out in a few weeks, but it will only have 4 elements and I am concerned that it may not be as flat as my 5-element Mk I.

Steve

[DaveS]

Thanks for the link Steve, looking at the diagrams it's definitely the "too close" situation, as I thought.

Dilemma for tonight, do I take the time to refine the spacing and tilt on this, or use the time to get 2 hours of H-alpha on M101 with the 130 PDS that's on the mount ATM? The moon's too near full and too close to the Rosette to do anything there, even in H-alpha.

Will think on it.

[ultranova]

Looking forward to seeing more pics with this set up

Paul

[DaveS]

Dur.....

It wasn't just the wrong spacing, the wrong filter as well . I had the [NII] filter in the train instead of the H-alpha. I only found out when swapping over to the 130 PDS for a H-alpha run on M101.