Sky-Watcher Evostar 72ED DS-Pro

[alexbb]

Quick shot from last 2 nights, after main target and before dawn. 2 panels, each 36 or 38 x120s. IDAS-LP D2 filter was used. The halos (to the left) seem to appear also with other filters. I didn't notice them with the Esprit 80. Perhaps the flattener has a quite flat last element so it reflects more?!

I will keep adding to this, but for a preview you get the point.

Half res:

 

Full res:

[Skyline]

Which Flattener did you use may I ask please ?

[alexbb]

Just now, Skyline said:

Which Flattener did you use may I ask please ?

The "dedicated" one with ~60mm backfocus.

[Skyline]

I am thinking of buying some M48 spacer rings so I can test them out with the SW Reducer and a Canon 1100D

[Skyline]

2 minutes ago, moise212 said:

The "dedicated" one with ~60mm backfocus.

Which one is the Dedicated one ? the 0.85x ?

[alexbb]

Be careful that 60mm is the max backfocus with which you can still reach focus. I got acceptable results with an extra 2-3mm "spacer" with a DSLR. I believe the 1100D pixels are the same size as the 550D pixels are, 4.2um. With the ASI1600, the aberrations are a little more visible with 3.8um pixels.

[alexbb]

2 minutes ago, Skyline said:

Which one is the Dedicated one ? the 0.85x ?

Yes. There's a "dedicated" flattener for the 72ED. I say "dedicated" because it's actually a flattener for the 80ED with a custom adapter for the focuser.

https://www.firstlightoptics.com/reducersflatteners/skywatcher-85x-reducerflattener-for-evostar-72ed-pro.html

Edited September 21, 2018 by moise212
link

[Skyline]

The 1100D has a pixel size of 5.2um

[alexbb]

2 hours ago, Skyline said:

The 1100D has a pixel size of 5.2um

You'll notice even less then the aberrations.

[Susaron]

So do you think that with an APS-C size sensor the distance to the sensor will be really around 55mm?, I still have not tried my set-up, QHY168C+ED80+flatener+SW72.

[alexbb]

You will notice more the wrong backfocus distance as the sensor size increases. An APS-C sensor is wider than the ASI1600/QHY168, but it also has larger pixels. However, I'd say you'll notice it.

The good thing is that you can increase the backfocus by adding some spacers between the flattener and a DSLR adapter, you have a few mm of thread available for that. I made one from a beer carton, it's 2-3mm thin and the stars shapes improved.

[Susaron]

Actually I measured the distance and it is around 57mm. As soon as I could make a picture I will let you know.

[Skyline]

I too was guessing around 57-58mm, I know 55mm does not cut it. As I took a image of the double cluster and I had some aberrations in the corners. I post it up later. 

I think with the particular draw tube collar I am using the focus will not be a problem (touch wood), we'll see.

Edited September 21, 2018 by Skyline

[Susaron]

Well I need to post it, yesterday I did some basic focusing test and with the ed80 reducer I was not able to focus my qhy168c, having a distance of 57mm. I changed the m54 to m48 adapter from.one with 32mm length to another of 21mm assuming it is not the correct distance, and at least I was able to reach focus, but very very close to the inward focuser limit.

To me it is a shame from Skywatcher not having tested properly the limits of the scope, they are selling it for AP and the reducer is so thick than it is not possible at the same time to reach focus while having the correct reducer distance. I am.about to send the reducer back and I will buy a simple flattener

[alexbb]

1 hour ago, Susaron said:

I changed the m54 to m48 adapter from.one with 32mm length to another of 21mm

You mean the adapter to connect the flattener to the focuser?

The one provided by SkyWatcher at least is thinner, it adds maybe 5-8mm, I say this from memory.

[Susaron]

8 hours ago, moise212 said:

You mean the adapter to connect the flattener to the focuser?

The one provided by SkyWatcher at least is thinner, it adds maybe 5-8mm, I say this from memory.

No I mean the adapter from the M48 thread in the reducer to the QHY, the QHY168C has a M54 female to M48 female adaptor which I connect to the reducer, it measures 32mm, but I have another one of 21mm.

The problem here is that I am not able to fulfill the requeriments of focus and correct distance reducer-sensor at the same time.

My imaging train will be Tube with Geoptik ring + FLO M56 to 2'' adaptor + ED80 reducer + 32mm M48 to M54 + QHY.

I need to make another test, but I am almos sure that I will have to send back the reducer to the seller.

Edited September 24, 2018 by Susaron

[Susaron]

Finally I will send back the Reducer to the seller, it is impossible to me to reach focus. With the adapter to thread the reducer directly to the focuser drawtube I achieve focus but it is only at 4-5mm from the beginning of the focuser. So as in visual this scope focus point has not been calculated properly. A shame IMHO for SW.

I will try with the Ts x0.79 reducer, it has M48 filter thread and and a M54 thread so I will be able to put both filter and reducer directly to the drawtube, moreover the TS reducer body length is only 27mm against the 44mm from the SW, so I figure out that I will get around of 18mm margin to reach focus.

I will let you know.

Regards.

Mario.

[Skyline]

I had a result last night using an additional 1.5mm spacing with a canon 1100d, will post later.

[Skyline]

I took this image of NGC 7789 Star cluster, I use a 1.5mm spacer with the 0.85x reducer making it 56.5mm, I think I might have to add another 0.5mm.

[Stub Mandrel]

13 hours ago, Skyline said:

I took this image of NGC 7789 Star cluster, I use a 1.5mm spacer with the 0.85x reducer making it 56.5mm, I think I might have to add another 0.5mm.

Interesting... with an ED 66 and a field flattener I needed a similar size spacer for my Canon 450D which is nominally 55mm spacing.

I see you also have an astro modified Canon. I would have expected removing the blocking filter to increase the effective optical path by about 1mm, but perhaps it increases it for flatteners and reducers for some reason?

[billyharris72]

I've recently purchased the Skywatcher reducer and I'm getting pretty bad curvature with 55mm backfocus. Thinking that it needs more space myself, so interesting to hear that 57-58mm range seems to work for some people. I can see a lot more fiddling in store before I get this right. I've been trying with the OVL flattener and have not been able to get an acceptable result - it performs oddly, with curvature bad at 53mm, getting worse at 55mm and then getting better at 58mm. Best performance I have got from it was with 38mm of t-extension tubes (so about 64.5mm back focus taking the filter wheel into account). That was getting there but there was still a little curvature, and the focuser does not rack in any further to allow additional spacers. Test shot attached is the best I have managed to date; could use people's views on that - I don't hink it's catastrophic but should I be aiming for better?

Billy.

[Stub Mandrel]

8 minutes ago, billyharris72 said:

I've been trying with the OVL flattener and have not been able to get an acceptable result - it performs oddly, with curvature bad at 53mm, getting worse at 55mm and then getting better at 58mm.

I've got an OVL flattener. At 55mm it was pretty bad, at 56mm I can barely see anything although CCD inspector said there was still about 1/3 of the curvature at 55mm.

But the OVL flattenr is meant for f5 to f6 I'm working with an f6 scope isn't the ED72 an f7.5 scope?

The lesson has to be experiment with thin spacers!

Edited September 27, 2018 by Stub Mandrel

[Skyline]

ED72 is a F/5.8 Neil

[Cozzy]

3 hours ago, Stub Mandrel said:

I've got an OVL flattener. At 55mm it was pretty bad, at 56mm I can barely see anything although CCD inspector said there was still about 1/3 of the curvature at 55mm.

But the OVL flattenr is meant for f5 to f6 I'm working with an f6 scope isn't the ED72 an f7.5 scope?

The lesson has to be experiment with thin spacers!

I think the SW ED72 is F5.8

[billyharris72]

Yep - f5.8 so should be right in the sweet spot. I reckon it's usable with a bit more tweaking; it's proving a bit of a painful experience.

On an altogether more positive note, I've just tested the Skywatcher flattener at 58mm and it's a massive improvement. Not perfect, but much closer.

Billy.