Achro Frac for solar imaging

[Rob63]

Hi All,

I'm looking for opinions on my plan to buy a cheap Achro frac (probably a skywatcher Evostar 120 f8) - for solar imaging.

I am aiming to use it for some close up work in Ha with the quark on AR's and proms and on WL features with my Lunt wedge and Solar Continuum filter.

It's for those occasional days when the seeing is great and I'm hoping if I stop it down to 100mm  it can work well on the good days too.

CA shouldn't be an issue and I don't think the evostar has a bad reputation for SA either.

I know the focuser on the Evostar is not the best but it can easily be replaced.

 

So the questions is would I get any major benefit from waiting a several months and buying a higher quality Apo frac e.g. ED120 or Equinox 120 (at 5x the price)

 

Thoughts?

[xtreemchaos]

id go for a ed if you can, it would be cheaper in the long run, ive the st 120 and it isn't a patch on my ed80 with the Quark. don't know about the evostar but I think the optics at simler to the st. charl. 

[michael.h.f.wilkinson]

The ST120 is very different from the Evostar. The fact that the Evostar is much slower means aberrations are much less. I myself am considering a bigger frac than the little APM 80mm triplet, and will most likely go for an ES AR127, which is F/6.5, which works perfectly with my H-alpha set-up, and has a decent focuser included. It also has a good rep in terms of overall sharpness (something that cannot be said of the ST120).  Chromatic aberration isn't an issue in H-alpha, Ca-K, or even the Solar Continuum band. Spherical aberration is an issue, however, and the ST120 should suffer much more from it than the Evostar or the ES AR127

[Rob63]

52 minutes ago, michael.h.f.wilkinson said:

The ST120 is very different from the Evostar. The fact that the Evostar is much slower means aberrations are much less. I myself am considering a bigger frac than the little APM 80mm triplet, and will most likely go for an ES AR127, which is F/6.5, which works perfectly with my H-alpha set-up, and has a decent focuser included. It also has a good rep in terms of overall sharpness (something that cannot be said of the ST120).  Chromatic aberration isn't an issue in H-alpha, Ca-K, or even the Solar Continuum band. Spherical aberration is an issue, however, and the ST120 should suffer much more from it than the Evostar or the ES AR127

Thanks Michael,

the  f6.5 ES AR127 looks a good alternative and should suit the Quark well (around f28 with it's  barlow).

It's a little bit more expensive than the Evostar 120 + replacement focuser but I will have a look at reviews and decide if it's worth the extra.

[Ben the Ignorant]

I haven't tried these instruments myself, but I'll pass along info I've found about them: the Celestron version of the 120mm f/8.3 achro has aspheric figuring, it should have less spherical aberration than its Sky-Watcher counterpart (and justify its higher price).

The Bresser 127/1200 is said to be better corrected than the Sky-Watcher 120/1000, however it is 200mm longer.

[michael.h.f.wilkinson]

12 minutes ago, Rob63 said:

Thanks Michael,

the  f6.5 ES AR127 looks a good alternative and should suit the Quark well (around f28 with it's  barlow).

It's a little bit more expensive than the Evostar 120 + replacement focuser but I will have a look at reviews and decide if it's worth the extra.

 

For me the F/6.5 is near perfect, as I get F/26 with the 4x tele-centric. F/7 would be ideal, but those are few and far between. F/8.3 of the Evostar is a bit long, as I get F/33.3. If desired I could stop down the ES to 118mm and hit F/7, without huge loss in resolution. 

 

[Rob63]

5 minutes ago, michael.h.f.wilkinson said:

 

For me the F/6.5 is near perfect, as I get F/26 with the 4x tele-centric. F/7 would be ideal, but those are few and far between. F/8.3 of the Evostar is a bit long, as I get F/33.3. If desired I could stop down the ES to 118mm and hit F/7, without huge loss in resolution. 

 

As far as I know Daystar quote F/27-F/32 as ideal but I think that for imaging you can go higher (or at least I am sure I read that on SGL or CN).

27 minutes ago, Ben the Ignorant said:

I haven't tried these instruments myself, but I'll pass along info I've found about them: the Celestron version of the 120mm f/8.3 achro has aspheric figuring, it should have less spherical aberration than its Sky-Watcher counterpart (and justify its higher price).

The Bresser 127/1200 is said to be better corrected than the Sky-Watcher 120/1000, however it is 200mm longer.

I looked at that one Ben, just wondering if it's too long (around f40 with the quark) but as I saif to Michael, it may be fine for imaging.

Hmmm, decisions decisions.

[michael.h.f.wilkinson]

2 minutes ago, Rob63 said:

As far as I know Daystar quote F/27-F/32 as ideal but I think that for imaging you can go higher (or at least I am sure I read that on SGL or CN).

I looked at that one Ben, just wondering if it's too long (around f40 with the quark) but as I saif to Michael, it may be fine for imaging.

Hmmm, decisions decisions.

Going beyond F/30-ish means you are oversampling the image. This means signal to noise goes down, and the amount of surface captured on the chip is smaller than it should be, meaning even more panes in a mosaic, for me. Yes, you should be able to get decent images, but a focal reducer will be needed to match to the pixel size of the camera

[Freddie]

If it helps, these were taken with a ST120 and quark. These are just two random shots I happen to have on this lappy. I have posted others in the past.

[Rob63]

Very nice Freddie, that's the sort of close up I am aiming at.

[Rob63]

1 hour ago, michael.h.f.wilkinson said:

Going beyond F/30-ish means you are oversampling the image. This means signal to noise goes down, and the amount of surface captured on the chip is smaller than it should be, meaning even more panes in a mosaic, for me. Yes, you should be able to get decent images, but a focal reducer will be needed to match to the pixel size of the camera

Don't think I'd try attempting one of your superb 60 pane mosaics Michael but you never know! 

The ES AR127 is looking like my favourite at the moment, slightly above the recommended 120mm aperture for the UV/IR filter but should be fine without resorting to an ERF.

As an aside from the main topic, do you take flats for each individual panel (i.e. doubling the work!) or just a master flat from a featureless part near the center of the sun?

 

Rob.

[michael.h.f.wilkinson]

6 minutes ago, Rob63 said:

Don't think I'd try attempting one of your superb 60 pane mosaics Michael but you never know! 

The ES AR127 is looking like my favourite at the moment, slightly above the recommended 120mm aperture for the UV/IR filter but should be fine without resorting to an ERF.

As an aside from the main topic, do you take flats for each individual panel (i.e. doubling the work!) or just a master flat from a featureless part near the center of the sun?

 

Rob.

I just take a single flat, taken at the centre of the disk, and out of focus.

[xtreemchaos]

2 hours ago, Ben the Ignorant said:

The Bresser 127/1200 is said to be better corrected than the Sky-Watcher 120/1000,

I'm still waiting for the seeing too be good to use mine with my Quark "3 months and counting" the mag is very high with the f9.5 I'm thinking about getting a shorter ed just for the Quark maybe near F6 still around 120mm. charl.

[Freddie]

5 hours ago, Rob63 said:

Very nice Freddie, that's the sort of close up I am aiming at.

Then why spend three times as much for the ES? Images from the Tecnosky 152 scopes that a couple of the guys are using are definately better but I don't see any improvement in images from more expensive 120mm scopes compared to the ST120.

[Rob63]

1 hour ago, Freddie said:

Then why spend three times as much for the ES? Images from the Tecnosky 152 scopes that a couple of the guys are using are definately better but I don't see any improvement in images from more expensive 120mm scopes compared to the ST120.

Hi Freddie, It's a valid point but f5 is not the best for a quark and  build quality of the ST range is not that great  ( I have an st80 for guiding on the dark side)  I have been comparing lots of images with various 120+ scopes and feel I can see definite  improvement over ST120's  The evostar is only £30 more expensive but both would need a better focuser, however, the ES AR127 wouldn't and it is built like a tank..

I'm still considering  an ED scope which of courses much more expensive but  has the advantage that it works well for my deep sky and planetary work - although it won't get dark enough here until September. 

 

As as usual I will decide/change my mind several times before I finally commit but it's great to get opinions. 

Rob

[Freddie]

Got any links to other ST120 images as I would be interested to compare them to mine. Thanks.

[celestron8g8]

12 hours ago, Rob63 said:

Hi All,

I'm looking for opinions on my plan to buy a cheap Achro frac (probably a skywatcher Evostar 120 f8) - for solar imaging.

I am aiming to use it for some close up work in Ha with the quark on AR's and proms and on WL features with my Lunt wedge and Solar Continuum filter.

It's for those occasional days when the seeing is great and I'm hoping if I stop it down to 100mm  it can work well on the good days too.

CA shouldn't be an issue and I don't think the evostar has a bad reputation for SA either.

I know the focuser on the Evostar is not the best but it can easily be replaced.

 

So the questions is would I get any major benefit from waiting a several months and buying a higher quality Apo frac e.g. ED120 or Equinox 120 (at 5x the price)

 

Thoughts?

 

 

Are you meaning a setup kind of like this : https://www.flickr.com/photos/mwise1023/33736254970/

 

http://photography-on-the.net/forum/showthread.php?t=1468723&page=5

 

[Rob63]

9 hours ago, Freddie said:

Got any links to other ST120 images as I would be interested to compare them to mine. Thanks.

Basically I spent many many hours on forums, facebook groups etc. there aren't a lot of solar images with the ST120 but quite a lot of lunar.

I'm pretty much sick of it now and can't make a decision (which is usually how it goes with new gear) .

[Rob63]

9 hours ago, celestron8g8 said:

 

 

Are you meaning a setup kind of like this : https://www.flickr.com/photos/mwise1023/33736254970/

 

http://photography-on-the.net/forum/showthread.php?t=1468723&page=5

 

Change the scope to whatever I eventually decide on and swap the mount to an AVX and we have an exact  match. Weather is better on your side though 

 

[Freddie]

Obviously if you plan to use the scope for other things then it is a different case but for solar Ha it is such a narrow band on a small ish chip that the optical issues do not show themselves. Planetary or DSO work is a different case and would obviously be a far from ideal scope. On solar Ha, I struggle to see the difference between the ST and other more expensive scopes of the same aperture.

[Rob63]

1 hour ago, Freddie said:

Obviously if you plan to use the scope for other things then it is a different case but for solar Ha it is such a narrow band on a small ish chip that the optical issues do not show themselves. Planetary or DSO work is a different case and would obviously be a far from ideal scope. On solar Ha, I struggle to see the difference between the ST and other more expensive scopes of the same aperture.

The reason I started this thread  was to confirm my thinking that I didn't need an expensive APO for solar work so your contribution has been very welcome - I hadn't even considered the ST120. I still think the Evostar 120 at f8 is a better bet as opposed to f5 on the ST and price difference is not significant.

Cheers,

Rob.

 

[Freddie]

Glad it has been of some help.

Daystar suggest F4-F8 so the ES is at the suggested limit and at 1m FL I think you will need some VERY good seeing to get good results.

[michael.h.f.wilkinson]

35 minutes ago, Freddie said:

Glad it has been of some help.

Daystar suggest F4-F8 so the ES is at the suggested limit and at 1m FL I think you will need some VERY good seeing to get good results.

Some impressive images with the ST120, Freddie, and I agree it should be fine for Quark usage. My Solar Spectrum filter has to be operated at F/25 or slower, preferably F/28-30, so the ST120 is too fast for my use, unless I get a new 5x or 6x tele-centric (and the 5x PowerMate is not very tele-centric). Adding the cost of this and a new focuser into the equation means the ES AR127 (not to be confused with the Evostar) is the better option for me. The Technosky 152mm F/5.9 would also need stopping down for my needs. I would certainly not go for an ED, let alone triplet for just solar use.

[Freddie]

Thanks Michael. Yes, the ST120 is a very good match for a Quark but much less so for anything else. There are quite a number of different requirements being discussed now in this thread so it will be important to keep an eye on what is being suggested for what.

For WL I've recently started to use my CPC925 with 2.5 barlow which is a whole new challenge !!!!!!!! 

[Tony G]

On 6/7/2017 at 08:22, Freddie said:

If it helps, these were taken with a ST120 and quark. These are just two random shots I happen to have on this lappy. I have posted others in the past.

Those are awesome images.