ST120 First Light

[Kef9]

Hi all

I have read this thread with intrest because I am thinking of getting a st120 or 150 one question I would like to ask. is the ca worse in the 150 than the 120. I would be using it for dso's with probably some general viewing.

If you can afford the extra get the 150

[John]

In theory the larger aperture of the 150 refractor should show more detail and contrast than the 125 mak but from personal experience I believe that CA and spherical abberation (another common abberation with chinese achromats) do diminish both resolution and sharpness to an extent which would undermine the aperture advantage here.

[Moonshane]

I suppose we're a bit off the original topic but will the 150mm achro show anything more than the 150mm newt he already has? the newt may have a central obstruction but will have no CA at all.

[John]

Not neccessaraly better on moon and planets!!! ST150 will have a higher resolution with more aperture!

I don't think it will - as I said in my earlier post, the CA and spherical abberation that the ST150 will possess will reduce the resolution. I've witnessed this first hand with F/8 150mm achros - remove the bulk of the CA and other abberation and the resolution and contrast is very noticably improved.

[Timbo]

Hi Alowen,

I spotted your thread on the AZ4 with the ST120, which is just up my street. Are you really happy with your ST despit the CA?

I can see my self heading down the AZ4/ST120 route very soon.

A lot of gazers seem to own them in some form or other.

Oh, BTW I'm in Plymouth; are you east or west Cornwall? I can imagine that skies must be quite dark in Kernow

I tend to pop off to Dartmoor for better views.

Keep us informed with more reports - please

Tim

[Kef9]

I don't think it will - as I said in my earlier post, the CA and spherical abberation that the ST150 will possess will reduce the resolution. I've witnessed this first hand with F/8 150mm achros - remove the bulk of the CA and other abberation and the resolution and contrast is very noticably improved.

I feel a test is neccessary. Anybody near manchester got a ST120?

[alowen]

Hi Alowen,

I spotted your thread on the AZ4 with the ST120, which is just up my street. Are you really happy with your ST despit the CA?

I can see my self heading down the AZ4/ST120 route very soon.

A lot of gazers seem to own them in some form or other.

Oh, BTW I'm in Plymouth; are you east or west Cornwall? I can imagine that skies must be quite dark in Kernow

I tend to pop off to Dartmoor for better views.

Keep us informed with more reports - please

Tim

Hi Tim, i genuinely don't find the CA to be too much of an issue. Mainly because I only use the 120 for DSO'S. As for an AZ4, its a definate when funds allow.

I live near Penzance and we have some amazing dark skies here.

Al

[thommo10]

I suppose we're a bit off the original topic but will the 150mm achro show anything more than the 150mm newt he already has? the newt may have a central obstruction but will have no CA at all.

The reason I am looking at the st150 is that my 150 newt is so far out of collamation and I have tried in vain to get it right the problem is the primary mirror is fixed, so the only adjustment I have is on the secondry, the adjustment screw's are rather crude, the other problem I had is that the primary came away slightly from the back plate, I have clued it back but wether it went back in the right position I don't know, the scope is a old astro systems scope. I have asked advice on SGL and have had good feed back, but with out making a new mirror cell I cannot see another way, also I do get a bit fed up of keep moving the OTA to get it into a position for easy viewing

[Moonshane]

if you like, I'll be passing near Birmingham when I go down south next weekend. I could meet you somewhere and have a look at the collimation etc and show you how? it would be no problem as long as you meet me somewhere en route (maybe one of the services places off the M6?).

the offer's there anyway.

[thommo10]

Thanks Moonshane, I Live about 2 miles from Junction 7 M6

[Moonshane]

pm sent

[lw24]

I've got the ST120 f/5 on an AZ4, and I love the combination.

Recently purchased a Baader Fringe Killer to remove some of the CA, and it seems to be doing a decent job at the moment.

A good purchase I feel

[nightfisher]

i have the 150 st, the older helios version and ca is horrendous when viewing the moon especially at edge of ep but i took some images using digital camera afocal and the ca had all but gone!

[Ags]

This thread has got me thinking... Would the st120 work well as a spotting scope when it is not doing DSO duties?

[alowen]

This thread has got me thinking... Would the st120 work well as a spotting scope when it is not doing DSO duties?

Hi, yeah the online shops say its good for both 'terrestial' and astronomy use.

Al

[Kef9]

I looked through mine in the day and it was not good news. Try looking at a tree with leaves. Urrrgghhhhhh!!!!

The C100ED on the other hand was perfect.

[Ags]

Kef9, what mag were you using in your daytime test? An idea - sticking a 40mm ep in during daylight would stop the scope down to something like f10 (due to the 8mm exit pupil) which should largely eliminate CA. Does the scope take 2" eps?

[Kef9]

Kef9, what mag were you using in your daytime test? An idea - sticking a 40mm ep in during daylight would stop the scope down to something like f10 (due to the 8mm exit pupil) which should largely eliminate CA. Does the scope take 2" eps?

Hi

Oh yes it was high mag so it was to be expected but it looked awful.

Sticking in a longer focal length eyepiece does not increase the focal ratio. It stays f5 unless you put the partial lens cap on which reduces the aperture, hence this increases the focal ratio. Is this what you mean't?

A longer focal length eyepiece however does reduce CA but that doesn't allow you thed mag you want. Finally yes my scope has a 2" diagonal.

When it comes to daytime viewing you really do need an apo I find.

[Ags]

My understanding is that your pupil acts as an aperture stop when the exit pupil exceeds pupil size.

[Kef9]

Ummmm interesting theory

After thinking about it though this does not affect the focal ratio of the instrument. All this would do is restrict the amount of light going into the eye dependent on the individuals pupil in question so the image would be dimmer. I dont think it would reduce CA but I may be wrong?

[Ags]

Hmmmm, I'm getting less certain by the minute. Hopefully someone more knowledgeable can weigh in.

[Ags]

Here is a thread on CN where it is stated that large exit pupils cut CA when they exceed pupil size, acting as an aperture mask:

Telescope Reviews: Three new Scopes!

So in daylight with an F5 achro, use a long focal length EP....

[Kef9]

Umm interesting find.

Maybe we are the only people interested since no one else seems to be bothered!!!