Stopping down a 120mm Achro Refractor

[Mark at Beaufort]

I have never used a Skywatcher Achromatic and I am wondering the effect of stopping down to gain a higher f number. The Skywatcher Evo Star 120 OTA has a focal length of 1000mm making it f/8.33. If it's stopped down to 80mm is becomes f/12.5 although the focal length remains 1000mm so magnification does not change.

Has anyone tried this procedure and does it improve contrast of brighter objects like Jupiter/Saturn and Moon etc.

[michael.h.f.wilkinson]

I have looked through the smaller, slower 100mm F/10 achro, and that had pretty good contrast on Jupiter. CA was there, but quite well controlled. Stopping down a 120mm F/8.33 to 100mm would give you the same image quality, but stopping down further might well reduce resolution to such a degree that any boost in contrast is negated.

[vlaiv]

Yes, and it works well. I've done it on short & fast ST102 F/5. I made couple of aperture masks, and did some testing. Even managed to take a decent deep sky photo without any blue bloat around stars - by using 66mm aperture mask and yellow #8 filter.

Just be aware that putting aperture mask on will reduce maximal attainable magnification - good rule as always is aperture in mm x2. 80mm scope will be good for up to x160. Of course image will be dimmer.

Btw, I've now got Evostar 102 F/10 and Baader contrast booster filter removes almost all CA while keeping things almost natural looking (there is small color shift). If you go for something like 70-75mm aperture mask, you will in principle eliminate chromatic aberration pretty much completely (CA index >5).

Btw, here is "a study" for photographic purposes I did at a time with ST102:

Rows contain same image, but stretched to different level (first one the most obviously as hot pixels / noise start to show). From top to bottom - no mask, no mask+#8, 80mm, 80mm+#8, 66mm, 66mm+#8, 50mm, 50mm+#8. Exposure times were scaled appropriately.

[Mark2022]

On 04/09/2019 at 16:49, vlaiv said:

Yes, and it works well. I've done it on short & fast ST102 F/5. I made couple of aperture masks, and did some testing. Even managed to take a decent deep sky photo without any blue bloat around stars - by using 66mm aperture mask and yellow #8 filter.

Just be aware that putting aperture mask on will reduce maximal attainable magnification - good rule as always is aperture in mm x2. 80mm scope will be good for up to x160. Of course image will be dimmer.

Btw, I've now got Evostar 102 F/10 and Baader contrast booster filter removes almost all CA while keeping things almost natural looking (there is small color shift). If you go for something like 70-75mm aperture mask, you will in principle eliminate chromatic aberration pretty much completely (CA index >5).

Btw, here is "a study" for photographic purposes I did at a time with ST102:

http://serve.trimacka.net/astro/Forum/2016-01-25/post_01/Montage.png

Rows contain same image, but stretched to different level (first one the most obviously as hot pixels / noise start to show). From top to bottom - no mask, no mask+#8, 80mm, 80mm+#8, 66mm, 66mm+#8, 50mm, 50mm+#8. Exposure times were scaled appropriately.

That #8 filter does a fine job Vlaiv (I just recently bought one myself but not had a chance to use it on my ST120). Even with no mask it has good effect but I think stopping down 20% aperture (i.e. to 80mm in your example) gives an excellent result. Not quite  ED quality but nearing it, would you say?

[vlaiv]

12 hours ago, Mark2022 said:

That #8 filter does a fine job Vlaiv (I just recently bought one myself but not had a chance to use it on my ST120). Even with no mask it has good effect but I think stopping down 20% aperture (i.e. to 80mm in your example) gives an excellent result. Not quite  ED quality but nearing it, would you say?

I can't really remember as it was quite some time ago.

Currently I have 102 / 1000 achromat (as far as achromats go) and I don't need neither aperture mask nor yellow filter with for the most part. Jupiter shows some CA, but I use Baader Contrast Booster to tame it.

There is a chart for achromats:

So you have F/ratio and aperture size and this ratio gives you level of chromatic aberration. If you want to see how the scope will perform without filter - just calculate this CA index.

For example if you have 120/600 scope and you want to stop it down to say CA index of 3 (just as comparison, my F/10 achromat has CA index of 2.5 as it is 4inch scope with F/10 so 10/4 = 2.5, and yes it is indeed in filterable range - but very minor CA).

Then you would need say 70mm mask. With that you have 2.75" aperture and 600/70 = F/8.6 and their ratio is 3.11 (CA index)

That will give you pretty much ED experience. You can certainly get similar experience with CA index of 2.5 and Baader Contrast Booster, so aperture mask of 80mm (CA index of ~2.4).

[Mark2022]

2 minutes ago, vlaiv said:

I can't really remember as it was quite some time ago.

Currently I have 102 / 1000 achromat (as far as achromats go) and I don't need neither aperture mask nor yellow filter with for the most part. Jupiter shows some CA, but I use Baader Contrast Booster to tame it.

There is a chart for achromats:

So you have F/ratio and aperture size and this ratio gives you level of chromatic aberration. If you want to see how the scope will perform without filter - just calculate this CA index.

For example if you have 120/600 scope and you want to stop it down to say CA index of 3 (just as comparison, my F/10 achromat has CA index of 2.5 as it is 4inch scope with F/10 so 10/4 = 2.5, and yes it is indeed in filterable range - but very minor CA).

Then you would need say 70mm mask. With that you have 2.75" aperture and 600/70 = F/8.6 and their ratio is 3.11 (CA index)

That will give you pretty much ED experience. You can certainly get similar experience with CA index of 2.5 and Baader Contrast Booster, so aperture mask of 80mm (CA index of ~2.4).

Yess, I've seen this chart and  it's very useful.  Cheers.

[Mark2022]

Just now, Mark2022 said:

I use Baader Contrast Booster to tame it.

By  the way, how would you rate this filter? I was going to buy one some time back but I have read so much about the #8 being able to tame CA and was never sure or confident that the Baader would really do as I expected so £10 for a yellow #8 rather than 10x that for the Baader made sense.

[vlaiv]

Just now, Mark2022 said:

By  the way, how would you rate this filter? I was going to buy one some time back but I have read so much about the #8 being able to tame CA and was never sure or confident that the Baader would really do as I expected so £10 for a yellow #8 rather than 10x that for the Baader made sense.

I think it is rather good. I can't say if it's worth the money or not to be honest. There are other Baader filters that I think are worth the money - for example Baader Solar Continuum filter. That one is expensive - but it is rather interesting piece of kit - it works on white light solar but it also works for lunar imaging to minimize seeing effects.

It can also help with chromatic aberration (although gives extremely green view, but if you can look past that - view is razor sharp). Can be used for telescope testing as it passes very narrow range of wavelengths where the eye is most sensitive and so on - so you see, it is versatile piece of kit and for that reason, I think it's worth the money (for anyone wishing to do above).

On the other hand - Baader Contrast Booster is well - just contrast booster and it tames a bit of chromatic aberration. It's not wonder filter. It does not remove it all. I still see bluish halo on very bright stars with it. Cast that it imparts on the image is rather subtle. Yes, it is there but after a bit, when the eye/brain adapts - view looks normal.

I guess, it being worth the money will depend on financial situation. It's not clear cut case - like for some bits that are definitively worth or not worth no matter how much extra cash you have lying around. Here - if you can afford, then it's worth, but if you can't, then I would not sweat too much about it.