F5 Vs F4

[Pitch Black Skies]

I'm considering upgrading my F5 130PDS to a 6" TS F4.

I know F4 will reduce the exposure time, but by how much? How is it calculated?

Also, does it reduce my total integration time too, or do I still need the same integration time but now I have more subs to stack so better SNR?

 

[alacant]

Hi

Be careful. The bottom of the range TS is a nightmare.

Cheers

**edit: for those not following the link, that's because of its design. Not because it's f4.

Edited March 30, 2022 by alacant

[Clarkey]

Personally I would stick with your 130PDS. I have the TS F4 and it took quite a bit of additional expense to get it to give decent images. Firstly, the Baader MPCC I had for my F5 scope was not up to the task of giving good stars to the edge of the sensor. I ending up paying for a Skywatcher F4 aplanatic CC. In addition, the focuser was not really up to the job so I replaced it with a Steeltrack. There were also a number of other changes required to make it an effective imaging scope.

Yes it is quite fast, but for the amount of work needed to get a 'good' scope I would look at something different. The F4 also requires very careful collimation - MUCH tighter than the F5. The other problem I had was awful internal reflections. Although not directly linked to the scope - it was the filters - to use the mono camera would require me to buy a whole new set of filters. I will try it with a DSLR as the reflections may not be an issue.

I don't know what mount you have, but you could look at a 150 or 200mm F5.

I bought a StellaMira F6 90mm ED triplet instead..... but I am nearly £1000 worse off!

[Nik271]

45 minutes ago, Pitch Black Skies said:

I'm considering upgrading my F5 130PDS to a 6" TS F4.

I know F4 will reduce the exposure time, but by how much? How is it calculated?

Also, does it reduce my total integration time too, or do I still need the same integration time but now I have more subs to stack so better SNR?

 

To answer your question, the difference of the speed of F4 vs F5 scope (using the same camera) is the square of their F-ratios. So the F4 scope will be 16/25= 64% faster:

all other things being equal you will need only 64% of the total integration time with the F4 scope. If you use a dedicated astro camera it's slightly better to keep the same length of individual frames, but decrease their number. If you use DSLR which usually generate thermal noise in long exposures it might be better to decrease the length of the frames instead (only applies if you use frames of length several minutes).

Most people find the difference in speed between F4 and F5 is not worth the extra need for manufacturing precision, collimation and corrections of aberrations the F4 mirror will require.

 

Edited March 30, 2022 by Nik271

[Pitch Black Skies]

On 30/03/2022 at 19:23, Clarkey said:

Personally I would stick with your 130PDS. I have the TS F4 and it took quite a bit of additional expense to get it to give decent images. Firstly, the Baader MPCC I had for my F5 scope was not up to the task of giving good stars to the edge of the sensor. I ending up paying for a Skywatcher F4 aplanatic CC. In addition, the focuser was not really up to the job so I replaced it with a Steeltrack. There were also a number of other changes required to make it an effective imaging scope.

Yes it is quite fast, but for the amount of work needed to get a 'good' scope I would look at something different. The F4 also requires very careful collimation - MUCH tighter than the F5. The other problem I had was awful internal reflections. Although not directly linked to the scope - it was the filters - to use the mono camera would require me to buy a whole new set of filters. I will try it with a DSLR as the reflections may not be an issue.

I don't know what mount you have, but you could look at a 150 or 200mm F5.

I bought a StellaMira F6 90mm ED triplet instead..... but I am nearly £1000 worse off!

Okay so that's yourself and Alacant warning against it. Will probably give it a miss so. There's a true saying, you get what you pay for.

Maybe a Quattro 8s? I have the HEQ5, not sure if it could handle it?

I have the Aplanatic CC. The 150 and 200 will only bring me a bit closer to the target though won't they. What I'm really after is shorter imaging times.

Edited April 7, 2022 by Pitch Black Skies

[Pitch Black Skies]

2 hours ago, Nik271 said:

To answer your question, the difference of the speed of F4 vs F5 scope (using the same camera) is the square of their F-ratios. So the F4 scope will be 16/25= 64% faster:

all other things being equal you will need only 64% of the total integration time with the F4 scope. If you use a dedicated astro camera it's slightly better to keep the same length of individual frames, but decrease their number. If you use DSLR which usually generate thermal noise in long exposures it might be better to decrease the length of the frames instead (only applies if you use frames of length several minutes).

Most people find the difference in speed between F4 and F5 is not worth the extra need for manufacturing precision, collimation and corrections of aberrations the F4 mirror will require.

 

Cool, thanks Nik.

I'm using a 533MC with Aplanatic CC.

64% faster, so does that mean less than half the total integration time?

Edit - 25/16 = 1.56 times faster. Is that correct?

So a 300s sub with F5 would be 200s with F4.

 

Do you recommend sticking with the F5 then?

 

Edited March 30, 2022 by Pitch Black Skies

[Nik271]

Sorry! my statement  was not very precise, I meant to achieve the same SNR with the new F4 scope you need  64% of total integration time of the old F5 scope. So 100 hours  total at F5 equals 64 hours at F4. This new camera looks very sensitive and with extremely low read noise so the length of the individual frames should not matter too much.

With the F4 scope you will be saving 36% of the  total time compared to  the130PDS, to me this does not seem worth the extra hassle.

[Clarkey]

19 minutes ago, Pitch Black Skies said:

The 150 and 200 will only bring me a bit closer to the target though won't they. What I'm really after is shorter imaging times.

The 200 is around 1000mm focal length which is quite a bit above the 130. Also, in terms of pixel scale, above this you would usually be binning data anyway so there is not a huge amount to gain from the longer FL. However, a 200pds or quattro will be pushing the limits of a HEQ5.

In reality F5 is pretty quick, especially at 650mm FL. F4 would theoretically slightly better but more difficult. There are newtonians produced by Sharpstar that go as fast as F2.8 but they are expensive. They are also not the easiest to work with and would require some modification. I now have an F4 that I will endeavour to work with in the autumn when the nebula's come back out to play. However, with hindsight I should have gone for an F5 or a better scope such as the quattro. But these would have gone an an AZ-EQ6.

[Pitch Black Skies]

1 hour ago, Nik271 said:

Sorry! my statement  was not very precise, I meant to achieve the same SNR with the new F4 scope you need  64% of total integration time of the old F5 scope. So 100 hours  total at F5 equals 64 hours at F4. This new camera looks very sensitive and with extremely low read noise so the length of the individual frames should not matter too much.

With the F4 scope you will be saving 36% of the  total time compared to  the130PDS, to me this does not seem worth the extra hassle.

I never had an F4 so not sure how much hassle it would be, but I'll take your word for it. I can only imagine what something like f2.8 is like then.

[Pitch Black Skies]

1 hour ago, Clarkey said:

The 200 is around 1000mm focal length which is quite a bit above the 130. Also, in terms of pixel scale, above this you would usually be binning data anyway so there is not a huge amount to gain from the longer FL. However, a 200pds or quattro will be pushing the limits of a HEQ5.

In reality F5 is pretty quick, especially at 650mm FL. F4 would theoretically slightly better but more difficult. There are newtonians produced by Sharpstar that go as fast as F2.8 but they are expensive. They are also not the easiest to work with and would require some modification. I now have an F4 that I will endeavour to work with in the autumn when the nebula's come back out to play. However, with hindsight I should have gone for an F5 or a better scope such as the quattro. But these would have gone an an AZ-EQ6.

Nice, I was looking at the 8" Quattro too but probably too much to ask for the HEQ5. Knew I should've got the EQ6R 😂

[iantaylor2uk]

I had a 12" Orion Optics (DX300) f/4 scope and personally wouldn't go more than 60 sec exposures with an astro camera. I got some good photos of M51 with just 10 sec exposures, after stacking two or three hundred of them.

[Pitch Black Skies]

5 minutes ago, iantaylor2uk said:

I had a 12" Orion Optics (DX300) f/4 scope and personally wouldn't go more than 60 sec exposures with an astro camera. I got some good photos of M51 with just 10 sec exposures, after stacking two or three hundred of them.

Nice, have you still got the pictures?

[Adam J]

5 hours ago, Pitch Black Skies said:

I'm considering upgrading my F5 130PDS to a 6" TS F4.

I know F4 will reduce the exposure time, but by how much? How is it calculated?

Also, does it reduce my total integration time too, or do I still need the same integration time but now I have more subs to stack so better SNR?

 

Don't do it unless you really are not bothered by imperfect stars. 

 

[Pitch Black Skies]

5 hours ago, Adam J said:

Don't do it unless you really are not bothered by imperfect stars. 

 

Cheers Adam, do you mean the TS or going to f4 in general?

[Clarkey]

I think to add some context, here are a couple of images taken with the F4 6" TS Newtonian. This is with a new focuser and a number of mods to give OK results. Make your own judgement. There are also lots of images on line. It should be added that I am no expert and my processing skills still need work....

[Pitch Black Skies]

24 minutes ago, Clarkey said:

I think to add some context, here are a couple of images taken with the F4 6" TS Newtonian. This is with a new focuser and a number of mods to give OK results. Make your own judgement. There are also lots of images on line. It should be added that I am no expert and my processing skills still need work....

Really nice, although I wonder if the 130 could produce something similar? I'm guessing yes, but it would require more time.

[Adam J]

3 hours ago, Pitch Black Skies said:

Cheers Adam, do you mean the TS or going to f4 in general?

My opinion in general on any scope is that you need to spend the big cash to get reliable performance below F5. So for a reflector that means going to something like a hyperbolic mirror Newtonian or a RASA etc. There are better F4 newts available but I dont know of any in the 6 inch class. 

Adam 

[Clarkey]

The 130 is certainly capable. I only ever image one target per night and am able to leave my kit out so never have less than 6 hours integration time. Quite often for faint stuff it will be more than one night. Makes all the difference. F4 does help in this case.

[Pitch Black Skies]

40 minutes ago, Clarkey said:

The 130 is certainly capable. I only ever image one target per night and am able to leave my kit out so never have less than 6 hours integration time. Quite often for faint stuff it will be more than one night. Makes all the difference. F4 does help in this case.

In a similar situation myself but I try to get about 3 nights per target. I'm aiming for 16-24 hrs total integration time.

[Pitch Black Skies]

42 minutes ago, Adam J said:

My opinion in general on any scope is that you need to spend the big cash to get reliable performance below F5. So for a reflector that means going to something like a hyperbolic mirror Newtonian or a RASA etc. There are better F4 newts available but I dont know of any in the 6 inch class. 

Adam 

Do you think the Quattro 8s would be a more sensible upgrade?

[alacant]

15 minutes ago, Pitch Black Skies said:

the Quattro 8s

If you had an eq6 it would be good.

Remember though that with all low end reflectors, there is still quite a bit of work to do to get them to astrograph standard. The difficulties with adjusting and retaining collimation are a result of not addressing the necessary upgrades; one collimates an f4 in EXACTLY the same way as an f5. The upgrades are outlined here. Apart from the dovetail plate, none are particularly expensive, but you will need time to pull the thing apart to replace the defective parts.

Even though it's still far from ideal, if you want to stay at 6" and comfortable on your mount, the TS UNC is the nearest sensibly designed f4 we've seen, but even then, the enormous secondary is blocking so much light that you may as well stay with your 130.

Cheers and HTH

[iantaylor2uk]

11 hours ago, Pitch Black Skies said:

Nice, have you still got the pictures?

There are some at the links below - note though that I didn't use a coma corrector or UV/IR filter for these photos, so they have been cropped a bit. I used an APS-C sized sensor (a ZWO 071 Pro camera). You can still see a bit of coma towards the edges. I now have a coma corrector for my reflectors! Also I collimated the reflector before imaging using a laser collimator.

M3 & M51: 10, 30 & 60 sec exposures: https://chesterastroblog.wordpress.com/2021/04/11/m3-and-m51/

M101: 60 sec exposures: https://photos.google.com/share/AF1QipOnmkqD1kWZYsvqMm8W8diHwSoXJn47oD7ZgiVB2iS3j6m_KeyJCI-lWtSR6Ss36g?pli=1&key=cnZtb3RGbHE3aWxNSGQ5OS1CN2dva1YyYzVnanpR

Hope this helps

 

[Pitch Black Skies]

2 hours ago, alacant said:

If you had an eq6 it would be good.

Remember though that with all low end reflectors, there is still quite a bit of work to do to get them to astrograph standard. The difficulties with adjusting and retaining collimation are a result of not addressing the necessary upgrades; one collimates an f4 in EXACTLY the same way as an f5. The upgrades are outlined here. Apart from the dovetail plate, none are particularly expensive, but you will need time to pull the thing apart to replace the defective parts.

Even though it's still far from ideal, if you want to stay at 6" and comfortable on your mount, the TS UNC is the nearest sensibly designed f4 we've seen, but even then, the enormous secondary is blocking so much light that you may as well stay with your 130.

Cheers and HTH

Yeah it's probably too much for the HEQ5. I think I'll stick with the 130 for now. It's a great little scope when tuned up.

Would you recommend replacing the 130s dovetail with a Losmandy plate?

Actually, if I post a picture of my setup in the 130 thread, would you take a look and recommend some improvements or mods?

[Pitch Black Skies]

1 hour ago, iantaylor2uk said:

There are some at the links below - note though that I didn't use a coma corrector or UV/IR filter for these photos, so they have been cropped a bit. I used an APS-C sized sensor (a ZWO 071 Pro camera). You can still see a bit of coma towards the edges. I now have a coma corrector for my reflectors! Also I collimated the reflector before imaging using a laser collimator.

M3 & M51: 10, 30 & 60 sec exposures: https://chesterastroblog.wordpress.com/2021/04/11/m3-and-m51/

M101: 60 sec exposures: https://photos.google.com/share/AF1QipOnmkqD1kWZYsvqMm8W8diHwSoXJn47oD7ZgiVB2iS3j6m_KeyJCI-lWtSR6Ss36g?pli=1&key=cnZtb3RGbHE3aWxNSGQ5OS1CN2dva1YyYzVnanpR

Hope this helps

 

Nice detail in them.

[alacant]

1 hour ago, Pitch Black Skies said:

Losmandy plate?

Every bit helps. 

1 hour ago, Pitch Black Skies said:

post a picture

Yes, of course.