Couple more Newtonian questions

[TiffsAndAstro]

Would a coma corrector be required on an F5 or f4.5 114mm Newtonian with an imx533 sensor 11 x 11 mm ? On a 1.25" focuser? 

Is the primary mirror fan only for bringing primary down to ambient or does it clear warmer air from in front of primary? Does it need to push air out the top of the tube? Is vibration an issue?

Does an eaf need a good focuser as a base or can an eaf make for say single speed? Some possibly fair bit of back lash

Edited Sunday at 21:30 by TiffsAndAstro

[ONIKKINEN]

1.25'' coma correctors dont really exist at the moment, at least sensible ones. There is a 2.7x barlow/coma corrector but that would be way too much for long exposure if that's what you were after. Radius of the coma free* field of view is roughly f_ratio^3 / 90, so for f/5 would be 1.39mm or a diameter of twice that so still going to see plenty of coma. I would say required still.
*Coma free as in not actually free of coma since coma starts to appear as soon as we are off center, but at this distance it will not be noticeable.

The purpose of a mirror fan is both, but mainly to bring the scope down to ambient. It can be turned off if the night doesn't have a big drop in temperature as it progresses. Pushing air up the tube is more effective, as warm air will rise so the fan is just helping it along the way. You'd need to mount it so that there are no vibrations, say using velcro or silicone glue. Not on the mirror itself of course, on the mirror cell or the back of the tube itself.

Good focusers aren't really about the focusing part but its about the stability of the drawtube. Bad focusers dont hold orientation and will have slop at different tube orientations, making good flats impossible since the optics move in relation to the camera during the night. With bad flats you get wild gradients that are almost impossible to remove, although with a 533 you might get away with it since the sensor is likely small enough to be fully illuminated.

[TiffsAndAstro]

1 hour ago, ONIKKINEN said:

1.25'' coma correctors dont really exist at the moment, at least sensible ones. There is a 2.7x barlow/coma corrector but that would be way too much for long exposure if that's what you were after. Radius of the coma free* field of view is roughly f_ratio^3 / 90, so for f/5 would be 1.39mm or a diameter of twice that so still going to see plenty of coma. I would say required still.
*Coma free as in not actually free of coma since coma starts to appear as soon as we are off center, but at this distance it will not be noticeable.

The purpose of a mirror fan is both, but mainly to bring the scope down to ambient. It can be turned off if the night doesn't have a big drop in temperature as it progresses. Pushing air up the tube is more effective, as warm air will rise so the fan is just helping it along the way. You'd need to mount it so that there are no vibrations, say using velcro or silicone glue. Not on the mirror itself of course, on the mirror cell or the back of the tube itself.

Good focusers aren't really about the focusing part but its about the stability of the drawtube. Bad focusers dont hold orientation and will have slop at different tube orientations, making good flats impossible since the optics move in relation to the camera during the night. With bad flats you get wild gradients that are almost impossible to remove, although with a 533 you might get away with it since the sensor is likely small enough to be fully illuminated.

Ty for all this. The need for a coma corrector rules out a small newt as none have 2" focusers.

I'll ignore them  and maybe look for a 5" with a 2" focuser. 

 

[Elp]

The 130pds has a 2 inch focuser, as do the Stellarlyras, how much smaller do you want to go? Percentage size in relation to aperture of central obstruction starts to become an issue the smaller you go but for what's on the market I don't think this will pose a problem, far more important is how well they're built.

Edited Sunday at 23:56 by Elp

[PeterC65]

I do EAA with an Explorer 150PDS and an Explorer 200PDS using an IMX585 camera (11.2mm x 6.3mm). I've looked for coma several times and found almost none, just a slight elongation of stars when I really zoom in, so I don't use one (I do use a field flattener with my refractor).

I've recently fitted fans to my Newtonian's to keep the secondary dew free and that seems to be working. I have a dew heater band around the primary which alone keeps that dew free, and the fan blows some of the warmed air up towards the secondary keeping that also free from dew. I use a Noctua NF-R8 redux-1200 fan which is designed for quiet running (lowers speed) and doesn't cause any vibration.

 

[TiffsAndAstro]

114mm primary mirror probably light enough to try on my current mount.

130pds is too heavy without extra bits and pieces, so I'll need a new mount first.

If I need a new mount might as well get a 6" newt much as 130pds appeals.

Edited Monday at 10:02 by TiffsAndAstro

[TiffsAndAstro]

39 minutes ago, PeterC65 said:

I do EAA with an Explorer 150PDS and an Explorer 200PDS using an IMX585 camera (11.2mm x 6.3mm). I've looked for coma several times and found almost none, just a slight elongation of stars when I really zoom in, so I don't use one (I do use a field flattener with my refractor).

I've recently fitted fans to my Newtonian's to keep the secondary dew free and that seems to be working. I have a dew heater band around the primary which alone keeps that dew free, and the fan blows some of the warmed air up towards the secondary keeping that also free from dew. I use a Noctua NF-R8 redux-1200 fan which is designed for quiet running (lowers speed) and doesn't cause any vibration.

 

I have a noctua inside my 8 year old pc

Maybe if I can find a very cheap 114mm newt I'll try for practice, but I'll need a new mount to handle a 130pdd or bigger. 8" probably too big as I have to take it down every session.

[PeterC65]

23 minutes ago, TiffsAndAstro said:

Maybe if I can find a very cheap 114mm newt I'll try for practice, but I'll need a new mount to handle a 130pdd or bigger. 8" probably too big as I have to take it down every session.

Mounts that can handle up to a 5kg load (which most of the smaller GOTO mounts can) can usually handle a 130PDS but for anything bigger you will need a higher load capacity mount. I have an AZ-EQ5 which has a load capacity of 15kg (per side when in AZ mode) and it easily handles the 200PDS but would struggle with the 250PDS. The 200PDS is quite a thing to handle physically and is probably the largest OTA that can be fitted to a mount single handed.

 

[Backyardscope]

I believe TS offer a 114mm Newtonian, Carbon Fibre with an 2" Focuser.

Teleskop-Express: TS-Optics Photon 4.5" f/4 Advanced Newtonian Telescope with Carbon Tube-TPC45F4

[Elp]

That looks like a nice little scope. What I would say, I forget what tripod you've got currently, you'd need a good one or at least have walls or shielding around your setup to stop wayward breezes as Newtonians catch them like nothing else (even my C6 barely responds to them unlike my 130pds did).

[TiffsAndAstro]

6 hours ago, Backyardscope said:

I believe TS offer a 114mm Newtonian, Carbon Fibre with an 2" Focuser.

Teleskop-Express: TS-Optics Photon 4.5" f/4 Advanced Newtonian Telescope with Carbon Tube-TPC45F4

I really really like the look of this. But by cheap I was thinking more dented tube on eBay for £50.

Wish you hadn't linked this scope it looks amazing. Not expensive either

It looks really nice. Really nice

[Elp]

You'd have to weigh up whether that 500mm FL and F4 gives you a significant advantage, though the aperture is more important in this regard once you remove the percentage size of the central obstruction.

[TiffsAndAstro]

3 minutes ago, Elp said:

That looks like a nice little scope. What I would say, I forget what tripod you've got currently, you'd need a good one or at least have walls or shielding around your setup to stop wayward breezes as Newtonians catch them like nothing else (even my C6 barely responds to them unlike my 130pds did).

Sw SA GTi stock tripod which I'm much happier with after tightening leg hinge screws. I image in a reasonably sheltered place.

I really like look of that scope. 

[Elp]

Fast newts generally need CC too, so you have to factor that price in too.

[TiffsAndAstro]

Just now, Elp said:

You'd have to weigh up whether that 500mm FL and F4 gives you a significant advantage, though the aperture is more important in this regard once you remove the percentage size of the central obstruction.

F5.83 on 3" compared to f4 on 4.5'

Going to see if they do a 6" 

 

[TiffsAndAstro]

Just now, Elp said:

Fast newts generally need CC too, so you have to factor that price in too.

Yeah £99 is budget version of cc. I think supposed to be ok.

0.95 one would be nice but will cost more than scope

[Elp]

The cheapest decent quality one I sourced was the GSO (Guan Sheng Optical) one from ali, also comes rebranded. It's a solid chunk of glass.

[TiffsAndAstro]

1 hour ago, Backyardscope said:

I believe TS offer a 114mm Newtonian, Carbon Fibre with an 2" Focuser.

Teleskop-Express: TS-Optics Photon 4.5" f/4 Advanced Newtonian Telescope with Carbon Tube-TPC45F4

sadly it has a 1.25" focuser  so no cc corrector

Edited Monday at 11:51 by TiffsAndAstro

[TiffsAndAstro]

1 hour ago, PeterC65 said:

Mounts that can handle up to a 5kg load (which most of the smaller GOTO mounts can) can usually handle a 130PDS but for anything bigger you will need a higher load capacity mount. I have an AZ-EQ5 which has a load capacity of 15kg (per side when in AZ mode) and it easily handles the 200PDS but would struggle with the 250PDS. The 200PDS is quite a thing to handle physically and is probably the largest OTA that can be fitted to a mount single handed.

 

I've seen some videos of people trying to put a big scope on a mount single handed and I'm not keen.

I think 6" newt is ideal for me, but would need a better mount first.

I could risk a 5" on my current mount, but if it can't handle it, I'd still need a better mount but I'd be "stuck" with a 5" rather than my preferred 6"

 

[Backyardscope]

https://www.teleskop-express.de/en/gso-guan-sheng-optical-14/imaging-correctors-for-telescopes-138/komakorr1-18160

[TiffsAndAstro]

Just now, Backyardscope said:

https://www.teleskop-express.de/en/gso-guan-sheng-optical-14/imaging-correctors-for-telescopes-138/komakorr1-18160

What is this witchcraft?

I've googled for 1.25" cc for a month never found one, Ty

[TiffsAndAstro]

19 minutes ago, Backyardscope said:

https://www.teleskop-express.de/en/gso-guan-sheng-optical-14/imaging-correctors-for-telescopes-138/komakorr1-18160

Theoretically this  could also work with my camera and a 2" adapter on 6" newt in the future?

[PeterC65]

5 minutes ago, TiffsAndAstro said:

I've seen some videos of people trying to put a big scope on a mount single handed and I'm not keen.

I cradle the 8" Newtonian horizontally in both arms, lift the dovetail into the saddle, then use my right hand (whilst still cradling the scope) to tighten the clamp. It's straightforward as long as I'm careful. The hardest part is getting the OTA through the patio door. The 6" Newtonian is a lot easier.

8 minutes ago, TiffsAndAstro said:

I think 6" newt is ideal for me, but would need a better mount first.

I could risk a 5" on my current mount, but if it can't handle it, I'd still need a better mount but I'd be "stuck" with a 5" rather than my preferred 6"

I started with a Sky-Watcher SynScan AZ GOTO mount which had a 5kg weight limit and found that it was restricting the scopes I was buying. First I bought a Skymax 127, then a 72mm refractor, both of which it handled adequately. I wanted more aperture though so after much deliberation I bought the Explorer 150PDS. This was pushing my luck with the 5kg mount and it showed. The mount axle needed tightening after using the 6" and it was hopeless in any amount of wind. It's what drove me to upgrade to the AZ-EQ5 which handles the 200PDS fine even in wind.

[TiffsAndAstro]

16 minutes ago, PeterC65 said:

I cradle the 8" Newtonian horizontally in both arms, lift the dovetail into the saddle, then use my right hand (whilst still cradling the scope) to tighten the clamp. It's straightforward as long as I'm careful. The hardest part is getting the OTA through the patio door. The 6" Newtonian is a lot easier.

I started with a Sky-Watcher SynScan AZ GOTO mount which had a 5kg weight limit and found that it was restricting the scopes I was buying. First I bought a Skymax 127, then a 72mm refractor, both of which it handled adequately. I wanted more aperture though so after much deliberation I bought the Explorer 150PDS. This was pushing my luck with the 5kg mount and it showed. The mount axle needed tightening after using the 6" and it was hopeless in any amount of wind. It's what drove me to upgrade to the AZ-EQ5 which handles the 200PDS fine even in wind.

6" I think is the sweet spot for me for all those reasons you mention.

As I'd need a new mount for it, only question (apart from funds) is whether I experiment with a a cheapo eBay 4.5" first or not. The existence of a 1.25" cc means I have a decision to make. 

[PeterC65]

There is a big jump in mount price when you go beyond a 5kg mount payload, particularly for AZ mounts, so I'd recommend trying to decide whether you need to make this jump before investing in kit.