cheap f4 reflector: am I crazy?

[laser_jock99]

2 hours ago, ollypenrice said:

The ones that have been difficult for me (difficult to the point of being impossible) have been the ones in which changing one parameter changes another when it shouldn't. This usually arises from inadequate maechanical integrritiy somewhere in the system. Somewhere - but where? The GSO residing in our loft, for instance, has an insufficiently rigid tube. Short of changing that, I don't see it working. 

Olly

I had to make three major modifications to the GSO 12" F4 to overcome the scopes' short comings 'as supplied'. You can buy pre-modified GSO optical sets but these are not cheap. I took the DIY route:

1) Primary Mirror cell- replacement of springs and adjuster knobs. Leave the secondary and spider well alone if possible as messing about with this is the direct cause of most collimation woes you read about....

2) Beef up the tube support- the provided dovetail will be inadequate. A heavy mirror at one end and heavy camera at the other end of a steel tube will cause the tube to flex. Move the tube rings appart, closer to the loading and ADD A TOP BAR!

The standard 14" Losmandy bar (middle) my GSO 12" F4 came with vs the one I had milled out at work.

 

 

3) Focuser + Camera - these are heavy items to hang off the tube and may deform it. I riveted a strengthening plate inside the tube around the focuser end.

 

 

None of the of the above modifications are particularly difficult- and you might argue you shouldn't even have to do this to a new scope. However we are talking about budget scopes here and that is the OP's point. The Revelation Astro (GSO) 12" F4 retails at a bargain £575, exactly the same optics in the Teleskop Service moddified version is another £900 more. My modifications cost me <£20 (I had pay for the springs & adjuster knobs). Sure a carbon tube would be lighter but the EQ6 can actually cope!

[alacant]

Hi lj99, hi everyone. Thanks for the detailed guide. I see that the modifications are for your 12" f4. Did you modify your 8" in the same way? I have the 208 (8"?)f3.9 coming next week. Of course, I'll try it out of the box first but could you have a quick look at the design? It has the top bar you mention, although maybe at not wide enough a separation. Would your parts list e.g. the springs apply equally to the 8"? I don't have machining facilities but am OK with drilling dovetails, removing mirrors -note taken about the secondary- and replacing bolts. It would be good to be up before next new moon; the parts seem quite economical but I fear that shipping to Spain may be slow... TIA and clear skies.

[laser_jock99]

On 14/01/2017 at 13:41, alacant said:

Hi lj99, hi everyone. Thanks for the detailed guide. I see that the modifications are for your 12" f4. Did you modify your 8" in the same way? I have the 208 (8"?)f3.9 coming next week. Of course, I'll try it out of the box first but could you have a quick look at the design? It has the top bar you mention, although maybe at not wide enough a separation. Would your parts list e.g. the springs apply equally to the 8"? I don't have machining facilities but am OK with drilling dovetails, removing mirrors -note taken about the secondary- and replacing bolts. It would be good to be up before next new moon; the parts seem quite economical but I fear that shipping to Spain may be slow... TIA and clear skies.

Replacing the primary mirror springs/knobs is a bit of no-brainer, just do it as make collimation a breeze.

Full details for the spring replacment mod for the 8" F4 GSO is here or you might be able to obtain a kit for the Explorer from Bobs Knobs?

Looking at the photo of the Explorer 8" the supplied dovetail is a rather short 200mm Skywatcher one so I'd be tempted to fit say a 14" Losmandy bar and a matching piece of aluminium profile for the top bar.

I first tested the tube/focuser re-inforcing mod on my 8" F4 scope. But to be fair it's probably not really worth while on this scope as the shorter, smaller diameter tube is more rigid than than the 10" & 12" already. I'd probably use the scope a bit first as you may find it's not required.

[Adam J]

The one I have my eye on is the 6 inch model....I am assuming that tube flex is not going to be such an issue in such a short tube. 

[alacant]

18 hours ago, laser_jock99 said:

I'd probably use the scope a bit first

Yes. Good advice. As this is the 3rd incarnation of the ES, I'm hoping that they've got at least some of the stuff you encountered with the gso sorted out. I know they've had to change the secondary due to a design flaw and users of the same over on astrobin tell me that the latest has yet another type of secondary. Who knows, a cheap f4 usable out of the box... What's that they say about pigs and aeronautics?!

[Adam J]

8 hours ago, alacant said:

Yes. Good advice. As this is the 3rd incarnation of the ES, I'm hoping that they've got at least some of the stuff you encountered with the gso sorted out. I know they've had to change the secondary due to a design flaw and users of the same over on astrobin tell me that the latest has yet another type of secondary. Who knows, a cheap f4 usable out of the box... What's that they say about pigs and aeronautics?!

Its all a matter of thrust.

[alacant]

Hi. It arrived a few days ago and so has been cloudy ever since. First impressions are good. The aluminium seems a notch up from the rolled steel skywatcher type tube and it can be adjusted without tools. It holds collimation well with strong springs on the primary and the type of conical adjusting screw advocated by laser_jock99. In the hope of getting a break in the cloud, I've knocked it, left it out all night, brought it inside, got condensation on it and slewed it around for the last few nights and it's kept collimation so far, but that's just beginners luck.

**Oh, and don't assume anything upon delivery. I wasted ages trying to work out why the collimation reflection always ended up concentric , as per slow reflectors. 

[iapa]

On 11/01/2017 at 22:40, laser_jock99 said:

The GSO F4 Newtonians are okay budget scopes (I  own the 6", 8", 10" & 12" versions).

http://www.teleskop-express.de/shop/product_info.php/info/p52_TS-Optics-GSO-8--f-4-Newtonian-Telescope---optical-tube-assembly.html

The optical quality is good and the mechanics can be easily modified to make the scope as stable as the more expensive 'tuned' F4 scopes.

 

GSO????

[laser_jock99]

40 minutes ago, iapa said:

GSO????

Guan Sheng Optical- a Taiwanese manufacture that supply the optics and often the mechanics for many re-branded scopes (eg Revelation Astro, Teleskop Service and many others).

http://www.gs-telescope.com/content.asp

 

[iapa]

1 minute ago, laser_jock99 said:

Guan Sheng Optical- a Taiwanese manufacture that supply the optics and often the mechanics for many re-branded scopes (eg Revelation Astro, Teleskop Service and many others).

http://www.gs-telescope.com/content.asp

 

Thanks.

So, find out who they supply to, checks prices and the mechanics then decide who you want to buy from

can this hobby not get just a bit simpler - nah, I expect not

?

[Adam J]

cant wait to see some images from this scope. So pictures of the scope would be awesome too. 

[alacant]

Well, 5 days in and of course it's still raining. Not just me either -you should hear the tourists. Not a very exciting snap I know but one of the best bits so far is the secondary adjusting system; it's almost predictable. That has to be a first. Looks like we have a few hours tomorrow before the cloud returns for the weekend.

[Adam J]

7 hours ago, alacant said:

Well, 5 days in and of course it's still raining. Not just me either -you should hear the tourists. Not a very exciting snap I know but one of the best bits so far is the secondary adjusting system; it's almost predictable. That has to be a first. Looks like we have a few hours tomorrow before the cloud returns for the weekend.

Yes I like the looks of that, more substantial than the one on my 130PDS so most likely an upgrade in accuracy, can only hope its the same on the 6inch model. I am going to grab one just as soon as I have sorted my obsy out over this summer. I am hoping that I will have an easier time of it if I am not having to move the scope in and out of the house all the time. How is the focuser? I hang quite some weight off of mine.

[alacant]

38 minutes ago, Adam J said:

How is the focuser?

It's a 2.5" no nonsense rack and pinion with low ratio; easy to fine focus but pain if you're doing visual and changing eyepieces. There are adaptors from 2.5 to 2 and 2 to 1.25 and a 2.5" extension tube thrown in. It's the same as on my 6". It takes my Canon dslr fine. HTH.

[alacant]

Thanks everyone for getting me to first light. One or two issues, but it's looking promising. This is what it can do in 20 minutes:

[Adam J]

2 hours ago, alacant said:

Thanks everyone for getting me to first light. One or two issues, but it's looking promising. This is what it can do in 20 minutes:

Looking good, stars look good.

What are the exact details? Camera / exposure / mount etc.

What coma corrector are you using?

[Astrosurf]

Lovely image.

[alacant]

3 hours ago, Adam J said:

exact details

Hi. 17 60s light frames and 20 1/4000s bias frames. My daytime canon 1200d and iso800, baader coma corrector and moon and skyglow filter on eq6 phd2 apt eqmod windows 10. That's as exact as I can do as I'm in the middle of another clear night with a note stuck to the camera to remind me to do flat frames this time.

I've a few coma issues -I belive the baader is sensitive to spacing- if anyone has experience of the baader at f3.9 I'd really appreciate any advice on this. So far, it's not turned out to be the horror story you read elsewhere. I'm sure worse is to come!

Thanks for your interest.

Edit: forgot, deepskystacker

[Adam J]

30 minutes ago, alacant said:

Hi. 17 60s light frames and 20 1/4000s bias frames. My daytime canon 1200d and iso800, baader coma corrector and moon and skyglow filter on eq6 phd2 apt eqmod windows 10. That's as exact as I can do as I'm in the middle of another clear night with a note stuck to the camera to remind me to do flat frames this time.

I've a few coma issues -I belive the baader is sensitive to spacing- if anyone has experience of the baader at f3.9 I'd really appreciate any advice on this. So far, it's not turned out to be the horror story you read elsewhere. I'm sure worse is to come!

Thanks for your interest.

Edit: forgot, deepskystacker

Actually for 60s frames that is an amazing result.

[alacant]

27 minutes ago, Adam J said:

60s frames

Just did another: 18x45s with the same bias frames. <smug>It makes f5 look miserably dark</smug> Only joking!

[Adam J]

I am absolutely getting a F4 scope once I sort out my role off roof observatory, will make life easier if the scope is not being continually being mounted and unmounted. Very nice results. 

What spaceing are you using on the baader CC?

Would be good to see some longer exposures too. The results should be impressive given what you got from M45 with only 60 seconds. Are you guiding? 

 

 

[alacant]

2 hours ago, Adam J said:

What spaceing are you using on the baader CC?

Hi. I'm using it with a wide 48 t adaptor and a 3mm spacer ring which I think gives me the correct spacing, but the light frames show coma in the corners. It's not bad and easily correctable in software but still it would be good to eliminate it. I wish I could find info on what spacing I need; I believe even 1mm out can be critical... Maybe it's gonna have to be trial and error. Does anyone know what the spacing should be? TIA.

2 hours ago, Adam J said:

Are you guiding? 

Sometimes, but I'm lazy. My EQ6 is good up to around 30s at this focal length. Beyond that I need to drift. But I need phd for that anyway so I usually and up guiding anyway.

I only have my daytime camera ATM. As soon as I get my modified camera back I'm keen to try nebulae using longer exposures.

[alacant]

It seems quite happy on my old rusty eq6. The USB hub fixed with Velcro to the tube avoids my usual mess of cables back to the computer. 

 

[ollypenrice]

That is giving good results. No question.

Olly

[alacant]

Almost forgot. Secondary collimation: you have to get the primary donut on the Cheshire cross-hairs. Why do guides recommend having the tube horizontal? It's impossible to do like that. With the tube pointed vertically, the secondary adjusters behave in a totally predictable manner and it takes just a few seconds. At any other angle, you're up against gravity pulling against the secondary and it becomes a frustrating mess. Of course, you must have tool-less adjusters. HTH.