Building a dual rig

[geordie85]

Hi

I've almost got all the pieces for a dual rig apart from a 2nd scope. I've been holding out for an esprit 80 but they're very rare on the 2nd hand market which has got me thinking of other potential scopes for the job.

My main imaging scope is a William optics flt98 carbon fiber OTA with 0.8 flattener, sx filter wheel, astrodon filters and a qhy183mm. I have a ASI zwo 183mm and zwo mini filter wheel which will be connected to my 2nd scope mounted on a skywatcher guide scope mount for alignment which is why I need a smaller slave scope, hence thinking of the esprit 80.

I've been wanting a triplet to reduce CA but then I thought, is CA mainly due to blue bloating? Could I get a doublet but keep the blue filter in my flt98 and almost eliminate the ca that way?

Another question I have is does glass matter? I know FPL-53 glass is superior to FPL-51 but does it matter as I'm only collecting colour data and all the detail will be collected with my main scope?

Is there anything else I should be thinking of? Or maybe suggestions of potential scopes?

 

[SamAndrew]

If you're only shooting colour with a mono camera, then you can refocus for each colour, so colour correction is no longer important (although field flatness is still important) 

[shirva]

Skywatcher evostar ed80 and 0.85 reducer 

[geordie85]

2 hours ago, SamAndrew said:

If you're only shooting colour with a mono camera, then you can refocus for each colour, so colour correction is no longer important (although field flatness is still important) 

I did think that but I wasn't 100% sure hence me asking. Thank you.

 

1 hour ago, shirva said:

Skywatcher evostar ed80 and 0.85 reducer 

That was a consideration but I think it'd be too heavy for the skywatcher guide scope mount.

[shirva]

I  don't get your meaning,,,  twin scopes,, would require double saddle or stacked scopes,,  a second dovetail fitted to the top of first scopes  mount rings.. Is usually the way to go, 

[geordie85]

43 minutes ago, shirva said:

I  don't get your meaning,,,  twin scopes,, would require double saddle or stacked scopes,,  a second dovetail fitted to the top of first scopes  mount rings.. Is usually the way to go, 

That is my plan, but the 2nd scope will need to be aligned so it is pointing to the same place as my main scope. I already have a skywatcher guide scope mount for alignment, but skywatcher haven't said how much weight it can carry without any slippage. I could just use a standard dual saddle but that would result in a big crop after image integration.

Here is my main imaging scope, guide scope and the guide scope mount waiting to be filled.

[inFINNity Deck]

I have adapted an ADM clamp and dovetail to do those adjustments. It works really well, adjusting the scopes does not take more than 10 minutes.

For images see figures 4 and 5 at the bottom of my C11 web-page: http://www.dehilster.info/astronomy/celestron_c11_xlt_edgehd.php

Nicolàs

 

[shirva]

On 23/08/2020 at 02:17, geordie85 said:

That is my plan, but the 2nd scope will need to be aligned so it is pointing to the same place as my main scope. I already have a skywatcher guide scope mount for alignment, but skywatcher haven't said how much weight it can carry without any slippage. I could just use a standard dual saddle but that would result in a big crop after image integration.

Here is my main imaging scope, guide scope and the guide scope mount waiting to be filled.

Got you now 👍

[shirva]

1 minute ago, shirva said:

Got you now 👍

What about a mini guide scope,, orion mini for example 

[ollypenrice]

On 23/08/2020 at 02:29, shirva said:

I  don't get your meaning,,,  twin scopes,, would require double saddle or stacked scopes,,  a second dovetail fitted to the top of first scopes  mount rings.. Is usually the way to go, 

I don't think it's the way to go and don't know of anybody mounting dual rig scopes one on top of the the other. You need to keep the system as stiff as possible so side by side is best. (Otherwise you're using the lower tube rings to carry the upper scope with a huge moment working against the system. You also need to be able to align the scopes. The device of choice is the Cassady T-Gad but it's only available used. FLO offer a good alternative to the T-Gad.

This worked perfectly from the off and never dropped a sub. It was working at 3.5 arcsecs per pixel, however.

But... when we replaced the two Tak FSQ106Ns with a pair of TEC140s working at 0.9 and 1.1"PP respectively we find we sometimes get slight trailing on the 'slave' scope - ie the one not carrying the autoguider. There is plenty of internet experience of getting high resolution dual rigs to work but Peter Goodhew seems to have nailed it by using an active optics unit on the slave scope. Our dual TEC version of the rig in the picture is workable but it would be nice to elmiminate flexure all of the time. We still sometimes get a bit of it.

Olly

[geordie85]

35 minutes ago, shirva said:

What about a mini guide scope,, orion mini for example 

I think you're missing my point of this thread. I was asking if CA would be an issue when using a mono camera and if it would matter if the 2nd scope used FPL-51 glass.

[tooth_dr]

I do find it can quite complicated having a dual ring, and if you pop the blue in another scope it can become a nightmare to keep track - just my experience! I’ve now added a third CCD and lens so have now labelled everything to keep tabs.

In my set up I have one scope taking luminance and the other RGB. Or one taking ha and the other oiii. I have all seven filters in each EFW so it’s a bit of a waste really. But they are bought now and arent eating much grass so I can leave them for the future. 
 

Don’t cut corners on your second scope is my opinion, wait for the triplet. 

Edited August 24, 2020 by tooth_dr

[ollypenrice]

On 21/08/2020 at 12:22, geordie85 said:

Hi

I've been wanting a triplet to reduce CA but then I thought, is CA mainly due to blue bloating? Could I get a doublet but keep the blue filter in my flt98 and almost eliminate the ca that way?

 

 

Blue bloating is a form of CA but might have varying origins. It might be that the scope's focal length in blue doesn't closely match its focal length in red and green. This would be because it wasn't sufficiently apochromatic. However, the blue bloat might come from a failure of the filters to block she shortest blue wavelengths as they approach the UV. Or bloat might arise from a combination of the two.

I think the ED80 might be an awfully big scope to put on the guide scope carrier. You really do need a rigid attachment of the slave scope. This would be a lot better. https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html  I know it's expensive but our Cassady cost nearly twice that second hand!

Olly

[david_taurus83]

For the price of that JTD saddle and a triplet apo, a second mount and an ED80 would be a good alternative...

[geordie85]

On 25/08/2020 at 08:27, ollypenrice said:

Blue bloating is a form of CA but might have varying origins. It might be that the scope's focal length in blue doesn't closely match its focal length in red and green. This would be because it wasn't sufficiently apochromatic. However, the blue bloat might come from a failure of the filters to block she shortest blue wavelengths as they approach the UV. Or bloat might arise from a combination of the two.

I think the ED80 might be an awfully big scope to put on the guide scope carrier. You really do need a rigid attachment of the slave scope. This would be a lot better. https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html  I know it's expensive but our Cassady cost nearly twice that second hand!

Olly

My original plan is well out of the window now as I bought an esprit 100 so definitely need something alot stronger than a guide scope mount.

Thanks for the explanation.

[Davey-T]

31 minutes ago, geordie85 said:

I think the ED80 might be an awfully big scope to put on the guide scope carrier. You really do need a rigid attachment of the slave scope. This would be a lot better. https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html

A very nice well made bit of kit but rather over engineered with masses more adjustment than needed (  IMHO )  better  and cheaper to make something.

Dave

[geordie85]

56 minutes ago, Davey-T said:

A very nice well made bit of kit but rather over engineered with masses more adjustment than needed (  IMHO )  better  and cheaper to make something.

Dave

I'm really bad at DIY so I'll just buy something