Small APO on top of GSO RC8, tube deformation?

[lowsfer]

My first thread here :)

I plan to get a SharpStar 65Q and a GSO RC8+CCDT67.

I want to bind them together, so this single setup covers three focal lengths: 420mm, 1072mm and 1600mm. When one scope is used for photography, the other one is used for guiding.

But I'm concerned about the 2.65kg weight of 65Q. If it's mounted on top of RC8, will that weight deform the tube of RC8?

Any experience?

If it does, are there any other lighter small APOs at <$500 margin? I would like a 300-500mm focal length.

[Waxing_Gibbous]

Been a while since I did any imaging but I had, among other stuff, an 80mm SV triplet - quite heavy for its size -as a guide scope riding atop my Mak. I experienced no deformation despite the tube being thinner than most other compound 'scopes(weight saving apparently).
From memory - examining one in the shop - I would think you could mount almost anything up to about 7kg atop the GSO, without experiencing any ill effects.

[blink138]

yes i had a celestron 102 f5 achro on my c11, no probs, at f6.3 anyhow, but maybe at f10 a bit of flexure showed up
yours should be ok
pat

[lowsfer]

Thank Waxing_Gibbous and blink138 for the comments. But Mak's have a lens in the front of the tube, which may help support the tube, I guess?

RC8 is open like newtonians. Would that make the situation worse? Do you have experience of mounting heavy scopes on top of newtonians?

[DJT]

I have the carbon fibre RC8 and at one point did have a Tak FS60 riding on the top with similar intentions.

The thing with the RC8 though is that it is heavy at the back and it comes with this huge amount of back focus (240mm give or take) in the form of extension rings which whilst its great for putting things in the imaging train it does mean that balance is fun. Start adding STL CCD's and filter wheels and it's an absolute hoot. I added a set of losmandy counter weights originally on the rail on the ota at the front to balance then relocated them to an extra long mount saddle at the front so there was no addition force on the OTA itself.

I was never really happy with using the little apo for guiding at the 1600 fl. There were still guiding issues and undoubtedly some flexure problems so I went with an OAG and because I wasn't using the TAK much, took it off. It's just unnecessary weight if not used which again has to be balanced across multiple axies and in an effort to get everything just so and simplify things, I stripped the rig down to only what I need to image at the native focal length. (I had the reducer as well but never used it as I like the FOV as it is) If I want to do widefield, it's straight forward to put the Tak back on and guiding is still covered by the OAG setup.

This works well for me personally. I couldn't tell you if the OTA was affected by the additional scope and the extra weights you need but I went with an approach of eliminating any possible causes of problems across the setup rather than point fixes and as a result my data is that much better.

The RC8 is great little scope. My one holds focus incredibly well (carbon fibre tube but an after market focuser was also added...),is quick to cool down, the OAG is awesome and I am getting good data with it. just a pity I am not that smart at processing..ho hum.:help:

[lowsfer]

OK, another issue I never considered. SCT has a front lens which helps balancing. Have you ever considered an ADM side-by-side mounting plate? I'm considering buying this.

[graham.hobart]

I had a metal tube Rc 8 and a CF Rc 10, both of which I placed a 110mm refractor on the top and sometimes a small guide scope mini guider on the top of that. I used an OAG with the Rc 10 but not with the rc 8. I never noticed any change in star shapes that would you expect from weight deforming the tube, i.e pointing at different parts of the sky.
I think if the weight is spread and well balanced you will be OK.
Graz

[lowsfer]

WOW that's a lot of weight. I was planning to buy a side-by-side saddle.

Thanks for the information! BTW, what reducer do you use for them? CCDT67 or 27TVPH? Any flattener?

[graham.hobart]

I had the CCDT67 but sold it as I never used it as I only used a KAF 8300 chip on the RC's.

[lowsfer]

Why? I thought CCDT67 was suitable for 8300. Am I wrong?

[graham.hobart]

Yep its suitable, I just didn't mind the edges of the images cropped so I never bothered using it.