Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Trying to decide how to upgrade OTA for ALPY600

  • Please log in to reply
11 replies to this topic

#1 mborland

mborland

    Viking 1

  • *****
  • topic starter
  • Posts: 617
  • Joined: 17 Mar 2020
  • Loc: Tucson

Posted 25 February 2024 - 11:17 PM

I have an ALPY 600 with guiding and calibration modules that I presently use on an Esprit 120 at f/7. It works reasonably well, but the ALPY 600 is designed for f/4 and I'd like to take advantage of that to get 3x faster data acquisition and reach fainter objects.

 

I have the Starizona 0.65x APEX ED L reducer, but I can't use it because, even with the Starizona Esprit 120 adapter, it requires so much focuser extension that my camera will hit the ground when imaging at high altitudes. Raising the mount isn't possible because it is in a very small dome.

 

For the same reason, I don't use the SW field flattener on the Esprit 120. That means my guider images show very distorted stars, which means plate solving doesn't work. I've worked around that using a separate guide scope for plate solving only, which works but there are slight offsets that are hard to tune out.

 

I'm considering various upgrade options to get f/4 with a flat field guide image

  1. 8" SCT with Starizona 0.4x Night Owl reducer. This would be great, except the Night Owl is not available now and, more critically, backfocus is only 38.5 mm. The ALPY 600 with guiding and calibration modules requires at least 64mm. I assume that even if I could achieve focus in the center, stars in the guiding image would be badly distorted.
  2. 8" SCT with Starizona 0.63x SCR reducer. This has 90 mm backfocus, but f/6.3 isn't enough of an improvement over f/7 to justify the purchase.
  3. 8" f/4 imaging Newtonian (e.g., GSO or one of the rebrands) with Baader 2" Rowe Coma corrector. The coma corrector gives 90mm backfocus. However, I'm concerned about the stability of the focuser and its connection to the scope with all the gear hanging off it at varying angles. I suppose I could always dig deeper into my pocket and upgrade the focuser (to what?).
  4. High-end 8" f/4 imaging Newtonian, e.g., one of TS German-made OTAs. Although the scope and focuser are probably far superior, the price is much higher. Shipping alone is almost equal to the cost of a high-end focuser!

I'm leaning toward option 3, but wonder if there are options or issues that I've missed.

 

Any suggestions or comments will be appreciated.

 

--Michael

 

 



#2 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 26 February 2024 - 08:34 AM

A fast Newtonian "light bucket" is optically and cost-wise the best solution. (No need for a coma corrector for spectroscopy as we are operating on the optical axis) Mechanically it is more of a problem though and back focus more limited as you say. (It is a shame RC are not faster). One thing to watch out for is the effect of modern dialectric mirror coatings  which can roll off very fast at the UV end in the quest for a few percent improvement in the visible. Unfortunately published figures are hard to come by but this is an example of a published curve for an RC from TS

 

https://www.cloudyni...ons/?p=12591904

 

Cheers

Robin



#3 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 26 February 2024 - 08:43 AM

Plate solving to find the target is probably best done using a separate finder. I star hop rather than plate solve and  have recently upgraded my old webcam on the back of the standard SCT finder to  a small low cost 30mm  finder plus an ASI120mm and I am amazed how much deeper it can go so can get to the target quicker



#4 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 26 February 2024 - 09:03 AM

A fast Newtonian "light bucket" is optically and cost-wise the best solution. 

Freedom from chromatism is a big plus when trying to get an accurate flux calibration. See this scary study by Christian Buil for example

http://www.astrosurf...ersion/atmo.htm

I use a reducer with a C11 SCT but I have to be careful that focus does not drift between target and reference and when accuracy of the continuum is particularly important I take duplicate references before and after the target



#5 mborland

mborland

    Viking 1

  • *****
  • topic starter
  • Posts: 617
  • Joined: 17 Mar 2020
  • Loc: Tucson

Posted 26 February 2024 - 09:38 AM

Robin,

 

Thanks for the detailed reply. The UV reflectivity issue is a new one to me.

 

I sent an email to GSO (makers of most of the affordable Newtonians, it seems) to ask if they can supply reflectivity data in the UV for their mirrors. There's nothing on their web site, but I imagine they must have such information. We'll see if they are willing to share it.

 

--Michael



#6 mborland

mborland

    Viking 1

  • *****
  • topic starter
  • Posts: 617
  • Joined: 17 Mar 2020
  • Loc: Tucson

Posted 04 March 2024 - 12:51 AM

I didn't hear anything from GSO or Teleskop-Service about the spectral reflectivity of their mirrors. I didn't even get a polite acknowledgement of my inquiry. I had hoped that with TS's ONTC "high-end Newtonians", they'd have some data to share.

 

SkyWatcher at least responded, but said that they don't have such information as they only look at visible wavelengths.

 

--Michael



#7 pvdv

pvdv

    Vostok 1

  • -----
  • Posts: 186
  • Joined: 06 Mar 2022

Posted 04 March 2024 - 07:29 AM

There is some generic info about the spectral response of different mirror coatings here

https://www.edmundop...irror-coatings/

As far as most Internet astro shops are concerned, I think only questions such as "Can I have blue ZWO cameras?" have a chance to be answered. ;)



#8 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 04 March 2024 - 06:47 PM

There are some discussions on coatings in various threads on the ARAS forum about the UVEX spectrograph which was designed to work down to the ozone absorption limit.

http://www.spectro-a...wforum.php?f=45

for example

http://www.spectro-a...opic.php?t=2277

 

Cheers

Robin



#9 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 05 March 2024 - 07:43 AM

I didn't hear anything from GSO or Teleskop-Service about the spectral reflectivity of their mirrors. I didn't even get a polite acknowledgement of my inquiry. I had hoped that with TS's ONTC "high-end Newtonians", they'd have some data to share.

 

 

 I found TS are generally pretty good at responding to enquiries but you are not the first to try and fail to get data from GSO about this.

http://www.spectro-a...?p=12419#p12419

It seems seems GSO may make different models with different coatings for different sellers. A simple SiO coating works better in the UV. A manufacturer like Orion Optics UK make their own mirrors and coating so might be more flexible in making a scope to order with a specific coating 

 

Cheers

Robin


  • mborland likes this

#10 mborland

mborland

    Viking 1

  • *****
  • topic starter
  • Posts: 617
  • Joined: 17 Mar 2020
  • Loc: Tucson

Posted 07 March 2024 - 09:09 PM

I got a response from TS. I asked if they had mirrors for Newtonians giving ~50% or better reflectivity between 300 and 800nm. They said that their GSO-made mirrors satisfied this requirement. They didn't provide a reflectivity curve, though.

 

Based on this, I think GSO-made scopes or scopes with GSO-made mirrors will be good for amateur spectroscopy.

 

--Michael



#11 robin_astro

robin_astro

    Mercury-Atlas

  • *****
  • Posts: 2,567
  • Joined: 18 Dec 2005
  • Loc: UK

Posted 08 March 2024 - 06:52 AM

I got a response from TS. I asked if they had mirrors for Newtonians giving ~50% or better reflectivity between 300 and 800nm. They said that their GSO-made mirrors satisfied this requirement. They didn't provide a reflectivity curve, though.

 

Based on this, I think GSO-made scopes or scopes with GSO-made mirrors will be good for amateur spectroscopy.

 

--Michael

Hmm..... Perhaps the Newtonians with traditional coatings, but the reflectivity curve on their website for this GSO RC with a special coating for TS definitely does not satisfy this requirement

https://www.teleskop...624-mm-ota-5222

https://www.teleskop...ektrum-1000.jpg



#12 mborland

mborland

    Viking 1

  • *****
  • topic starter
  • Posts: 617
  • Joined: 17 Mar 2020
  • Loc: Tucson

Posted 08 March 2024 - 11:13 AM

Robin,

 

Perhaps GSO uses different processes for the RC mirrors. It does seem a bit odd..

 

The TS rep was pretty definite about this. He said the reflectivity is ~20% at 200 nm and ~80% at 400 nm, for example.

 

--Michael




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics